Zoonotic Potential of the Microsporidiaappropriate serodiagnostic tools seems feasible (298). No tests are available for the serodiagnosis of E. bieneusi. Many but not all microsporidia

CLINICAL MICROBIOLOGY REVIEWS, July 2005, p. 423–445 Vol. 18, No. 30893-8512/05/$08.00�0 doi:10.1128/CMR.18.3.423–445.2005Copyright © 2005, American Society for Microbiology. All Rights Reserved.

Zoonotic Potential of the MicrosporidiaAlexander Mathis,1* Rainer Weber,2 and Peter Deplazes1

Institute of Parasitology, Vetsuisse Faculty, University of Zurich,1 andDivision of Infectious Diseases and Hospital Epidemiology,

University Hospital,2 Zurich, Switzerland

INTRODUCTION .......................................................................................................................................................423The Organisms........................................................................................................................................................424Diagnostic Techniques ...........................................................................................................................................424Phylogeny .................................................................................................................................................................425Natural History .......................................................................................................................................................426

ENTEROCYTOZOON BIENEUSI ..............................................................................................................................426Infections in Humans.............................................................................................................................................426Infections in Animals .............................................................................................................................................428Animal Models ........................................................................................................................................................428Molecular Epidemiology ........................................................................................................................................428Sources of Human Infections and Transmission ...............................................................................................429

ENCEPHALITOZOON ................................................................................................................................................430ENCEPHALITOZOON CUNICULI............................................................................................................................431

Infections in Humans.............................................................................................................................................431Infections in Animals .............................................................................................................................................432

Rabbits .................................................................................................................................................................432Rodents.................................................................................................................................................................432Carnivores............................................................................................................................................................433Monkeys ...............................................................................................................................................................433

Molecular Epidemiology ........................................................................................................................................433Sources of Human Infections and Transmission ...............................................................................................434

ENCEPHALITOZOON HELLEM ..............................................................................................................................434Infections in Humans.............................................................................................................................................434Infections in Animals .............................................................................................................................................434Molecular Epidemiology ........................................................................................................................................435Sources of Human Infections and Transmission ...............................................................................................435

ENCEPHALITOZOON INTESTINALIS ....................................................................................................................435Infections in Humans.............................................................................................................................................435Infections in Animals .............................................................................................................................................436Molecular Epidemiology ........................................................................................................................................436Sources of Human Infections and Transmission ...............................................................................................436

OTHER MICROSPORIDIA......................................................................................................................................436Vittaforma spp. .........................................................................................................................................................436Pleistophora spp. ......................................................................................................................................................437Trachipleistophora spp.............................................................................................................................................437Brachiola spp. ..........................................................................................................................................................437Microsporidium spp. ................................................................................................................................................438

Microsporidium ceylonensis .................................................................................................................................438Microsporidium africanum ..................................................................................................................................438

CONCLUDING REMARKS......................................................................................................................................438ACKNOWLEDGMENT..............................................................................................................................................438REFERENCES ............................................................................................................................................................438

INTRODUCTION

Microsporidia are an exceptionally diverse group of or-ganisms, comprising more than 1,200 species which parasit-ize a wide variety of invertebrate and vertebrate hosts.These organisms have long been known to be causative

agents of economically important diseases in insects (silkworms and honey bees) (14), fish (179, 256), and mammals(rabbits, fur-bearing animals, and laboratory rodents) (37),and they emerged as important opportunistic pathogenswhen AIDS became pandemic (320). Thus, the question ofwhether animal reservoirs are the sources of human infec-tions is reasonable.

The focus of this review is to discuss the most recent per-ceptions on the zoonotic potential of the various microsporidiawith proven vertebrate hosts. Furthermore, we provide a brief

* Corresponding author. Mailing address: Institute of Parasitology,Winterthurerstr 266a, CH-8057 Zurich, Switzerland. Phone: 41 (0)44635 85 36. Fax: 41 (0)44 635 89 07. E-mail: [email protected].

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update on other microsporidia which have no known verte-brate host or an invertebrate host and cause rare infections inhumans.

The Organisms

Microsporidia are unicellular, obligate intracellular eu-karyotes. Their life cycle includes a proliferative merogonicstage, followed by a sporogonic stage resulting in characteris-tically small (1 to 4 �m), environmentally resistant, infectivespores (Fig. 1) (95). The spores contain a long, coiled tubularextrusion apparatus (“polar tube”), which distinguishes mi-crosporidia from all other organisms and has a crucial role inhost cell invasion: Upon extrusion from the spore, it injects thesporoplasm along with its nucleus into the cytoplasm of a newhost cell after piercing the plasmalemma of the host cell or themembrane of the phagosomes containing the endocytosedspores (58, 106).

Before the onset of the AIDS pandemic, only eight cases ofhuman microsporidial infections had been reported (reviewedin reference 320). In most cases, species identification of thecausative agents was not conclusive. In 1985, as early as 2 yearsafter the identification of human immunodeficiency virus(HIV) as the causative agent of AIDS, the microsporidial spe-cies Enterocytozoon bieneusi was discovered in HIV-infectedpatients with chronic diarrhea (76). Subsequently, several newgenera and species of microsporidia were found to be impor-tant opportunistic pathogens in humans, infecting virtually ev-

ery organ in the body and a broad spectrum of cell types (217)(Table 1).

The most common microsporidial infections in humans aredue to E. bieneusi and Encephalitozoon intestinalis. Both spe-cies have been found worldwide, mainly in HIV-infected pa-tients with chronic diarrhea but also in immunocompetentpersons with acute, self-limited diarrhea. Encephalitozoon cu-niculi and Encephalitozoon hellem have been diagnosed, withvery few exceptions, in immunocompromised patients and ascausing local (e.g., ocular) or disseminated infections. Otherrare microsporidial species infecting immunodeficient patientsinclude Vittaforma corneae (formerly Nosema corneum), Pleis-tophora ronneafiei, Trachipleistophora spp., and Brachiola spp.Furthermore, V. corneae, Trachipleistophora hominis, Brachiolaalgerae, Nosema ocularum, Microsporidium ceylonensis, and Mi-crosporidium africanum have been described in single cases asagents of ocular infections in immunocompetent persons.

Due to the administration of antiretroviral combinationtherapy, which restores immunity in HIV-infected persons, thenumber of clinically manifest microsporidial infections hasmarkedly decreased in affluent countries (322). However, it isestimated that two-thirds of all HIV-infected persons live insub-Saharan Africa, where antiretroviral therapy is not widelyavailable due to the costs, and consequently, opportunisticcomplications continue to cause severe morbidity and mortal-ity. Indeed, a recent study showed that 13% of HIV-positivediarrheic adults in Mali were positive for E. bieneusi, whichthus was the most prevalent intestinal parasite in this Africancountry (10). Furthermore, microsporidial infections are in-creasingly being diagnosed in affluent countries in immunosup-pressed patients who undergo organ transplantation (39, 115,119, 130, 132, 156, 167, 197, 200, 228, 229, 244, 260, 283), aswell as causing ocular infections in nonimmunocompromisedpersons (47, 171, 206, 259, 274, 284).

Diagnostic Techniques

Considerable progress has been made in improving the rep-ertoire of techniques for detection of microsporidia. Previ-ously, diagnosis was based on laborious electron-microscopicexaminations because of the small size of the organisms. Theintroduction of staining techniques allowed routine, light-mi-croscopic diagnosis of microsporidial spores, at least to thegenus level (319). Diagnosis to the species level is achieved byusing antibodies (polyclonal or monoclonal) and by molecularmethods based on the PCR (reviewed in references 116, 322,and 327). This latter sensitive and specific method has, inaddition, the intrinsic advantage that upon further analysis ofthe PCR products with various methods (restriction fragmentlength polymorphism, SSCP, or sequencing), identification atthe subspecies level (strains or genotypes) can be achieved (86,154).

Serologic tests have been useful in detecting antibodies of E.cuniculi in several species of animals, but the value of detectingantibodies against Encephalitozoon spp. in humans has beencontroversial because of possible nonspecificity of the testswhen spore walls are used as the antigen (322). By employingrecombinant antigens of the polar tube of E. cuniculi, a highspecificity was recently demonstrated, and the development of

FIG. 1. Electron micrograph of the developmental stages of themicrosporidian species Encephalitozoon cuniculi in a host cell-derivedvacuole in in vitro-cultivated human fibroblast cells. K, nucleus of hostcell; M, meront; P, sporont, which divides into two sporoblasts; B,sporoblast (2 �m in length), with cross sections of the polar tube; S,mature spore.

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appropriate serodiagnostic tools seems feasible (298). No testsare available for the serodiagnosis of E. bieneusi.

Many but not all microsporidia infecting humans can becontinuously cultivated in vitro in various cell lines (184, 306).This facilitates investigations of their basic biology (102, 127)and also allows for easy assessment of drugs (80) or disinfec-tion schemes (118, 147, 150, 173), which has become an issuesince human-pathogenic microsporidia have been discoveredin surface waters (57, 89, 90, 103, 150, 255, 272).

Phylogeny

Investigations on the basic biology have unearthed highlyexceptional characteristics of the microsporidia. Although theyare true eukaryotes (i.e., they possess a typical eukaryotic nu-cleus, endomembrane system, and cytoskeleton), they also dis-play molecular and cytological characteristics reminiscent ofprokaryotes, including features of the translational apparatus,genome size (which is in the range of that of bacteria), and lack

of recognizable mitochondria, peroxisomes, and typical Golgimembranes (reviewed in reference 189). Because of their pro-karyotic features, microsporidia were initially classified withinthe Archezoa, together with other amitochondriate organisms(Giardia, trichomonads, and Entamoeba) which were thoughtto have evolved prior to the acquisition of mitochondria byendosymbiosis and consequently to represent the most primi-tive eukaryotes (42). However, the genome sequence of E.cuniculi revealed that it contains genes related to some mito-chondrial functions, implying that microsporidia have retaineda mitochondrion-derived organelle (152). Indeed, tiny or-ganelles with double membranes could be demonstrated byusing antibodies against a mitochondrial protein (Hsp70) inthe human microsporidial parasite T. hominis (330).

