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CLINICAL MICROBIOLOGY REVIEWS, Oct. 2008, p. 626–638 Vol. 21, No. 40893-8512/08/$08.00�0 doi:10.1128/CMR.00021-08Copyright © 2008, American Society for Microbiology. All Rights Reserved.

Current World Status of Balantidium coliFrederick L. Schuster1* and Lynn Ramirez-Avila2

California Department of Health, Viral and Rickettsial Disease Laboratory, Richmond, California,1 andSan Francisco General Hospital, University of California Medical Center, San Francisco, California2

INTRODUCTION .......................................................................................................................................................626History ......................................................................................................................................................................627

MORPHOLOGY .........................................................................................................................................................628TAXONOMY AND MOLECULAR BIOLOGY.......................................................................................................629

Balantidium spp. Defined .......................................................................................................................................629Molecular Biology ...................................................................................................................................................629Balantidium coli versus Balantidium suis..............................................................................................................630

PHYSIOLOGY ............................................................................................................................................................630CULTIVATION ...........................................................................................................................................................630

Temperature Range for Growth............................................................................................................................631HOST-PARASITE INTERACTIONS .......................................................................................................................631

Immune Response...................................................................................................................................................631Immunosuppression ...............................................................................................................................................631Crossing the Species Barrier: Transfection Attempts .......................................................................................632

DISEASE POTENTIAL..............................................................................................................................................632Factors Contributing to Disease...........................................................................................................................633

EPIDEMIOLOGY.......................................................................................................................................................633Outbreaks of Balantidiosis ....................................................................................................................................633Virulence versus Avirulence in Balantidium ........................................................................................................633

PREVALENCE OF BALANTIDIUM COLI..............................................................................................................633Infections in Nonhuman Mammals......................................................................................................................633

Swine.....................................................................................................................................................................634Simians and apes................................................................................................................................................634Amphibia..............................................................................................................................................................634

Infections in Humans.............................................................................................................................................634Areas of endemicity ............................................................................................................................................635Other possible pathways for transmission......................................................................................................635Risk factors..........................................................................................................................................................635Institutional balantidiosis..................................................................................................................................635

LABORATORY DIAGNOSIS....................................................................................................................................635Pulmonary Infections .............................................................................................................................................635Balantidiosis versus “Dysentery” .........................................................................................................................636Balantidium and Laboratory Infection .................................................................................................................636

PREVENTION OF INFECTION..............................................................................................................................636ANTIMICROBIAL THERAPY..................................................................................................................................636CONCLUSIONS .........................................................................................................................................................636ACKNOWLEDGMENTS ...........................................................................................................................................637REFERENCES ............................................................................................................................................................637

INTRODUCTION

Protozoan parasitism covers a broad spectrum of diseases.Balantidium coli and balantidiosis, the subjects of this review,barely register among infectious protozoan diseases.

Balantidium is the only ciliated protozoon known to infecthumans and is the largest protozoon infecting humans andnonhuman primates. Balantidiosis is a zoonotic disease andis acquired by humans via the fecal-oral route from thenormal host, the pig, where it is asymptomatic. Water is the

vehicle for most cases of balantidiosis. Human-to-humantransmission may also occur. Balantidium’s habitats in hu-mans are the cecum and colon. Humans may remain asymp-tomatic, as does the pig, or may develop dysentery similar tothat caused by Entamoeba histolytica. Death is an infrequentconsequence of balantidiosis, but in developing countrieswith undernourished and overparasitized populations, it canmake the difference between a healthy life and chronic de-bilitation.

The organism is cosmopolitan and can be found whereverpigs are found. Disease appears to be a problem mostly ofdeveloping countries, where water sources may be contam-inated with porcine or human feces. B. coli can become anopportunistic parasite in immunosuppressed hosts living inurban environments, where pigs are not a factor in infection.

* Corresponding author. Mailing address: California Department ofHealth, Viral and Rickettsial Disease Laboratory, Richmond, CA94804. Phone: (510) 307-8651. Fax: (510) 307-8599. E-mail: [email protected].

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Balantidium is an often-neglected pathogen. Research onBalantidium has been sparse. Zaman (80) published an inclu-sive review on Balantidium 30 years ago, but recently the or-ganism has come to be regarded as an emerging protozoanpathogen and has been reviewed by Garcia (22).

Balantidium has a simple life cycle, as follows: dormant cystto trophozoite and trophozoite to cyst. Transmission is direct,from a contaminated water or food supply to humans (Fig. 1).No intermediate host, as occurs with many other parasiticspecies, is needed.