Furthermore, sophisticated phylogenetic analyses revealedthat the microsporidia evolved from within the fungi, beingmost closely related to the zygomycetes (43, 155). Microspo-ridia share additional features with fungi, e.g., the presence ofchitin (although chitin is also found in other phyla, e.g., mol-

TABLE 1. Microsporidial species infecting humans

Species

Discovery No. of confirmed reported patients

Site(s) of infection (reference[s]) Animalhost(s)Host Yr Reference(s) Immuno-

compromisedaImmuno-

competentb

Enterocytozoon bieneusi Human 1985 76 �1,000 �20 Intestine, biliary tract, respiratorytract (322)

Vertebrates

Encephalitozoon hellem Human 1991 78 �50 3 Eye, respiratory tract, urinarytract, systemic infection (322)

Birds

Encephalitozoonintestinalis (originallynamed Septataintestinalis)

Human 1993 30, 139 �200 2 Intestine, biliary tract, respiratorytract, bone, skin, systemicinfection (322)

Mammals

Encephalitozooncuniculi

Rabbit 1923 170 �20 —c Systemic infection, eye,respiratory tract, urinary tract,liver, peritoneum, brain (322)

Mammals

Vittaforma corneae(originally namedNosema corneum)

Human 1990 65, 258 1 3 Eye, urinary tract (75, 101, 231) Unknown

Vittaforma-liked Human 2003 278 22 3 Intestine (278) UnknownPleistophora ronneafiei

(originally namedPleistophora sp.)

Human 1985 32, 169 1e — Muscle (169) Unknown

Trachipleistophorahominis

Human 1996 145 1 1 Muscle, eye (100, 231) Unknown

Trachipleistophoraanthropophthera

Human 1998 305 3 — Systemic infection, eye (151, 335) Unknown

Brachiola algerae(originally namedNosema algerae)

Mosquito 1970 304 1f 1 Eye, muscle (61, 307) Mosquito

Brachiola connori(originally namedNosema connori)

Human 1974 33, 273 1g — Systemic infection (273) Unknown

Brachiola vesicularum Human 1998 33 1 — Muscle (33) UnknownNosema ocularum Human 1991 31 — 1 Eye (31) UnknownMicrosporidium

ceylonensisHuman 1973 11, 12, 36 1 (unknown) Eye (12) Unknown

Microsporidiumafricanum

Human 1981 223 1 (unknown) Eye (223) Unknown

a HIV-seropositive persons, AIDS patients, and organ transplant recipients.b Immunocompetent, otherwise healthy.c Two patients with unknown immunostatus, presumably cellular immunodeficiency (see Table 4).d PCR/sequencing results only, needs confirmation.e Cellular immunodeficiency (HIV antibody negative).f Patient was taking immunosuppressive agents for rheumatoid arthritis for decades.g Thymic aplasia.

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lusks) and trehalose, similarities between the cell cycles, andthe organization of certain genes (35, 211). Therefore, micro-sporidia are nowadays considered to be highly derived fungithat underwent substantial genetic and functional losses result-ing in one of the smallest eukaryotic genomes described todate.

The placement of microsporidia among the fungi might haveimplications for the discovery of new therapeutic strategies.Although microsporidiosis in general can be successfullytreated with albendazole and fumagillin, therapy for the mostprevalent species, E. bieneusi, is difficult (129, 204). Indeed, afew studies have demonstrated that inhibitors of chitin syn-thase enzymes are effective against microsporidia (19, 270).

Natural History

A fundamental question that arose with the discovery of newmicrosporidial species in humans was that of their naturalorigin. The phylogram generated with small subunit ribosomalRNA (ssrRNA) gene sequences of microsporidia infecting hu-

mans (Fig. 2) illustrates their polyphyletic nature. Of particularinterest is the fact that the closest relatives of three speciesinfecting humans and belonging to the genera Trachipleisto-phora and Vittaforma are microsporidia that infect insects, andit is tempting to speculate that the insects might serve asreservoirs for these microsporidia. Indeed, another microspo-ridian of arthropod origin, Brachiola (formerly Nosema) al-gerae, was demonstrated to be a causative agent in humaninfections (61).

The two new species of the genus Encephalitozoon, E. intes-tinalis and E. hellem, are very closely related to the widespreadparasite of mammals E. cuniculi. The most prevalent species,E. bieneusi, is most closely related to a fish pathogen. For allfour major microsporidial species infecting humans (E. bie-neusi and the three Encephalitozoon spp.), animal hosts areknown (Table 1; Fig. 2) implying a zoonotic nature of theseparasites. Molecular studies have identified phenotypic and/orgenetic variability within these species, indicating that they arenot uniform. Whereas the significance of this variability re-mains largely unknown for E. hellem and E. intestinalis, strainvariation in E. bieneusi and E. cuniculi provided new insightsinto the biology, origin and distribution of these parasites andhas allowed the question of their zoonotic potential to beaddressed.

ENTEROCYTOZOON BIENEUSI

There are two genera in the family Enterocytozoonidae: (i)Nucleospora, with the two characterized species N. salmonis, anintranuclear microsporidian of salmonid fish (87), and N. se-cunda, a parasite of a warm-water African fish (180), and (ii)Enterocytozoon, with E. bieneusi, infecting the cytoplasm ofenterocytes and other epithelial cells in humans and mammals(76).

E. bieneusi, the most common microsporidial species knownto cause human disease, was first described as an HIV-associ-ated opportunistic intestinal pathogen in 1985 and was mor-phologically characterized using electron microscopy (76). In1996, morphologically identical spores were detected in fecesof pigs (74), and subsequently they also were detected in fecalsamples and intestinal tissue of other mammals.

Infections in Humans

Several hundred HIV-infected patients with chronic diar-rhea attributed to this organism have been reported from allover the world. The prevalence of E. bieneusi infections amongHIV-infected patients reached up to 50% as documented byselected studies in Table 2. Human infections are well docu-mented in all inhabited continents. In most studies, preva-lences were significantly higher in patients with chronic diar-rhea (92, 99, 128, 271, 324, 332). The prevalences presented inTable 2, however, do not allow for comparative analyses be-cause there were considerable differences with regard to theselection of the patient groups, the patients’ characteristics(age, sex, sociodemographic data, degree of immunodefi-ciency, and clinical features), and the specimens investigated(biopsies or stool samples). Furthermore, the improvements ofdiagnostic methods achieved over the last 15 years have to beconsidered.

FIG. 2. Dendrogram generated from the small subunit ribosomalRNA (ssrRNA) gene of microsporidian species identified in humans(underlined) and selected other species (Kimura’s distance, un-weighted pair group method of analysis). Known animal hosts areindicated in brackets; the brewer’s yeast Saccharomyces cerevisiaeserves as an outgroup. No corresponding gene sequences are knownfor the human-infecting microsporidian species Pleistophora ronneafiei,Trachipleistophora anthropophthera, Brachiola (formerly Nosema) con-nori, B. vesicularum, Nosema ocularum, Microsporidium ceylonensis,and M. africanum (Table 1).



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A few studies from affluent countries indicate that the prev-alence of E. bieneusi in HIV-infected patients is decreasing (53,324). Recent studies have shown that administration of anti-retroviral combination therapy can result in remission of HIV-

associated intestinal microsporidiosis (40, 54, 120, 198, 214). Adecrease of 50% in E. bieneusi cases in Switzerland was alsointerpreted as being associated with antiretroviral therapy(324).

TABLE 2. Selected studies on prevalence rates of E. bieneusi in HIV-infected persons

Geographic area Specimen; diagnostic methoda No. of patients examined; patients’characteristics Prevalence (%) Yr (reference[s])

AfricaCameroon (Yaunde) Stool; LM 66; chronic diarrhea 9b 1997 (241)Niger (Niame) Stool; LM 60; 41 with diarrhea 7 1993 (27)Zimbabwe (Harare) Stool; LM 129; hospitalized with diarrhea 13 1995 (300)Zimbabwe Formalin-fixed stool; PCR 74; 45 with diarrhea 46 1997 (41)Zambia (Lusaka) Stool; LM 69; chronic diarrhea 23 1995 (91)Mali (Bamako) Stool; LM, partially

confirmed by TEM88; 80% with chronic diarrhea 32 1997 (185)

Tanzania Stool; LM, TEM 86; chronic diarrhea 3 1999 (44)Zimbabwe (Harare) Stool; LM, PCR 88; diarrhea longer than 1 week 18 (LM), 51 (PCR) 1999 (133)Mali (Bamako) Stool; LM, IFAT, PCR 61; diarrhea 13.1 2002 (10)

AmericasUnited States (Washington, D.C.) Intestinal biopsies; EM, TEM 67; chronic unexplained diarrhea 30 1990 (218)United States (Texas) Duodenal biopsies; TEM 55; chronic diarrhea 33 1993 (227)

51; without chronic diarrhea 25United States (Atlanta, Ga.) Stool; LM 65; diarrhea 11 1993 (128)

65; without diarrhea 2United States (New York) Intestinal biopsies; TEM 34; AIDS patients 44b 1994 (160, 161)

194; diarrhea 39b

United States (New York) Duodenal biopsies; PCR,confirmation by TEM

68; diarrhea 37 1996 (62)

43; without diarrhoea 2.3United States (California) Stool; LM 8,439; diarrhea, yr 1993, 1994,

1995, 1996.8.8, 9.7, 6.6, 2.9 1998 (53)

Brazil (Fortaleza) Stool; LM 79; with diarrhea 6b 1994 (332)82; without diarrhea 1b

Brazil (Rio de Janeiro) Stool; LM 13; chronic unexplained diarrhea 46b 1996 (25)Brazil (Rio de Janeiro) Stools, duodenal or ileal

biopsies40; chronic diarrhea 27.5 2000 (24)

Peru (Lima) Stool; LM 2672; diarrhea 3.9 2003 (275)

AustralasiaAustralia (New South Wales) Duodenal biopsies; LM 109; chronic diarrhea 29 1993 (99)

confirmed by EM 71; without diarrhea 1.4Australia (Victoria) Stool; LM, TEM 139; diarrhea 3.5 1993 (239)Thailand Stool; LM, TEM 66; chronic diarrhea 33.3 1998 (313)Thailand (Bangkok) Stool; LM; TEM 288; diarrhea 11 2001 (317)Thailand Stool; LM, TEM 95; children with diarrhea 25.3 2002 (312)India Stool; LM 120; diarrhea 2.5 2002 (199)

EuropeThe Netherlands Duodenal biopsies; LM

partially confirmed byTEM

55; unexplained diarrhea38: without diarrhea

27b

3b1991 (92)

The Netherlands Stool; LM 143; diarrhea 7.7 1993 (301)France Duodenal biopsies; LM 66; chronic diarrhea 2 1993 (56)France (Paris) Stool, intestinal biopsies; LM 18; chronic unexplained diarrhea 50 1993 (203)France (Nice) Stool; LM 46; chronic diarrhea 24 1995 (16)Italy (Apulia) Stool; LM 56; diarrhea 2b 1995 (187)Italy Intestinal biopsies; EM, TEM 72; chronic diarrhea 4.2 1996 (309)Germany (Cologne) Intestinal biopsies; PCR,

Southern hybridization46; diarrhea 22 1996 (110)

Germany (Hamburg) Stool; LM 50; diarrhea, hospitalized patients 32 1998 (271)47; without diarrhea 4

England (London) Intestinal biopsies; LM, EM 59; diarrhea 14b 1991 (221)England (northwest) Stool, intestinal biopsies; LM

confirmed by TEM63; diarrhea 14 1995 (166)

Spain (Madrid) Stool; LM, confirmation byPCR

48 children; chronic diarrhea 2 1997 (69)

Sweden (Stockholm) Duodenal biopsies; LM 72; abdominal symptoms ofunknown cause

3b 1998 (281)

Switzerland Stool; LM partially confirmed 164; chronic diarrhea (1992–1994) 10.7 1999 (324)by TEM and PCR 156; chronic diarrhea (1994–1996) 5.3

949; without diarrhea 0.4Portugal Stool; LM, PCR 215; diarrhea 42.8c 2001 (98)

a LM, light microscopy; TEM, transmission electron microscopy.b Intestinal microsporidia, species not stated.c A total of 92 samples were positive for microsporidia; 20 of 69 isolates that were further characterized by PCR were E. bieneusi, and 49 were E. intestinalis.



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Although predominantly described among adults sufferingfrom immunodeficiency due to HIV infection, E. bieneusi in-fections were also reported from HIV-negative patients whowere immunocompromised due to an underlying disease ordue to therapeutic immunosuppression when undergoing or-gan transplantation (119, 130, 197, 228, 229, 260, 318). Fur-thermore, a few E. bieneusi infections in HIV-negative, immu-nocompetent, and otherwise healthy persons that wereassociated with self-limited diarrhea were reported, mostly inthe context of traveler’s diarrhea in Europe (9, 71, 114, 181,182, 209, 242, 268, 282, 314) but also in single cases in Africa(44, 131). Hence, E. bieneusi was detected by PCR in stoolsamples from 7 of 148 travelers with diarrhea returning toGermany (209). A recent study performed in Spain revealedthat 8 of 47 (17%) geriatric persons with diarrhea were in-fected with E. bieneusi (182), and it was speculated that age-related diminishment of the immunological capacities mightpredispose elderly persons to microsporidial infections. On theother hand, no data so far indicate that children might be moresusceptible to E. bieneusi infections (300).

Over the last decade, evidence has accumulated that E.bieneusi might also persist as an asymptomatic infection inimmunocompetent humans. E. bieneusi was recovered in 8 of990 stool samples from African children who were not consid-ered HIV positive, suggesting enteric carriage among immu-nocompetent persons in tropical countries (27). Asymptomaticinfections in children were reported in another study fromAfrica (44) and in a study from Asia, where not only healthyorphans (5.9%) but also child-care workers (1.9%) were in-fected (213). In all these studies, light microscopy, partly com-bined with transmission electron microscopy for confirmationof positive cases, was the diagnostic method, which might notbe sensitive enough to detect subclinical infections under allcircumstances. Therefore, more sensitive diagnostic tools suchas PCR are required to elucidate the question of whether thisparasite is a common organism of the human intestinal flora,causing severe disease only under immunodeficiency, orwhether zoonotic transmission is of considerable importance(see below).

Infections in Animals

Eleven years after its discovery as a human pathogen, E.bieneusi was for the first time detected in animals, namely, inpigs (74), and a prevalence of 35% was determined by PCR ina subsequent investigation (26) of 109 randomly selected pigsfrom 22 farms located in different parts of Switzerland. Asignificantly (P � 0.05) higher occurrence of E. bieneusi wasfound in weaned piglets. The feces of three infected pigs, whichdid not show any clinical signs, were examined weekly by PCR,revealing excretion of E. bieneusi spores in 67% of the samples.Hence, E. bieneusi seems to be a common parasite in asymp-tomatic pigs. The low pathogenicity of E. bieneusi for pigs wasfurther substantiated by the lack of intestinal lesions in immu-nosuppressed and immunocompetent gnotobiotic piglets ex-perimentally infected with E. bieneusi of human or macaqueorigin (159). This study, however, demonstrated that immuno-suppression of piglets did lead to an increased excretion ofspores.

Subsequent studies have confirmed the occurrence of E.

bieneusi in pigs with high prevalence (32%) (29) and also incalves (9.5 to 11.5%) (237, 243, 277). The parasite has alsobeen detected in cats (72, 191), dogs (182, 191), a goat (182),a llama (72), a variety of species of wild fur-bearing mammals(beavers, foxes, muskrats, otters, and raccoons) (72, 276),hedgehogs (A. Mathis, unpublished data), and, recently, non-mammalian hosts (chickens and pigeons) (233; M. Haro et al.,unpublished data [GenBank accession number AY668953]).

Natural infections with E. bieneusi were documented in cap-tive monkeys, namely, rhesus macaques. Prevalences were16.7% (n � 131) in normal, asymptomatic animals, in whichthe infection persisted for 262 days, and 33.8% (n � 53) inanimals which were experimentally infected with the simianimmunodeficiency virus (186). A screening of 42 wild monkeysfrom Central Africa (Cameroon) by microscopy and PCR didnot yield a single E. bieneusi-positive result (225).

Animal Models

For various reasons including mass production of the para-site for basic research, development of diagnostic tools, drugscreening, and studies on disease pathogenesis, an animalmodel of enterocytozoonosis is desirable. E. bieneusi of humanorigin has been established in immunocompromised rhesusmonkeys (126, 293), immunosuppressed gnotobiotic piglets(159), Sprague-Dawley rats (2), and New Zealand rabbits (3).In all animals, only chronic asymptomatic infections were ob-served, similar to the infections in naturally infected, immuno-competent pigs (26). Many attempts to establish E. bieneusi inimmunocompetent and immunodeficient mice were unsuccess-ful (83). Hence, all hitherto-described experimental animalmodels appear not to be adequate to mimic the pathologicalintestinal situation in HIV-infected patients.

Molecular Epidemiology

Analyses of the single internal transcribed spacer of therRNA genes (ITS) have revealed that there is considerablegenetic variation within E. bieneusi isolates of human and an-imal origins, and more than 50 genotypes have so far beendescribed based on subtle differences within this 243-bp se-quence. An overview of human-infecting genotypes is providedin Table 3. In contrast to the situation with microsporidia ofthe genus Encephalitozoon (see below), no other genetic mark-ers are available. However, classification of isolates of E. cu-niculi and E. hellem based on ITS sequences has largely beenconfirmed by data for other genetic loci. Nevertheless, addi-tional independent markers for E. bieneusi are highly desirablein order to clarify the genetic structure of the parasite’s pop-ulations.

Five different ITS genotypes of E. bieneusi infecting humanshave been confirmed in independent studies and another 12were discovered in single studies, with one of these studiesaccounting for eight of these novel genotypes (275). Limitedinformation is available on the geographic distribution of hu-man-derived genotypes of E. bieneusi. Except for the above-mentioned eight genotypes identified in a study in SouthAmerica (275), all other genotypes have been found in Europe,where most of the studies aiming at genotyping this parasitehave been conducted (26, 72, 174, 235, 240). In additions,



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genotypes A, B, D, and IV have been identified in HIV-in-fected patients from South America and the United States(193, 275). In single studies from Asia and Africa, genotype Awas found in 20 asymptomatic children in Thailand (I. Sub-rungruang et al., unpublished data [GenBank accession num-bers AY357185 to AY357404]) and genotype IV in childrenwith diarrhea in Uganda (292).

Of all 17 human-infecting E. bieneusi genotypes identified sofar, four seem to have a zoonotic potential, as they have alsobeen discovered in vertebrate hosts (Table 3). For the threegenotypes A, B, and C, which account for the overwhelmingnumber of isolates from humans, no animal host is known, andone might speculate that it is simply a matter of time until suchhosts will be identified. On the other hand, several lines ofevidence suggest that there is a certain degree of host speci-ficity, at least for some of the E. bieneusi genotypes. First, adendrogram based on ITS sequences of human-infecting E.bieneusi genotypes (confirmed in independent studies) and se-lected genotypes with animal hosts reveals a clustering of ge-notypes according to host species, although this grouping is notabsolute (Fig. 3). Interestingly, the eight novel E. bieneusigenotypes recently identified in a single study of HIV patients(275) (Table 3) cluster within the branch containing all other

human-derived genotypes (for a detailed dendrogram, see ref-erence 275).