History

Malmsten (44) was the first to recognize the organism in twohumans with dysentery in the year 1857. Malmsten identified itas a species of Paramecium and named the organism Parame-cium coli. Leuckart described a morphologically similar speciesfrom the pig intestine in 1861 (39). Shortly thereafter, Steinequated the two organisms and placed them in the genusBalantidium (G. balanto-, bag), and because of priority, thespecies name (B. coli) was retained (64). Ironically, Malmsten

FIG. 1. Life cycle of Balantidium infection in humans. The trophozoites and cysts are shed in feces (1), and if the cysts, in particular,contaminate drinking water or food, the infection can be spread to other humans (2). Fruits and vegetables may also be contaminated by cysts andserve as a means of transmission. The bottom panel illustrates the pattern of encystment and asexual reproduction in trophic ciliates. (Reprintedfrom the CDC-DPDx Parasite Image Library [http://www.dpd.cdc.gov/dpdx/].)

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first described balantidiosis, usually regarded as a disease oftropic or subtropic regions, from patients in Sweden (44).

Morphologically similar organisms have been detected in avariety of mammals, including rats, chimpanzees, orangutans,and infrequently, dogs or cats. The species found in pigs, Bal-antidium suis, is likely identical to B. coli from humans. Otherspecies have been isolated from guinea pigs (Balantidiumcaviae), cockroaches (Balantidium blattarum), fish, birds, andamphibia. In all, there have been about 50 species described(4). Morphology has been the basis for identification to thespecies level, and some of the different species that have beencreated are in reality B. coli showing polymorphism in differenthosts and under varied growth conditions (41). The taxonomywill ultimately be resolved once these organisms undergo se-quencing of their small subunit rRNAs.

MORPHOLOGY

The trophic ciliate measures 30 to 150 �m in length by 25 to120 �m in width; the cyst, which may be spherical or slightlyovoid, measures 40 to 60 �m in diameter (41). Size, however,varies, with some balantidia being up to 200 �m in length. Themouth (oral apparatus) is located at the tapering anterior end,and the cytopyge (anus) is at the rounded posterior end (Fig.2). A sausage-shaped macronucleus and a rounded micronu-cleus are located in the cytoplasm (Fig. 3). Asexual division

occurs as it does in most ciliates. A transverse furrow forms,dividing the mother cell into two asymmetric daughter cells, ananterior (proter) and a posterior (opisthe) cell. The proterretains the oral apparatus, and the opisthe develops a newapparatus. An anterior stomatogenic field appears in the opis-the, leading to formation of the new oral apparatus, but the oldoral apparatus may also undergo reorganization (37). Theswimming organism exhibits a rotary-type motion by means ofits somatic cilia that may facilitate movement through thecontents of the colon.

Sexual reproduction as conjugation has been reported forBalantidium, but information is lacking about details of nuclearevents. Two successive fissions (meiosis) occur preceded byformation of the conjugants, by either equal or unequal divi-sions (37). The two conjugants attach at the oral apparatus andexchange micronuclear products of meiosis.

Although the organism lives in an anaerobic environment,Zaman (80) described mitochondrion-like bodies in electronmicrographs of Balantidium; in contrast, Entamoeba histolyticaand Entamoeba dispar, found in the human colon, are anaer-obes and lack mitochondria. Cristae or tubuli, however, werereportedly absent from the mitochondrion-like bodies, raisingthe possibility that these organelles are hydrogenosomes (80).Hydrogenosomes, relict mitochondria, have been identified inbalantidia as well as in other anaerobic ciliates (25, 27). Othercytoplasmic components include endoplasmic reticulum, ribo-somes, and numerous vacuoles filled with food particles. AGolgi apparatus was not seen, but vesicles of endoplasmicreticulum function in lieu of Golgi vesicles (49). Mucocysts arealso seen (61). Two contractile vacuoles, functioning as osmo-regulatory organelles, pulsate at a low rate even though thesurrounding environment is isotonic or close to it (79). Undi-

FIG. 2. Trophic stage of Balantidium. The surface of the organismis uniformly covered by cilia, and the lighter areas in the cytoplasmrepresent a contractile vacuole (CV) and the macronucleus (Mn). Theoral apparatus (OA) is at the apical end of the organism in thismicrograph. (Reprinted from the CDC-DPDx Parasite Image Library[http://www.dpd.cdc.gov/dpdx/].)

FIG. 3. View of intestinal lumen with two trophic ciliates. Theprominent macronucleus is seen in this stained section. The upperciliate shows the small micronucleus nestled against the macronucleus.(Reprinted from the CDC-DPDx Parasite Image Library [http://www.dpd.cdc.gov/dpdx/].)