Further evidence for some degree of host specificity origi-nates from experimental animal models using immunodeficientor immunosuppressed animals (see above). With E. bieneusi ofhuman origin, only chronic asymptomatic infections which donot appropriately mimic the pathological intestinal situation inHIV-infected patients were observed in rhesus monkeys, rats,piglets, and rabbits (see above), and many attempts to establishhuman-derived E. bieneusi in immunodeficient mice were un-successful (83). A minor role of animals as sources of humaninfections is also substantiated by epidemiological data (seebelow).

Taken together, the picture of E. bieneusi with respect to itszoonotic potential is reminiscent of that of another intestinalparasite, Giardia lamblia, which comprises zoonotic as well asnonzoonotic genotypes (205).

Sources of Human Infections and Transmission

Extensive intestinal and rare respiratory tract involvement asdescribed for patients with disseminated E. bieneusi infectionssuggest that different modes of transmission are possible, in-

TABLE 3. Enterocytozoon bieneusi genotypes in humans: number of described cases and animal hosts

E. bieneusi genotypedesignation

No. of reported human cases (reference)Animal host, genotype

designation (reference[s])HIV-infected or otherwiseimmunocompromised patients Immunocompetent patients

Genotypes identified inindependent studiesA 3 (235), 3 (26), 1 (193), 32 (275)B, I 3 (235), 8 (26), 66 (174), 2 (193), 11

(240)C, II 2 (235), 2 (26), 1 (transplant

recipient) (260), 9 (174), 7(transplant recipient) (174),

1 (174)

1 (lymphoma) (174), 1 (72)D 1 (236), 9 (275), 1 (240) Macaque (45, 126); pig,

EBITS9 (29); wildlife,WL8 (29, 45, 276)

IV 9 (174), 1 (transplant recipient),(174), 1 (myeloma) (174), 18 (275),1 (240)

1 (174), 10 (children with diarrhea,immunostatus not determined;estimated to be 18–29% amongthese children) (292)

Cat, K (72); cattle, BEB5(72, 277)

Genotypes identifiedin single studies

Q (identical to C, IIbut with 2 ntinsertion)

1 (72)

III 3 (174)V 1 (174)Peru3 1 (275)Peru4 1 (275) Pig, EbpC (26); pig E

(237); wildlife, WL13(276)

Peru5 3 (275) Wildlife, WL11 (276)Peru6 1 (275)Peru7 8 (275)Peru8 4 (275)Peru10 3 (275)Peru11 6 (275)



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cluding the fecal-oral or oral-oral route, inhalation of aerosols,or ingestion of food contaminated with fecal material. In ad-dition, direct human-to-human transmission is substantiated bystudies that implicate male homosexuality or having an HIV-infected cohabitant as risk factors for acquiring intestinal mi-crosporidiosis (148, 316). Person-to-person transmission wasalso suggested in a study which revealed that 9 of 13 infectedorphans, who were HIV negative, were confined to two houses,whereas HIV-positive children inhabiting another house werenot infected (213).

The detection of E. bieneusi in immunocompetent humancarriers indicates that this parasite, or at least some of itsgenotypes, could naturally persist in the human population.Infections in organ transplant recipients or otherwise immu-nocompromised HIV-negative patients as well as in immuno-competent individuals were probably unrelated to direct con-tact with infected patients with AIDS.

No seasonal variation was obvious in the prevalence of hu-man intestinal microsporidiosis in Brazil, as had been foundfor the intestinal parasite Cryptosporidium parvum (332). De-

spite differences in climate and sociodemographic factors, sim-ilar results were obtained in a study in southern Californiainvestigating 8,439 HIV-infected patients over a period of 4years (53). In both studies it was suggested that contaminateddrinking water was not likely to be the major source of micro-sporidial infections. However, other investigators have sug-gested that water contact may be an independent risk factor forenterocytozoonosis. In a prospective case-control study includ-ing 30 HIV-infected patients with intestinal microsporidiosis(28 with E. bieneusi infection and 2 with E. intestinalis infec-tion) and 56 HIV-infected controls (148), “swimming in apool” during the preceding 12 months was identified as one ofthree risk factors for intestinal microsporidiosis (besides malehomosexuality and a CD4 lymphocyte count of �100/mm3).Other factors, such as consumption of different sorts of bever-ages or undercooked food, exposure to animals (cats, dogs,birds, bees, or fish), or recreational activities (swimming infreshwater or in the sea, trips abroad in the past 36 months, orvisits to the countryside) were found not to be related toinfection risk (148). Another study done in the United Statesincluded 12 HIV-infected patients with intestinal microsporidi-osis and 54 CD4-matched controls. Risk factors for the acqui-sition of microsporidia were different kinds of water contacts(recreational swimming in rivers and lakes, drinking unfilteredtap water, or use of humidifiers) and close contact with anotherHIV-infected person (316). Lastly, a study investigating anurban cohort of HIV-infected patients revealed occupationalcontact with water or use of a hot tub or spa as risk factors foracquiring intestinal microsporidiosis (64), whereas contact withcompanion animals was not related to infection risk.

Infection risk associated with traveling was suggested for E.bieneusi infections of HIV-infected and noninfected travelersin one study (55) but not in another one (64). A comparativestudy on diarrheic HIV-infected patients from the Paris area(France), including 26 patients with intestinal microsporidialinfection (species not determined) and 28 patients with cryp-tosporidiosis, revealed that trips to tropical countries werestrongly associated only with microsporidial infections (55). Itis not known whether particular factors are associated withmicrosporidial transmission in tropical countries where HIV-negative adult and children were found to be infected (27).

Detection of E. bieneusi and confirmation to the specieslevel was achieved by PCR and subsequent sequence analysisof part of the ssrRNA gene in surface water but not in ground-water samples (89, 272) and recently, by species-specific PCR,also in zebra mussels from a river (123). Contamination ofsurface water may be from discharged domestic wastewater orfrom animal sources. As no genotyping was performed in thesestudies, the potential infection risk for humans from suchsources needs further clarification.

ENCEPHALITOZOON

Three species of the genus Encephalitozoon have been iden-tified as human pathogens: (i) E. cuniculi, which has a widehost range among mammals (37) and a worldwide distributionin domestic rabbits and is found in distinct geographic areas incarnivores and monkeys; (ii) E. hellem, which was distinguishedfrom E. cuniculi in 1991 (78) and which has been reported in afew cases in avian hosts in the United States and Indonesia;

FIG. 3. Dendrogram of ITS sequences of human-infecting E. bie-neusi genotypes (confirmed in independent studies) and selected ge-notypes with animal hosts. All sequences are deposited in GenBankwith the same designations (Kimura’s distance, unweighted pair groupmethod of analysis).



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and (iii) E. intestinalis (originally designated Septata intestina-lis), which first was described in 1993 (30) and which wasdiagnosed in feces of farm animals in Mexico and in gorillas inAfrica.

All three species have spores that are morphologically indis-tinguishable. Intraspecies genetic variation has so far beendescribed for E. cuniculi and E. hellem. In E. cuniculi, threestrains (I, II, and III) are recognized, which, according to thehost of the originally characterized isolates, are also designated“rabbit strain,” “mouse strain,” and “dog strain.” The fourthknown species from this genus, E. lacertae (37, 162), was iden-tified in reptiles only and is most closely related to E. cuniculi.

ENCEPHALITOZOON CUNICULI


The first Encephalitozoon infection reported in humans, in1959 (195), as well as a few subsequent cases were diagnosedbased on spore morphology only. Therefore, these cases can-not unambiguously be attributed to E. cuniculi, as species dif-ferentiation was not possible at that time. Recent findings of E.cuniculi infections, as determined by immunological and/or

molecular methods, in several patients (HIV infected, under-going organ transplantation, or with idiopathic CD4� T-lym-phocytopenia) from Europe and from the United States provethe infectivity of E. cuniculi for immunodeficient humans (Ta-ble 4).

In seroepidemiological studies with enzyme-linked immu-nosorbent assay (ELISA) and the indirect fluorescent-antibodytest (IFAT) using spore antigens of E. cuniculi or parasite cellsuspensions, prevalences of up to 42% have been reported forpatients with a history of tropical diseases or a stay in tropicalcountries and for patients with renal diseases, psychiatric dis-orders, or neurological disorders (15, 142, 146, 261). DespiteWestern blot analysis, which potentially increases the specific-ity of the ELISA (146), it is uncertain whether detection ofantibodies to E. cuniculi reflects true infections or antigenexposure without establishment of the parasite, cross-reactiv-ity, or reactions due to polyclonal B-cell stimulation, particu-larly in patients with tropical diseases. More recent studies (50,121, 135, 165, 224, 302) suffered from the same methodologicallimitations. However, all these serological studies suggest thathuman exposure to microsporidia may be common but withoutclinical significance. In a very recent serodiagnostic study of an

TABLE 4. Single cases of human infections due to E. cuniculi confirmed by molecular analyses

Immune status and country Clinical manifestations Site(s) of infection and/or specimen E. cuniculistrain Yr Reference(s)

HIV-infected patientsUnited Kingdom Abdominal pain, anorexia, nausea, vomiting,

fever, cough, renal failureKidney, urine III 1994 1, 141, 143

United States Fever, cough, emesis, insomnia, sinuscongestion, severe dry eyes, blurred vision

Urine, sputum III 1995 63, 66, 85

Germany Keratoconjunctivitis, sinusitis, rhinitis Urine, sputum stool, nasal discharge,duodenal biopsy

NDa 1995 111

Switzerland Headache, visual impairment, cognitiveimpairment, nausea, vomiting

Cerebrospinal fluid, stool, sputum,urine

I 1997 321

United States Dizziness, fever, nausea, abdominal pain,diarrhea

Adrenal glands, kidneys, brain,heart, trachea, urinary bladder,spleen, lymph nodes