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gested residue from food vacuoles is eliminated through thecytopyge.

Balantidium jocularum, from the herbivorous surgeonfish,was found to harbor endosymbiotic bacteria in its macronu-cleus (26). The bacteria were unusually large, measuring up to4 �m in length, but apparently without pathogenic potentialfor the ciliate. They are probably gram-positive organisms be-cause of an endospore-like inclusion within the bacterium. Noother reports of endosymbionts from Balantidium have ap-peared.

The cyst of Balantidium is the transmissive stage of theorganism. Because of its thickened wall, it is protected fromdesiccation and other environmental stress (Fig. 4). It survivesbest in humid surroundings protected from direct sunlight. Thetrophic ciliate is reportedly unable to survive passage throughthe stomach because of the low pH of gastric fluid, but Balan-tidium trophozoites inoculated into the stomach of guinea pigshave been found in the colon (56). The process of encystmentbegins in the colon and rectum of the host, and cysts aregenerally found in formed feces (56). Cysts, however, were notproduced in cultures of balantidia (32, 80), nor are they pro-duced in cases of acute dysentery. Attempts to simulate in vitrothe colorectal environment in which cysts form (resorption ofwater and increased salt concentrations) were unsuccessful ininducing encystations; overfeeding or starvation was also un-successful (32). Loss of the ability to encyst is seen in someother protozoa (e.g., soil amebae) maintained in culture, duein part to less than optimal growth conditions and/or limitingamounts of nutrients essential for encystment. E. histolytica, anagent of amebic dysentery, resembles Balantidium in produc-ing cysts in formed stools but trophozoites in dysenteric stools.In vitro encystment of E. histolytica depends upon a number offactors, including withholding rice starch, the composition ofthe bacterial flora, and the encystment medium, and may re-quire a complex protocol to induce cyst formation (10).

The diet of the ciliate is made up of bacteria and foodparticles present or passing through the colon. If extensivedamage has been done to the wall of the colon, red blood cellsmay also be seen in trophic organisms, as well as blood in thestool. The ciliates produce flask-shaped lesions in the submu-cosa, where they form clusters called nides or nests (4).

TAXONOMY AND MOLECULAR BIOLOGY

Protist taxonomy was recently revised by a committee of theInternational Society of Protistologists and allied groups,based on the wealth of new information that has accumulatedabout these organisms since the last “official” taxonomy pub-lished in 1980 (1). Since that time, an increased understandingof ultrastructural morphology, biochemical properties and re-lationships, studies of nuclear and mitochondrial DNAs, andprotist ecology has developed. The revised taxonomy does notuse the classic categories of phylum, class, order, etc., in orderto maximize taxonomic plasticity as additional information onphylogenetic relationships, particularly from genomic studies,becomes available (1).

Balantidium spp. Defined

The following definition is based on the new taxonomy andrepresents a characterization of the genus Balantidium, fromthe more general ciliate features to those of the genus Balan-tidium.

Balantidium spp. are ciliated binucleate protozoa with macro-and micronuclei (features of Ciliophora) covered by uniformrows of monokinetid somatic ciliation, i.e., rows of cilia arisingfrom single rows of subsurface kinetosomes and their associ-ated fibrils; a slit-like anteroventral oral cavity depressed belowthe surface (a feature of Litostomea); an oral apparatus withdense ciliation but lacking specialized oral membranelles, withfeatures of endosymbiosis in many animals (“hairy” mouths;features of Trichostomatia); a vestibular groove leading intothe oral apparatus (feature of Vestibulifera); a vestibulargroove of less than one-half the body length, with features ofcommensals of vertebrate, amphibian, and insect hosts or, insome cases, parasites that may attack the intestinal lining (fea-ture of the “family” Balantidiidae). Representative species inmammals are B. coli, B. suis, and B. caviae.

Molecular Biology

Nucleotide sequences from two studies on Balantidium areon file at GenBank, one based on small subunit rRNA(AF029763) and the second based on internal transcribedspacers 1 and 2 (AF045030) (65, 66, 73). Sequencing was usedto construct a phylogenetic tree placing Balantidium coli withthe trichostome ciliates Isotricha spp. and Dasytricha spp., butnot necessarily in the same subgroup (“order” Vestibulifera).These ciliates have a sunken oral apparatus but lack toxictrichocysts, which aid in subduing prey (51). As yet, a definitivetaxonomy of the group awaits additional information.

FIG. 4. Encysted ciliate. The cytoplasm is protected from environ-mental stress within a double-walled covering. (Reprinted from theCDC-DPDx Parasite Image Library [http://www.dpd.cdc.gov/dpdx/].)

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Balantidium coli versus Balantidium suis

Are B. coli from humans and B. suis from pigs the sameorganism? Persons infected with porcine strains of Balantidiumare usually assumed to have B. coli infections (72). The answerawaits further studies of the organisms’ DNAs. Although pigsare the major reservoir for balantidiosis in humans, laboratorystudies indicate that humans are not readily infected with B.suis or, for that matter, B. coli. The two organisms appear to bedifferent from one another—one is larger (B. coli) and one issmaller (B. suis)—which is evidence enough for some to regardthem as different species. According to Levine (41), however,a clonal isolate of B. coli showed both morphotypes after beingmaintained in culture. The size difference between the two wasattributable to growth conditions of the ciliates. Jameson (32)favored the view that there is a single species of Balantidiumthat splits into two size variations. Similarly, B. caviae from theguinea pig was described as being indistinguishable in vitrofrom B. suis from swine (56). An agar diffusion study of anti-gens of Balantidium spp. found only minor antigenic differ-ences between B. coli and B. suis (36).

PHYSIOLOGY

Few studies have examined the energy metabolism of theseorganisms. Balantidia are equally able to survive under anaer-obic as well as aerobic conditions. Carbohydrates are the chiefsource of energy for the ciliates growing in vivo (80).

In studies of B. coli combining ultrastructure with cytochem-istry, peroxisomes were identified in the ciliate. These vesiclescontain peroxidase, an enzyme affording protection from thedestructive effects of highly oxidative compounds, such as hy-drogen peroxide (60). A comparison of the cytoplasm of cili-ates from asymptomatic swine and those with acute balantid-iosis was made. Peroxisomes were more numerous but smaller(0.6 to 0.8 �m) in balantidia from asymptomatic pigs than inthose with acute disease (�0.8 �m). Likewise, nucleic acidcontents (particularly RNA, but also DNA) from symptomaticand asymptomatic hogs differ, with the former having morecontent (62). The difference may depend on the degree ofmetabolic activity of the ciliates and, in the case of RNA, maybe indicative of enhanced protein synthesis. Ciliates withhigher nucleic acid content produced more robust cultures, atleast in the initial stages of in vitro growth (62).

The enzyme glucose-6-phosphatase was present in small ves-icles attached to the endoplasmic reticulum or in the mem-brane itself (61). Alkaline phosphatase was found in the ciliatecortex, nuclear and ciliary membranes, and kinetosomes aswell as in vesicles of the endoplasmic reticulum (61). Phos-phatase enzymes are important for their role in making glucoseavailable as an energy source.

Balantidia produce no known toxins, but their ability toproduce ulceration of the colon wall is attributed to hyaluron-idase, an enzyme that digests hyaluronic acid, a component ofthe “glue” holding mucosal epithelial cells together (68). Thedissolution of group C Streptococcus hyaluronic acid-contain-ing capsules and the breakdown of potassium hyaluronate byliving B. coli were taken as evidence of hyaluronidase activity.Entamoeba histolytica, the classic protozoan dysentery agent,attacks the mucosal surface of the colon and was long claimed

to possess hyaluronidase activity. Attempts to demonstrate itspresence, however, using E. histolytica extracts, did not supportthe claim (50). In the case of B. coli, other factors may affectthe results, including associated bacteria, waning of virulence(see next paragraph) in long-term cultures, up- and downregu-lation of presumptive virulence genes, and strain differences.Thus, the matter of hyaluronidase production by Balantidiumwarrants further study. Levine (41) noted that invasion of co-lonic epithelium by Balantidium might be secondary to damagecaused by intestinal bacteria.

The term “virulence” is used here as the degree of patho-genicity of a parasite and involves substances elaborated by apathogen that facilitate infection and disease (e.g., toxins, en-zymes that promote invasiveness, and adhesive properties).

CULTIVATION

Balantidia are available from fresh pig feces, particularlyfrom animals with evidence of acute balantidiosis, and fromslaughterhouses. An insulated bottle is used for transport ofporcine intestinal contents to the laboratory. There were anumber of early attempts at maintenance or cultivation invitro. Gastric mucin media were developed for Balantidiumand maintained growth for as long as 30 months. Calf serumand rice starch or rice powder were also required. Starchgrains, if present in culture medium, are ingested and serve asa carbohydrate source (80). The addition of soluble sugars togrowth media encourages overgrowth of accompanying bacte-ria, whereas starch particles are efficiently ingested and di-gested by the phagotrophic ciliates (10). A defined mediumwith cysteine HCl and i-inositol was used for short-term phys-iologic experiments with the organisms, but little in the wayof results appeared in the literature (35). Trophic ciliates,however, did not survive agitation in attempts at manometricstudies.