III 1997 196

Switzerland None Urine I 1997 192Mexico Pneumonitis, otitis media Urine, respiratory specimen, stool III 1997 73, 192Switzerland None Urine I 1997 73, 192Switzerland Conjunctivitis, sinusitis, seizure disorder Urine I 1997 192Switzerland Renal insufficiency, leucocyturia,

erythrocyturiaUrine I 1997 192

Switzerland None Urine, respiratory specimen I 1997 192Italy Chronic sinusitis, bilateral keratoconjunctivitis Nasal epithelium I 1998 238France Visual impairment Cerebrospinal fluid, urine, sputum,

stool, duodenal biopsyND 2000 105

Chile Cough, fever Bronchoalveolar lavage,transbronchial biopsy

ND (IIIb) 2001 328

Spain Fever, asthenia, abdominal pain, diarrhea Stool, urine, sputum III 2001 68Italy Fever, myalgia, poor general condition Kidney, liver, lymph nodes, spleen,

adrenal medulla, brain, ovaryIII 2002 288

HIV-negative patients,immunocompromised(undergoing organtransplantation)

Canada Fever, keratoconjunctivitis, allografttenderness,

Urine, stool, sputum, conjunctivalscrapings

III 2002 200

Mexico Cough, fever, diarrhea, thoracic pain, extremeweakness

Liver, kidney III 2003 115

United States Respiratory distress Lung biopsy III 2004 283

HIV-negative patients,otherwiseimmunocompromisedc

Switzerland Iris tumor Tumor biopsy I 2005 158

a ND, not determined.b E. cuniculi strain III deduced from clinical and epidemiological findings (see the text).c Idiopathic CD4� T-lymphocytopenia.



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immunocompetent laboratory worker who was accidentally in-fected with E. cuniculi when drops containing spores werespilled in his eyes, cross-reactivity of his serum with spore wallsof E. hellem and E. intestinalis was demonstrated. However,only little cross-reactivity was observed when recombinant po-lar tube proteins of these parasites were used as antigens (298).


Beyond the numerous reports of E. cuniculi infections inrabbits, carnivores, and monkeys (see below), the parasite wasidentified as the causative agent of placentitis and abortion ina horse (220). Serological investigations (IFAT) revealed an-tibodies against E. cuniculi in goats and cattle (51, 136), but, asoutlined above, the test specificity was not ascertained.

Rabbits. Encephalitozoonosis in laboratory and pet rabbitsis of clinical significance worldwide. E. cuniculi usually causeschronic infections which can persist asymptomatically foryears. Severe neurological disease due to granulomatous en-cephalitis can occur unrelated to the age and sex of the animals(210) (Fig. 4). Until microsporidian-negative rabbit colonieswere established, encephalitozoonosis was an important con-founding variable in rabbit-based biomedical research on a

variety of diseases (reviewed in reference 315).In the past, high prevalences of encephalitozoonosis were

recorded for laboratory rabbit colonies (37, 315), but theseinfections can be controlled by serological screening and highhygienic standards. On the other hand, the disease is still highlyendemic in the pet rabbit population. In seroepidemiologicalsurveys in Switzerland and the United Kingdom, specific anti-bodies against E. cuniculi spores were detected in 7.5% (n �292) and 23% (n � 26) of healthy rabbits and in 85% (n � 72)and 71% (n � 65) of rabbits (mainly kept as pets) with neu-rological symptoms or with direct contact with symptomaticanimals (74, 137, 210), respectively.

In rabbits, horizontal transmission by ingestion of spores isregarded to occur most frequently, but intrauterine infectionhas also been documented (13, 37). After experimental oralinfections of rabbits, regular spore excretion in the urine wasobserved between days 38 and 63 postinfection and later in-termittently at very low density (59). Spore excretion was re-ported in 9 of 11 symptomatic rabbits (60), indicating that suchanimals may play an important epidemiological role.

Based upon serological evidence, it was suggested that wildrabbits (Oryctolagus cuniculus) represent the natural host of E.cuniculi (331). Other studies revealed seroprevalences of 3.9%among 204 wild European rabbits in France (46) and 25% inthe wild rabbit population in Western Australia (285). To ourknowledge, E. cuniculi infections have so far not been reportedfor other free ranging lagomorphs.

Rodents. E. cuniculi has been diagnosed in the past in nu-merous cases as a common parasite of laboratory rodents suchas mice, rats, hamsters, and guinea pigs (reviewed in references37 and 315), but nowadays, with high hygienic standards for themaintenance of laboratory rodents being applied, infectionswith microsporidia should no longer be a significant problem inthese animals. On the other hand, rodent models have gainedmore attention for immunological research in the field of mi-crosporidiosis (reviewed in reference 77).

Information about the possible significance of microsporidi-osis in rodents kept as pet animals and about the epidemiologyof microsporidiosis in wild rodents is scarce. Until 1986, onlythree reports of E. cuniculi infections in wild rats from Japanand the United Kingdom had been published (37). Recently,we isolated E. cuniculi strain II (“mouse strain”) from one of30 wild rats (Rattus norvegicus) caught in the city of Zurich,Switzerland (212) (Table 5).

In wild mice, specific antibodies against E. cuniculi spores

FIG. 4. Rabbit with torticollis (head tilt) due to cerebral infectionwith E. cuniculi.

TABLE 5. Hosts and geographical distribution of Encephalitozoon cuniculi strains

E. cuniculi straina Host Geographic area (no. of isolates) Reference(s)

I (“rabbit strain”) Rabbit Switzerland (21), United States (3), Germany (1), Australia (1),Italy (1), Japan (1)

86, 113, 153, 192, 210;P. Deplazes and A.Mathis, unpublisheddata.

Human Switzerland (6), Italy (1), United States (1) 73, 238, 321, 334II (“mouse strain”) Mouse Czech Republic (1), United Kingdom (1), United States (1) 86, 334

Wild rat Switzerland (1) 212Blue fox Norway (8), Finland (1) 8, 190

III (“dog strain”) Dog United States (10), South Africa (1) 86, 141, 264Prosimian

a As determined by the number of 5�-GTTT-3� repeats in the ITS (86).



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were found in Iceland in 4% and 9% of Apodemus sylvaticusand Mus musculus animals, respectively (140). The authorssuggested that mice are a potential “reservoir” of E. cuniculifor arctic foxes and feral minks. Indeed, recent molecular char-acterization of E. cuniculi isolates from Norway and Finland(8, 190) revealed that all foxes originating from four differentfarms were infected with strain II (“mouse strain”) of E. cu-niculi (see “Molecular Epidemiology” below).

Carnivores. The clinical manifestation of canine encephali-tozoonosis is an encephalitis-nephritis syndrome which hadpreviously been confused with canine distemper (23). En-cephalitozoonosis in domestic dogs, which is caused by strainIII (“dog strain”) of E. cuniculi, has been described from Tan-zania, South Africa, and the United States (23, 37, 252, 264).

In domestic cats, only three cases of E. cuniculi infectionshave been reported (37, 178). In captive carnivores, dissemi-nated infections similar to those found in dogs occurred in foxcubs (215). This disease, which is still a major problem andcauses heavy losses of blue foxes in northern European coun-tries (5), is caused by strain II (“mouse strain”) of E. cuniculi(190).

Outbreaks of encephalitozoonosis in carnivores in zoos werepreviously reviewed (37). Few data are available about thedisease in wild carnivores. Encephalitozoon-like organismswere detected by light microscopy in brain tissues from a wildhand-reared red fox (Vulpes vulpes) with neurological symp-toms from the United Kingdom (331) and from captive wilddog (Lycaon pictus) pups which died of a fatal disease resem-bling canine distemper 13 days after vaccination with a liveattenuated strain of canine distemper virus (303). Serologicalinvestigations with ELISA revealed no seropositive animals in86 wild red foxes from Switzerland (210). In Iceland, sero-prevalences were 12% among 372 wild arctic foxes (Alopexlagopus) and 8% in feral mink (Mustela vison) (140). His-topathological findings for a seropositive fox pup with a neu-rological disorder which died 2 days after capture were consis-tent with encephalitozoonosis described for farmed foxes. Theauthors suggested that encephalitozoonosis contributed to thedecline of the arctic fox population size by depressing fetal andpup survival and that mice may represent a potential reservoirfor microsporidia (140). Indeed, in Greenland, where rodentsare absent from the diet of these arctic foxes, none of 230 foxesinvestigated had antibodies to E. cuniculi (7).

In all mentioned reports of encephalitozoonosis in wild car-nivores (140, 303, 331), Encephalitozoon-like organisms weredetected by light microscopy only. Therefore, species andstrain determinations were not conclusive.

In dogs and foxes, encephalitozoonosis is being perpetuatedin the population by horizontal and vertical transmission (23,201). Dogs and foxes infected by ingestion of spores showedmoderate clinical symptoms, and the chronically infected ani-mals represented the main source of infections for the off-spring. In fur farms, food contaminated with spores from in-fected rodents or rabbits was assumed to be a possible sourceof infection for foxes and minks (140, 215).

Monkeys. Disseminated natural infections resulting in highmorbidity and severe encephalitis caused by Encephalitozoon-like organism have been reported for stillborn and young squir-rel monkeys (Saimiri sciureus) in the United States (28, 337).Although parasite identification was based on electron micros-

copy only, which does not allow one to distinguish E. cuniculifrom E. hellem, the neuropathological symptoms strongly sug-gested that E. cuniculi was the species involved. Only recently,strain III (“dog strain”) of E. cuniculi was identified in tamarincolonies (Saguinus imperator, Oedipomidas oedipus, and Leon-topithecus rosalia rosalia) in two zoos in Europe, causing severedisseminated infection with high mortality in infants (134, 234,329).