Biphasic media in tubes were often used with an agar, in-spissated egg yolk, or serum butt overlaid with nutrient me-dium. Bacteria present in the sample can overgrow in an en-riched medium, requiring the addition of antibiotics (e.g.,penicillin-streptomycin) to suppress bacterial proliferation.When growing in tubes, balantidia favor the bottom of thetube, where conditions are microaerophilic or anaerobic. Za-man maintained monoxenic cultures of B. coli with Escherichiacoli, using antibiotics to control bacterial overgrowth (80). Di-amond’s TYSGM medium for Entamoeba and other entericparasites will also support the xenic (with bacteria) growth ofbalantidia (10). The medium contains Trypticase (casein di-gest), yeast extract, serum, and porcine gastric mucin. Starchpowder is added to tubes at the time of inoculation of medium.Zaman (80) noted that B. suis from the pig is not as readilycultured as B. coli from humans. Others apparently encoun-tered no difficulties with cultivation of B. suis (62). Among thevariables involved in cultivation are the growth medium andpH (optimal range, 5.4 to 8.0) (32), associated undefined bac-terial flora, strain differences, and differences in nutritionalrequirements between ciliates from different hosts.

In the case of many pathogens maintained in vitro, pro-longed cultivation attenuates virulence, and animal passagemay be required to restore it. The difficulty in infecting labo-ratory animals (e.g., the guinea pig) and the absence of an

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animal model of disease are obstacles in attempts to study thepathogenicity of the ciliate.

Temperature Range for Growth

Balantidium grows at temperatures between 20°C and 40°C,a range that is adaptive for mammalian endosymbionts but canalso allow for survival and growth outside the host (10). Tro-phic ciliates did not survive longer than 24 to 48 h in culturesmaintained at temperatures over 40°C (12). The trophic or-ganism from pig intestine has been reported to survive roomtemperature and exposure to air for 24 h to as long as 10 days(56). There is no information about the ability of balantidiafrom poikilothermic animals to make the transition to mam-malian body temperature.

HOST-PARASITE INTERACTIONS

In pigs, as in some humans and other mammals, Balantidiuminfects but causes no serious disease of the gastrointestinaltract. It can thrive there in balance with its host without causingdysenteric symptoms, such as severe diarrhea and bloodystools. The problem with this detente is that malnutrition,alcoholism, or a compromised immune system (as in humanimmunodeficiency virus [HIV]/AIDS, etc.) can tip the balancein favor of the ciliate, leading to disease (3, 20, 70). In acutedisease, explosive diarrhea may occur as often as every 20 min(38). Perforation of the colon may also occur in acute disease,with life-threatening consequences.

Hosts vary in their susceptibility to Balantidium, and at-tempts at infecting humans have been disappointing (76). Thismay be due to virulence of the particular strain used for infec-tion, the intrinsic health of the host, and/or the dosage of theciliate administered to the host. There is no evidence thatother intestinal flora of humans, whether bacteria, protozoa, orviruses, render the host more susceptible to infection. It isknown, however, that the presence of pathogenic bacteria (e.g.,Salmonella) in the intestine can worsen an infection by invad-ing colonic lesions caused by balantidia (41, 57, 59, 60).

Nonhuman primates, particularly the great apes and OldWorld monkeys, can develop Balantidium infections. This is ofconcern since some of these animals are endangered species orclose to it because of disease, encroachment by humans ontheir natural habitats, and resultant density-dependent changesin their populations. Orangutans, for example, are less likely tobe infected by Balantidium in their natural setting, while thosein “rehabilitation” centers for injured or orphaned animalshave a higher prevalence of the ciliate (34). Reasons for thisinclude overcrowding at the centers, with increased stress onindividuals; contacts with humans and other animal speciesand their associated bacterial, viral, and protozoan organisms;and water sources contaminated by fecal material in confinedareas (34). In the wild, the orangutan population density is 2individuals km�2 (34).

Immune Response

Few intensive studies have been carried out on the host’simmune response to Balantidium. One study examined a pop-ulation of captive rhesus macaque monkeys with and without

chronic diarrhea (57). The study detected a broad array ofbacterial and protozoan organisms, as well as viruses. A sig-nificantly larger proportion of animals with chronic diarrheaharbored pathogenic bacteria (Campylobacter spp., Shigellaflexneri, and Yersinia enterocolitica) than those without diar-rhea. The prevalence of Balantidium was �12% (estimatedfrom the graph in reference 57). The presence of Balantidiumin the colon appeared not to be the cause of disease. Monkeyswith chronic diarrhea showed a significantly increased produc-tion of interleukin-1�, interleukin-3, and tumor necrosis factoralpha, to a level of 60 to 70% from about 20 to 30% innondiarrheic animals. This was not necessarily due to Balan-tidium, however, because of the variety of other intestinal or-ganisms.

Experiments to demonstrate an immune response to Balan-tidium were done by Zaman, who studied an immobilizationreaction to Balantidium (78). Using anti-Balantidium antibod-ies raised in rabbits, the serum immobilized the ciliates withinless than a minute (titers of 1:4 and 1:8); higher dilutions tooklonger to immobilize. Heat-inactivated rabbit serum did notimmobilize. Ultimately, the treated organisms disintegrated.Demonstrating a humoral response to the ciliates in patientswith balantidiosis by indirect fluorescent-antibody (IFA) stain-ing would be of interest. Dzebenski (18) tested pigs for anti-Balantidium antibodies by IFA staining but had difficulty indetecting any activity in pig sera. He attributed the lack ofresponse to the small sample of animals tested and the absenceof evidence of tissue invasion in the animals used.

Availability of an IFA test would sidestep the labor-intensiveexamination of stool samples (except for validation) and mightgive a better estimate of Balantidium exposure if applied topersons living in regions of endemicity and to groups at risk.

Populations with constant exposure to Balantidium may de-velop some degree of immunity (19). In areas where contactwith pigs is common, such as the Altiplano region of Bolivia,most of the schoolchildren examined had asymptomatic infec-tions, but few had diarrheic stools, suggesting resistance tofulminant disease (19). In an outbreak of balantidiosis on theWestern Pacific island of Truk (see below), immunity may havebeen a factor in the relatively rapid resolution (estimated at �6weeks) of the outbreak among inhabitants (72).

Immunosuppression

Immunocompromised individuals appear to be less resistantto balantidiosis. There have been no concerted studies, how-ever, to determine the prevalence of balantidia in immunosup-pressed hosts. Reports in the literature are more of an anec-dotal nature. Two reports of balantidiosis in HIV/AIDSpatients point to Balantidium as both a pathogen and an op-portunist (8, 11).

There have been several reports in the literature of Balan-tidium spreading to the lungs, causing a pneumonia-like dis-ease. Most of these infections have occurred in elderly orotherwise immunocompromised persons. A 71-year-old woman(Greece) with anal cancer, diabetes, fever, and intermittent di-arrhea was found to have Balantidium in her lungs when abronchial secretion was examined as a wet mount slide prep-aration (70). Although the patient was treated with metroni-dazole, a drug of choice for treating balantidiosis, and several

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other antimicrobials, she died of cardiac arrest. Computerizedtomography scans and X-rays showed lung lesions in a 58-year-old woman (Greece) with leukemia who also suffered weightloss, weakness, abdominal cramps, and a cough. Since a fungalinfection was suspected, bronchoalveolar lavage was per-formed, and balantidia were found in the wash fluid (3). Theauthors postulated that the ciliates were spread hematog-enously from the site of colonic ulceration to the lungs. Anti-microbial treatment with metronidazole was successful inresolving the infection.

Nonpulmonary infections have also developed in immuno-suppressed patients. A 54-year-old alcoholic pork butcher(France) with acute diarrhea suffered colonic perforation butrecovered after doxycycline treatment and colectomy (20). Bal-antidia were detected in stools of a 47-year-old female (Tur-key) with non-Hodgkin’s lymphoma, accompanied by abdom-inal pain and bloody diarrhea; she, too, was treated successfullywith metronidazole (75).

Except for the pork butcher in the preceding paragraph, theother patients had no contact with pigs and lived in urbanareas. The leukemic patient had also received corticosteroidsand chemotherapy, which may have increased vulnerability toopportunistic infections by muting the immune response.Other possible sources of infection may have been consump-tion of uncooked vegetables and/or food contaminated by ratdroppings or mechanically by cockroaches or flies.

A Barbary sheep (Korea) from a zoological park died aftershowing signs of emaciation, lethargy, and weakness (9). Atautopsy, B. coli was found in the gastric lymph ducts and in thesubmucosa of the abomasum of the animal. The sheep was alsoinfected with the coccidian Eimeria and with parasitic worms.Balantidia, however, were not found in the animal’s stool. Itwas hypothesized that balantidia initially infected the aboma-sum, where damage to the mucosal surface by Eimeria facili-tated invasion of the lymphatics.