In experimental infections of vervet monkeys with dog-de-rived E. cuniculi isolates, transmission from infected infants totheir nursing dams as well as transplacental transmission wasestablished (297). In naturally infected monkeys, transplacen-tal transmission was shown by the presence of multifocal gran-ulomatous encephalitis with invasion of the brain by Enceph-alitozoon in stillborn monkeys or monkeys only a few days old,as well as by the reported placentitis in an animal and theisolation of Encephalitozoon-like parasites from placental tis-sue of a baboon (reviewed in reference 37). The hypothesisthat animals with silent infections can perpetuate the disease ina colony is supported by a serological survey of a squirrelmonkey colony over 3 years. More than half of these 250animals tested positive at least once, and asymptomatic younganimals were also seropositive (251). Export of such seropos-itive asymptomatic animals represents a high risk for the in-troduction of the parasite in other colonies, as was recentlyobserved in an Emperor tamarin colony in Europe (134). Sofar, no sources of infection have been elucidated for E. cuniculiinfections in monkeys, and it is not known whether this mi-crosporidial species is prevalent in free-living monkeys.

In contrast to natural infections causing neonatal death orlethal infections in young monkeys, experimental infections ofimmunocompetent monkeys with dog- or rabbit-derived iso-lates resulted in asymptomatic infections (84, 254, 297). There-fore, many factors, including host species, E. cuniculi strain,age and immune status of the host, and mode of transmission(e.g., intrauterine or oral), may influence the outcome of in-fections in monkeys.


In E. cuniculi, the existence of three strains (I, II, and III;also named “rabbit strain,” “mouse strain,” and “dog strain”)was established using immunological and molecular methods(86). The repeated element 5�-GTTT-3� in the ITS is a reliableand widely used genetic marker: strain I contains three suchrepeats, strain II contains two repeats, and strain III containsfour repeats. A recent multilocus analysis yielded additionalmarkers for the three strains, namely, the genes coding for thepolar tube protein and spore wall protein 1 (333). This straindifferentiation helped to elucidate the complex epidemiologi-cal situation of E. cuniculi infections in different hosts and indifferent parts of the world.

Table 5 shows the host species and the geographical originsof those E. cuniculi isolates which were determined to thestrain level and for which comprehensive information aboutthe host animal was available. These data confirm earlier ob-servations which suggested from circumstantial evidence thatdifferent strains of E. cuniculi with different host preferencesmight exist in nature (reviewed in reference 37). Strain II(“mouse strain”) was identified in rodents (mouse and rat) and



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in farmed blue foxes, supporting the hypothesis that mice area reservoir of E. cuniculi for blue foxes (140). All E. cuniculiisolates from rabbits characterized so far belong to strain I(“rabbit strain”), and all isolates from dogs belong to strain III(“dog strain”).

Dogs indeed seem to be susceptible to strain III only. InSwitzerland, where strain I (“rabbit strain”) is highly prevalentin the rabbit population (210) and strain II (“mouse strain”)was detected in a rat (212), no positive antibody reactions weredetected by ELISA in 104 healthy dogs and in 108 dogs withneurological disorders (210). E. cuniculi strain II is known tohave a remarkable pathogenicity for the blue fox and mink inNorway (6, 37), but encephalitozoonosis of dogs has not beenreported from Norway, and a recent serological survey with1,104 canine serum samples revealed no indications for aninfection (6). The absence of canine encephalitozoonosis inEurope indicates that strains I and II of E. cuniculi do not playa significant role as pathogens in the dog population.

Host preference of the strains is further substantiated by theobservations that strain II (“mouse strain”) was recently iden-tified from a wild rat in an area in Switzerland (212) wherestrain I (“rabbit strain”) of E. cuniculi is highly endemic in petrabbits and that, vice versa, a considerable number of rabbitsfrom all over Switzerland all were infected with strain I butnever with strain II. It is unlikely that strains I and II simply areepidemiologically separated, as E. cuniculi-positive rabbitswere identified from stables where rodents were freely roam-ing.

On the other hand, a strict host specificity of the strains wasnot demonstrated under experimental conditions (reviewed inreference 192). E. cuniculi strains II (“mouse strain”) and III(“dog strain”), for example, were also infective to rabbits thatwere given spores from an in vitro culture by oral administra-tion (192; P. Deplazes, unpublished data).

The strains of E. cuniculi seem to differ in their geographicaldistributions. Hence, strain II has so far been identified inEurope only, strain I infects rabbits from at least three conti-nents (America, Australia, and Europe), and strain III wasidentified in dogs in America and South Africa. Strain III wasrecently identified for the first time in Europe in tamarin col-onies from zoos (134, 234, 329). However, it was suggested inone of these cases that this infection had been imported withancestor animals originating from the United States and hasbeen perpetuated in the population by transplacental infec-tions (134). A potential for spreading of this imported strain tothe autochthonous carnivore population in the future has to beconsidered.

Immunocompromised humans were found to be infectedwith strain I (“rabbit strain”) in Europe and with strain III(“dog strain”) in the Americas (Table 5). In Europe, too,several HIV-infected patients were diagnosed to be infectedwith E. cuniculi strain III (Table 4), but no travel histories forthese patients were provided.


It is unlikely that E. cuniculi is a natural pathogen in hu-mans, and its zoonotic origin is evident. Indeed did two of thesix patients from Switzerland, all of whom were infected withE. cuniculi strain I (“rabbit strain”), recall exposure to rabbits

in the past (192, 321). Two patients from the United Stateswho were infected with strain III (“dog strain”) did own a petdog, but microsporidial infection in the animals was not proven(85, 283). Seroconversion in one of three children exposed topuppies with overt encephalitozoonosis has been reported(79). In another case of an E. cuniculi infection in an HIV-infected person where no material was available for genotyp-ing, strain III (“dog strain”) was suspected to be the causativeagent based on geographical (Chile) and clinical reasons andon the fact that the patient had a high risk of exposure as shewas working as a pet dog groomer (328). Patient-to-patienttransmission was considered to be unlikely in the only study todate investigating several patients, because these patients nei-ther knew each other nor had obvious personal contacts suchas hospitalization at the same time (73, 192).

As spores of E. cuniculi are highly resistant in the environ-ment and can survive several months under humid conditions(172, 311) direct contact with infected animals or humans isnot required, and waterborne infections seem feasible. How-ever, none of the two E. cuniculi strains found in humans haveso far been detected in surface waters, in contrast to the casefor strain II (“mouse strain”), which in a single study wasidentified by PCR in one of a total 50 samples from Switzer-land (A. Mathis, unpublished data).

ENCEPHALITOZOON HELLEM


E. hellem has so far been diagnosed in around 50 HIV-infected persons in a relatively few countries: Most cases werereported from the United States (52, 78, 81, 248–250, 308). E.hellem was diagnosed in European patients from Italy (247),Switzerland (73, 109, 323), Germany (109), and the UnitedKingdom (144) and in one case in Africa (Tanzania) (75).

It is not clear whether epidemiological factors are responsi-ble for a restricted distribution of this microsporidian orwhether the relatively difficult identification of this species byimmunological or molecular methods hampers its detection. E.hellem causes disseminated and ocular infections in HIV-pos-itive patients, but asymptomatic infections of the respiratorytract have also been described (245). To our knowledge, E.hellem has so far been identified on two occasions in nonim-munosuppressed and HIV-seronegative patients, namely, frombronchoalveolar lavage of a patient with a simultaneous My-cobacterium tuberculosis coinfection (246) and in fecal samplesfrom two diarrheic travelers returning from Singapore, whowere coinfected with E. bieneusi (209) as diagnosed by PCRand confirmed by sequencing.


Before the description of E. hellem in 1991 (78), Encephali-tozoon-like microsporidia were described on several occasionsfrom psittacine birds (parrots). With the exception of a peach-faced lovebird from Australia, all cases were from birds kept inaviaries in the United States (37, 216, 230). Molecular analysesrecently allowed identification of E. hellem in psittacine birdsin the United States as the etiological agent of lethal infectionsin budgerigar chicks (Melopsittacus undulatus) in an aviary



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(20), in tissues from two eclectus parrots (Eclectus roratus)(226), and in droppings of a clinically normal peach-facedlovebird (Agapornis roseicollis) (267). The first detection of E.hellem in a wild psittacine bird was demonstrated by molecularanalyses of spores from the intestinal contents of a yellow-streaked lory (Chalopsitta scintillata) captured on Aru Island(Indonesia) (279). However, microsporidial infections havebeen reported from nonpsittacine birds in the last few years.Horned puffin (Fratercula corniculata) chicks, which werecaught in Alaska and which suffered a high mortality rate inquarantine at a zoo, were found to be infected with nonspeci-fied microsporidian-like organisms (287). An intestinal micro-sporidiosis was diagnosed postmortem in a clinically normalostrich (Struthio camelus) in Texas (125), and the causativeagent was identified as E. hellem (263). Several species of wild,migratory hummingbirds from California, which suffered fromenteritis, were infected with E. hellem (262). Finally, the par-asite was diagnosed in a Gouldian finch from a flock with highnestling mortalities in Australia (38). A single epidemiologicalsurvey reported a prevalence of 25% of 198 clinically normallovebirds from three flocks (265).

Experimental inoculations have resulted in disseminated in-fections in athymic nude mice (266). In an attempt to assesswhether avian species with close contacts with humans canserve as hosts of E. hellem, young chickens and turkey poults (3to 5 days of age) were inoculated by gastric gavage with 107

spores (97). All birds remained asymptomatic, and no micros-poridia were detected by microscopy in fecal smears and inhistological specimens of the intestine. However, PCR, whichwas shown to be considerably more sensitive for detection ofmicrosporidia in feces than microscopy (96), was positive withfecal material from chicks only. Taken together, these resultssuggest that several avian species might serve as asymptomaticcarriers of E. hellem.