Among possible pathways by which balantidia in the colonwall could colonize the lungs are the circulatory (hepatic portalcircuit) or lymphatic systems (3, 9, 70), perforation of the colonand spread through the peritoneal cavity (70), invasion of thelungs across the diaphragm (58), and colonization of the na-sopharynx with spread to the lungs, resulting from aspiration offluid from the oral cavity (58). It is interesting that there was noindication of Balantidium trophic ciliates or cysts in the stoolsamples of most of these individuals.

Crossing the Species Barrier: Transfection Attempts

A parasite passing from one species to another faces theproblem of species specificity, both for the host and for theparasite. Although humans are susceptible hosts for balantidia,efforts to infect humans have not been successful (76). Balan-tidium has no problem in passing from pigs to humans underappropriate conditions (e.g., the outbreak in Truk). For trans-mission to be successful, it would appear that proximity andpersistent contact between pigs and humans are factors.

Gelatin capsules containing human feces with active ciliatesand cysts were given to volunteers (76). The capsules contained250 trophic ciliates and 250 cysts. Volunteers were followedover a period of 10 years, but no evidence of infection based onstool examination was found. The study may have been ex-

tended to detect asymptomatic or cryptic infections that werenot readily apparent in stool samples from the volunteers. Inanother study, a human fecal homogenate containing consid-erably larger numbers of cysts of B. coli (1.2 � 104 to 4.8 � 104)was used to infect piglets and monkeys (74). Severe diarrheadeveloped in about half of the piglets (4 of 10) and in monkeysthat had been pretreated with hydrocortisone (2 of 4). Otheranimals suffered moderate diarrhea (piglets) or were asymp-tomatic (monkeys).

Attempts were also made to infect guinea pigs by usingporcine balantidia (56). Ciliates harvested from culture, con-taining starch grains as markers, were injected into the stom-ach of a guinea pig. Starch-filled ciliates were subsequentlyfound in its esophagus and cecum. A second attempt trans-ferred ciliates directly from the pig by use of a stomach tube,and trophic ciliates were detected in the ileum, jejunum, andcecum. Both animals died soon after infection as a result of theexperimental procedure.

DISEASE POTENTIAL

Balantidiosis has a range of mild to severe clinical presen-tations. The following three clinical manifestations of balan-tidiosis can occur (71): (i) asymptomatic hosts who are carriersof disease and serve as reservoirs of infection in the commu-nity; (ii) chronic infection that presents with nonbloody diar-rhea, cramping, halitosis, and abdominal pain secondary totrophozoite invasion of the large intestine (71, 75); and (iii)patients with fulminating balantidiosis passing mucoid, bloodystools.

The most severe presentation of B. coli occurs with weightloss, tenesmus, and bloody stools (71). Intestinal hemorrhageand perforation can also occur and are mediated by the pro-duction of B. coli proteolytic enzymes (3). Direct evidence forthe presence of proteolytic enzymes is lacking, but proteolysisis generally assumed to be a factor in digesting the mucouscoating of the colon and facilitating tissue invasion, abscessformation, ulceration, and perforation of the intestine (3).Entamoeba histolytica, which has a similar pattern of pathoge-nicity, has been shown to possess and secrete cysteine, serine,aspartic, and metallo-proteases, some of which can target themucus layer of the intestinal wall and aid in penetration of theunderlying tissue (43, 69).

Hemorrhage and perforation were reported for fatal cases ofB. coli infections (17). Fulminating balantidiosis has a casefatality rate of 30% (19). Vasquez and Vidal (71) described thecase of a 60-year-old pig farmer with pancolonic damage andmicroperforation who died despite antiparasitic treatment.Another fatal case of balantidiosis occurred in a 63-year-oldpig farmer, who died of dysentery and subsequent hemorrhageafter 8 days; an autopsy revealed ulcerative colitis (54). Fatalcases of balantidiosis have also been associated with sepsissecondary to intestinal disease (51, 58). A malnourished2-year-old girl with an anorectal malformation who was diag-nosed with balantidiosis developed septic shock and died de-spite antimicrobial treatment with ampicillin and amikacin forsepsis and metronidazole for balantidiosis (7).

Although the intestine is the most common site of B. colidisease, there are extraintestinal sites of infection. These in-clude the appendix but rarely the liver (16). Dodd (16) re-

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ported a case of a 16-year-old who presented with abdominalpain, fever, and elevated white blood cell count and who had agangrenous appendix. Pathological examination of the appen-dix revealed inflammation, ulceration, necrosis, and B. colitrophozoites (16). Genitourinary sites of infection, includinguterine infection, vaginitis, and cystitis, are thought to occur viadirect spread from the anal area or secondary to rectovaginalfistulas created from infection with B. coli (58). Lung infectionswith Balantidium are infrequent but noteworthy. A necrotizinglung infection was reported for a 42-year-old organic farmerwho routinely used pig manure to fertilize his vegetables, prob-ably as a result of aerosolizing the manure and inhaling air-borne cysts (58). Airborne transmission of cysts is unlikely.Cysts of Balantidium are large and would not be carried overgreat distances, either on air currents or in water droplets.Thus, infection by inhalation would require direct contact withaerosol droplets.