The means of transmission of E. hellem infections in birdsremain unexplored. As spores were found in kidney and intes-tine, spore excretion probably occurs with feces (20, 37, 216,279). Black and colleagues (20) observed severe outbreaks ofencephalitozoonosis with high mortality in young chicks,whereas adult birds in the same aviary appeared unaffected. Inseveral reports, avian microsporidiosis leading to high morbid-ity and mortality was observed shortly after birds were broughtinto a new environment (37, 279, 287). These observationssuggest that latent infections as observed with E. cuniculi inother animals also occur in adult birds and that the stress oftranslocation may reduce the degree of resistance to this in-fection. Immunosuppressive viral infections such as that withthe psittacine beak and feather disease virus were found in twounrelated cases of lethal microsporidiosis in eclectus parrots(226), and a significant correlation was identified betweenshedding of E. hellem spores and psittacine beak and featherdisease virus infection of lovebirds (265).


Comparable to the situation in E. cuniculi, different geno-types as assessed by the sequence of the ITS were identified inE. hellem (194). Genotype 1, which represents the overwhelm-ing number of E. hellem isolates characterized so far, wasdetermined for isolates from several patients from the United

States (153, 310, 333) and from Europe (109, 333) and for oneE. hellem isolate each from a parrot caught in the wild inIndonesia (279) and from a psittacine bird from the UnitedStates (267). Genotypes 2 and 3 (194) and 2b (333), whichdiffer from each other only by short insertions, consist of hu-man-derived E. hellem isolates from Switzerland (three iso-lates) and Tanzania (one isolate) (194, 333). Differencesamong the E. hellem genotypes were also detected by Westernblot analysis, but there was no absolute match between geno-type and antigen profile. Indeed, karyotyping (17, 70) andmultilocus analyses targeting coding (polar tube protein geneand ssrRNA gene) and noncoding (intergenic spacer) lociidentified a least six different human-derived genotypes (138,222, 333). From all these analyses, it appears that E. hellemisolates from the United States and from Europe are differentpopulations. No isolates from birds have as yet been subjectedto this more detailed genetic analysis; therefore, the biologicaland epidemiological significance of these findings remains tobe elucidated.


To our knowledge no epidemiological studies to demon-strate risk factors for E. hellem infections have been conducted,and the parasite has never been identified in surface waters.However, it is worth noting that some patients with ocularmicrosporidiosis owned or were exposed to pet birds (112, 219,336). The identification of E. hellem as etiological cause ofsevere renal and intestinal infections in parrots and the excre-tion of spores in feces indicate that birds are a potential, butyet unproven, source of zoonotic infections. Once introducedinto a susceptible human population, transmission of sporesfrom person to person might be of importance. In HIV-in-fected patients with disseminated E. hellem infections, sporeswere isolated from sputum, nasal secretions, sinus aspirate,and urine (219, 323). The occurrence of upper and lower re-spiratory tract infections suggests that this species can be trans-mitted via the aerosol route (248, 323). Furthermore, oral orocular autoinoculation, perhaps by contaminated fingers, mayoccur (249). For drug addicts, a hematogenous mode of trans-mission with contaminated syringes has been suggested but wasnot proven (117).

ENCEPHALITOZOON INTESTINALIS


E. intestinalis is the second most prevalent microsporidialspecies infecting humans. Infections in HIV-positive patientshave been reported from the Americas (62, 208, 257), fromEurope (21, 98, 107, 175, 177, 202, 271, 280, 299, 325), fromAustralia (88, 168, 208), and from Africa (10, 122, 157, 167).Most reports are descriptions of single case; in a few studieslarger groups were examined, and prevalences for E. intestina-lis were 7.3% for 68 AIDS patients with diarrhea from theUnited States (62), 2% for 97 consecutive HIV-infected pa-tients in Germany (271), 3% for 75 patients with chronic di-arrhea from Zambia (157), and 0.9% for 320 patients withchronic diarrhea in Switzerland (324). A study investigating216 AIDS patients with gastrointestinal complaints in Portugal



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revealed 92 patients being positive for microsporidia by mi-croscopy (trichrome stain, uvitex, and calcofluor; prevalence,42.8%). Of the positive specimens, 69 were further investigatedby species-specific PCR, resulting in the diagnosis of E. intes-tinalis in 49 of them and of E. bieneusi in 20 (98). When 63 stoolsamples from HIV-infected patients with diarrhea from En-gland were investigated by the calcofluor method, no E. intes-tinalis infections were found, whereas in 9 samples (14.3%) E.bieneusi was detected (166). Similarly, no E. intestinalis infec-tions were detected in 61 HIV-positive persons from Mali(Africa) by using a monoclonal antibody, but 8 (13.1%) werepositive for E. bieneusi (10).

Two studies have proposed that double infections with E.bieneusi and E. intestinalis might occur. Five such double in-fections were identified in 10 cases of E. intestinalis infectionsin AIDS patients as proven by PCR on biopsy material (109).van Gool and colleagues (299), when attempting to cultivate E.bieneusi from four stool samples from biopsy-proven patients,surprisingly ended up with cultures of E. intestinalis in all fourcases. They suggested that this microsporidian might be morecommon than suspected; however, laboratory contaminationof the cultures cannot be excluded.

E. intestinalis had been identified in two HIV-negative trav-elers with diarrhea (232). In a cross-sectional survey of tworural villages in Mexico, a monoclonal antibody (3B6) whichrecognizes E. intestinalis, E. hellem, E. cuniculi, and the othermicrosporidial species Nosema and Vairimorpha spp., but nei-ther Enterocytozoon bieneusi nor yeast from stool sample (93),was used to investigate 255 persons. Twenty (7.8%) of themwere positive, with 21% of 70 households having at least onemember who was positive (94). Indications for high preva-lences in immunocompetent persons were also found in an-other study (302). Using serologic techniques with E. intesti-nalis as antigens, high seroprevalences among 300 Dutch blooddonors (8%) and 276 pregnant French women (5%) werefound. However, the tests used were not strictly specific for E.intestinalis but presumably were genus specific.


Experimentally infected wild-type mice were shown to ex-crete spores intermittently and in low numbers (4), a situationthat is reminiscent of the one with E. bieneusi infections in pigs(26). In a survey using the monoclonal antibody 3B6 (which isnot E. intestinalis specific; see above) in IFAT, fecal samplesfrom domestic animals from a rural area in central Mexicowere investigated, resulting in 19 (11%) of 172 mammalian and16 of 99 avian samples being positive. Transmission electronmicroscopy on specimens from mammals revealed microspo-ridian-like structures. PCR using a contamination-prone pro-tocol (with a cloned E. intestinalis target sequence as positivecontrol) was done several months later on total DNA extractedfrom the formalin-fixed specimens, which generally is consid-ered not to be suitable for sensitive detection due to time-dependent, formalin-induced degradation of DNA (149, 291).Six of the eight investigated fecal samples from goat, pig, cow,dog, and donkey were positive for E. intestinalis but negativewhen tested with primers specific for E. cuniculi and E. hellem(22).

Such a high prevalence in animals has not been confirmed in

other studies. In a recent study, by employing microscopy andPCR on nonfixed material, low prevalences (3%) of E. intesti-nalis were identified in free-ranging gorillas and in people whoshare gorilla habitats in Uganda (122). Hence, a zoonotic po-tential was suggested, but further studies are needed to inves-tigate whether E. intestinalis indeed is a common pathogen ofa wide variety of (domestic) animals and whether animal-borneE. intestinalis is epidemiologically related to the human dis-ease.


In contrast to the situation in other human-infecting Enceph-alitozoon species, for which distinct strains with differences intheir biology and epidemiology were identified, E. intestinalisseems to be a very homogenous species. Analysis of five hu-man-derived isolates revealed only minor differences in sodiumdodecyl sulfate-polyacrylamide gel electrophoresis and West-ern blot profiles (67). Whereas isolates from both E. cuniculiand E. hellem show considerable intraspecies variability in theirkaryotypes (17, 18, 70, 269), isolates of E. intestinalis displayedidentical karyotypes when analyzed by pulsed-field gel electro-phoresis (17). No variation of the ITS sequences was detectedin 16 isolates of E. intestinalis (82, 176).


Modes of transmission of E. intestinalis spores are uncertain.Considering the various sites of infections (intestinal, respira-tory, and ocular) (108) and spore release into the environmentvia stool, transmission via the fecal-oral route, by inhalation, orby contaminated fingers seems plausible. Waterborne infec-tion, at least focally, can be suspected, as the parasite wasidentified by sequence analysis of PCR amplicons from surfacewater and groundwater (89, 286), from 6 of 12 investigatedsamples of source water used for consumption (90), and re-cently, by species-specific PCR, also in zebra mussels from ariver (123). Case-control studies indeed demonstrated thatcontact with water might be a risk factor for acquiring intesti-nal microsporidiosis (E. bieneusi and E. intestinalis), but eithermicrosporidial species were not specified or E. bieneusi was thepredominant one (see “E. bieneusi: Sources of Human Infec-tions and Transmission” above).

OTHER MICROSPORIDIA

Several microsporidial species have been identified in rarecases of patients with ocular infections, myopathies, intestinalinfections, or disseminated infections (Table 1). We brieflydiscuss the features of those microsporidia for which ultra-structural and/or molecular information is available. With theexception of B. algerae, which infects mosquitoes, no host otherthan humans is known for these species.

Vittaforma spp.

Vittaforma corneae (258), which originally was designatedNosema corneum (253), was the first human-derived microspo-ridial isolate which could be established in an in vitro culture.Up to now, four cases of human infections are on record, three



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of them related to eye infections in immunocompetent patientsfrom the United States and Europe (101, 231, 253) and onepresenting as an urinary tract infection in a patient with AIDSliving in Africa, who had a concurrent infection with E. hellem(75). Phylogenetic analysis based on rRNA gene sequences(Fig. 2) revealed arthropod-infecting species as being mostclosely related to V. corneae and the human-infecting E. bie-neusi as a close taxonomic relative. Although no common nat-ural hosts of Vittaforma have yet been identified, these micro-sporidia seem to be ubiquitous. Investigations of surface watersin the United States and in Europe by PCR and sequencingrevealed gene sequences with highest scores of identity (91 to98%) with V. corneae in, e.g., seven of eight samples analyzed(89, 104). The Vittaforma-like organisms (96% identity with V.cornea) identified in feces from 25 diarrheic patients (22 HIVpositive and 3 HIV negative) in Portugal (278) might thereforesimply reflect intestinal passages of microsporidial spores.

Pleistophora spp.

Three cases of myositis in immunodeficient patients wereidentified as being caused by Pleistophora spp. (49, 124, 169).The correct taxonomic position of the organisms in two of thecases (49, 124) was not possible but it was assumed (48) thatthey belong to the subsequently described genus Trachipleisto-phora (see below). Ultrastructural analysis of the third isolate(169) allowed its classification as the new species Pleistophoraronneafiei (32).

Pleistophora spp. have nearly exclusively lower (poikilother-mic) vertebrate hosts, in particular fish and reptiles, but noother Pleistophora species has spores with the dimensions of P.ronneafiei, and hence a possible animal reservoir remains ob-scure.

Trachipleistophora spp.

Closer examination of a morphologically Pleistophora-likemicrosporidian isolated from an patient with AIDS with infec-tion of skeletal muscle and also the nasal sinus cavities andconjunctiva (100) led to the establishment of the new genusand species Trachipleistophora hominis (145). Apart from thisfirst case from Australia, T. hominis was identified in an im-munocompetent African immigrant in the United Kingdom ascausal agent of a stromal keratitis (231).

Phylogenetic analysis of T. hominis based on rRNA genesequences revealed that its closest relationship is with speciesof the genus Vavraia, which are pathogenic to insects (48) (Fig.2). T. hominis indeed could be readily propagated in mosquitolarvae, and the spores produced were again infective for athy-mic mice. Furthermore, spores were transferred from infectedadult mosquitoes to a feeding substrate (326). Insects being apotential source of T. hominis was substantiated by experimen-tal infections of severe combined immunodeficient (SCID)mice. Whereas oral inoculation was ineffective for establishingan infection, injection of the parasites, mimicking insect stings,led to disseminated infections, including infection of urinarybladder, liver, and spleen but not the brain (163). Severe ocularand milder disseminated infections were also observed uponeye contamination with spores, providing a direct way of in-fection independent of insect bites.

A second species of this genus, Trachipleistophora anthro-pophthera, was differentiated based on ultrastructural featuresand clinical syndromes (305). In contrast to T. hominis infec-tions in humans and in SCID mice, T. anthropophthera alsoinfected the brain, as was reported for the first two infectionsin AIDS patients from the United States (335). This micro-sporidial species was identified in a single additional humancase in an HIV-infected person from Thailand suffering fromchronic keratitis, and the isolate was successfully cultivated inmouse fibroblast cells (151).

No genetic information allowing phylogenetic analysis isavailable for T. anthropophthera. Based on the common char-acteristic feature of bisporous sprorophorous vesicles, it wassuggested that this species might be related to the genus Te-lomyxa, which infects insects (mayflies) (289).

Brachiola spp.

Brachiola algerae (183) (formerly Nosema algerae) is has longbeen known as a pathogen of different orders of insects, e.g.,mosquitoes (304), and was a candidate agent for biologicalcontrol of these insects (296). Within this context, it was eval-uated in the 1970s whether B. algerae could pose a biohazardfor mammalians when injected by a mosquito during a bloodmeal. Later, during the AIDS pandemic and before the firstcase of a human infection with this parasite was reported (307),the potential of B. algerae to infect immunocompromised ver-tebrates was assessed. Hence, this microsporidial species wasshown to proliferate in mammalian cell lines, initially only attemperatures of below 37°C (294) but later also after adapta-tion to 37°C (207, 289). Intravenous injection of spores failedto induce infections in immunocompetent and immunocom-promised mice, and subcutaneous injections led to limited de-velopment close to the infection site in extremities only (290,295). Furthermore, ocular, but not oral, intraperitoneal, intra-muscular, or subcutaneous, inoculation of SCID mice withspores of B. algerae led to severe infection of the liver as theonly affected internal organ (164). It was concluded that aninitial infection of the eye allowed the parasite to adapt to theconditions within the mammalian host and to spread to inter-nal organs, presumably via macrophages.

In the first case of a human B. algerae infection, the parasitewas isolated from the cornea of an immunocompetent patient(307), and the infection was resolved after appropriate treat-ment. In the second case (61), a deep-tissue infection in mus-cles in a patient with diabetes and rheumatoid arthritis wasdocumented. The patient, who was HIV negative and whoeventually died from a massive cerebrovascular infarcation,was taking immunosuppressive agents for rheumatoid arthritis.Diagnosis from muscle biopsies was confirmed by light andelectron microscopy, by IFAT using rabbit antiserum againstB. algerae, and by PCR and sequencing. Rather perplexing isthe finding that the parasite isolate proliferated in RK13 (rab-bit kidney) cells in vitro at 30°C but not at 37°C, a fact that wasnot further discussed by the authors.

Two further species of the genus Brachiola have been de-scribed based on ultrastructural features from single humaninfections: B. connori (33) (originally named Nosema connori[273]), causing disseminated infection in an immunologicallycompromised child (188, 273), and B. vesicularum, causing



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myositis in an AIDS patient (33). The validity of the latterspecies name was doubted, and it was suggested that B. vesicu-larum, for which no genetic information was provided, may beidentical to B. algerae based on the ultrastructure of B. algeraewhen growing in human muscle fibroblasts at 38°C (289). How-ever, examination of muscle biopsies from the above-men-tioned B. algerae case (61) confirmed the existence of ultra-structural differences, implying that B. algerae and B.vesicularum indeed are different species (34).

Microsporidium spp.

Microsporidium ceylonensis. In the original case report (12)of this ocular infection of a Tamil boy, spores were the onlyparasite stages identified, and it was suggested that either E.cuniculi, which had been confirmed as a human pathogen atthat time (195), or Nosema helminthorum was the causativeagent. The latter species, a parasite of tapeworms of goats, wassuggested because the boy had suffered an injury of the af-fected eye when he had been gored by a goat. In a subsequentultrastructural study published in 1998 (36), a generic classifi-cation of this isolate could not be achieved and it was namedMicrosporidium ceylonensis. The ultrastructural details identi-fied should facilitate the recognition of this parasite in futurecases.

Microsporidium africanum. Microsporidial spores were iden-tified in histiocystes of a corneal ulcer of an African woman(223), with a tentative identification as a Nosema sp. The iso-late was later transferred to the collective group Microspo-ridium upon reexamination of the electron micrographs andwas designated Microsporidium africanum (37). There are noclues whatsoever on other hosts of this species.

CONCLUDING REMARKS

Microsporidia are identified as parasitic organisms of almostevery animal group, including numerous invertebrates and ver-tebrates. In humans, fewer than 10 human infections werereported until microsporidia had emerged as important oppor-tunistic pathogens when AIDS became pandemic. Meanwhile,their significance has dramatically decreased in areas whereantiretroviral combination therapy is available, but the preva-lence of opportunistic infections in HIV-infected patients re-mains still high or is increasing in resource-poor countries. Inaffluent countries, the focus has shifted to microsporidial in-fections in otherwise immunocompromised patients, particu-larly organ transplant recipients, and in immunocompetentpersons with cornea infection. Human microsporidial infec-tions might be underdiagnosed due to the difficulties of detec-tion.

The sources of most microsporidia infecting humans andmodes of transmission are still uncertain. Because microspo-ridial spores are released into the environment via stool, urine,and respiratory secretions, possible sources of infection may bepersons or animals infected with microsporidia. Furthermore,because some microsporidia infecting humans are morpholog-ically identical to organisms known in animal reservoirs, thequestion arose as to whether these intracellular parasites are ofzoonotic origin. Molecular techniques have been applied to

address this question, but at present, conclusive results areavailable for only a few microsporidial species.

Using genetic analyses, the zoonotic potential has clearlybeen documented for several Encephalitozoon species includ-ing E. cuniculi and E. intestinalis, which infects mammals andhumans, as well as E. hellem, which infects birds and humans.Epidemiological evidence, however, is lacking because thenumber of reported human infections is still low.

Most animal and human isolates of E. bieneusi, the mostprevalent human microsporidian, causing mainly severe andchronic HIV-associated diarrhea, are genetically distinct. Nev-ertheless, identical E. bieneusi genotypes were found in mam-mals and in a few human patients, suggesting that zoonotictransmission is possible. The molecular results strongly supportthe hypothesis that most human-infecting E. bieneusi geno-types are “human adapted” and not zoonotic.

Because other microsporidial species are only very rarelyfound even in severely immunocompromised humans, it ap-pears likely that humans do not represent a major parasitereservoir. Although the natural hosts have not been found yetfor several of these rare human pathogens, an arthropod or fishhost reservoir might be considered.

Ingestion of microsporidial spores is the most probablemode of acquisition, and it is assumed that infection is mostlikely transmitted either directly from human to human via thefecal-oral route or indirectly via water or food. Although someepidemiological studies indicated the possibility of human-to-human or waterborne infections, convincing epidemiologicalresults are still lacking. Transmission by the aerosol route hasalso been considered, because spores have been found in re-spiratory specimens of patients with Encephalitozoon spp. or E.bieneusi infection.

Patients who have severe cellular immunodeficiency appearto be at highest risk of developing microsporidial disease, butlittle is known about immunity to microsporidial infection. It isnot understood whether microsporidial infection in these pa-tients is primarily a reactivation of latent infection acquiredbefore the state of suppressed immunity or whether microspo-ridial disease is caused by recently acquired infection.

Based on the scarce current epidemiological knowledge con-cerning transmission of microsporidia, no recommendations toprevent infections can be suggested.

ACKNOWLEDGMENT

Work on microsporidia in the authors’ groups was supported bygrants from the Swiss National Science Foundation (3200-049751.96/1and 3237399.93).

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Zoonotic Potential of the Microsporidiaappropriate serodiagnostic tools seems feasible (298). No tests are available for the serodiagnosis of E. bieneusi. Many but not all microsporidia