Factors Contributing to Disease

Nutritional status, intestinal bacterial flora, parasite load,achlorhydria, alcoholism, or any chronic disease may affect theseverity of disease (19). As previously noted, some degree ofimmunity may be present in populations that are exposed toBalantidium on a regular basis (19, 72).

EPIDEMIOLOGY

B. coli infection is uncommon in humans despite its potentialfor worldwide distribution. The organism, though pathogenic,is of low virulence. The worldwide prevalence is estimated at0.02 to 1% (19), but it varies widely by geographic location (3).

Areas of high prevalence include regions of Latin America,the Philippines, Papua New Guinea and West Irian, and areasof the Middle East (63, 75). In New Guinea, the rate of infec-tion among pig farmers is as high as 28% (53), and in theAltiplano area of Bolivia, balantidiosis rates range between 6and 29% (19).

The major factors leading to human balantidiosis include (i)close contact between pigs and humans, (ii) a lack of appro-priate waste disposal such that swine and human excrementcontaminate drinking water sources (e.g., wells and streams)and food, and (iii) subtropical and/or tropical climatic condi-tions (e.g., warmth and humidity) favoring survival of cysts.Balantidia, however, are adaptive to less than ideal conditions,as evidenced by their ability to infect compromised hosts livingin urban settings and to survive in hogs in decidedly nontrop-ical locations, such as Denmark (31) and Poland (62). Human-to-human spread by the fecal-oral route can take place as withother enteric diseases.

Outbreaks of Balantidiosis

What factors are responsible for balantidiosis outbreaks inhumans? What is regarded as the largest outbreak of balan-tidiosis occurred following a typhoon that hit the island of Trukin the Caroline Islands (Western Pacific), where many of theresidents kept pigs. A typhoon destroyed homes and rooftopcatchment systems for collection of rainwater, leaving peoplewithout a source of uncontaminated water. Instead, they used

water from streams and wells that were contaminated with pigand human feces, resulting in balantidiosis in 110 persons onthe island, as well as an increase in E. histolytica and Ascarisinfections (72). Neither age nor gender was a factor in infec-tion and disease, as it appears to be in “normal” transmissionin areas of endemicity. Symptoms of disease appeared an av-erage of �6 days (range, 0 to 27 days) before laboratory diag-nosis was made by stool examination (72).

In institutional populations (mental hospitals, prisons, andorphan asylums), where pigs are an unlikely source of infec-tion, outbreaks are the result of asymptomatic carriers and thedifficulties involved in maintaining hygienic control (6). Casesdeveloping in urban areas generally occur in immunocompro-mised hosts and are self-limiting outbreaks (3, 9, 20, 70, 75).

The bacterial flora of the host can influence its susceptibilityand course of infection, particularly if pathogenic or poten-tially pathogenic bacteria are found in the gut. This has beenthe case for nonhuman primates (57) and supports a secondaryrole of Balantidium in causing disease (41, 59, 62).

Virulence versus Avirulence in Balantidium

Are there virulent and avirulent strains of B. coli and B. suis?What factor(s) triggered the balantidiosis outbreak in Trukdescribed above? Because the prevalence of Balantidium in thepopulation was low before the typhoon hit the island, Walzeret al. (72) postulated that the source of the infection was pigsrather than humans.

In theory, a virulent strain entering a population, whetherfrom contaminated water or human or pig contacts, is a likelysource of disease outbreak. In contrast, avirulent strains areeither ineffectual in causing disease or produce asymptomaticinfections. But no evidence for such a dichotomy in balantidiais available. Humans are not easily infected, and the preva-lence of Balantidium among humans (estimated at 1% world-wide) is lower than that in pigs, which can be as high as 100%in surveyed swine populations (31). The distribution of patho-genic Entamoeba histolytica and its nonpathogenic look-alikeE. dispar in human populations helps to explain the anomaly ofpersons having entamoebae in stools without symptoms ofinvasive disease (15). Many more persons are infected asymp-tomatically with E. dispar than with the pathogenic organism E.histolytica. Among those diagnosed as having Entamoeba intheir stools, only 10 to 20% exhibit diarrheic disease. Further-more, even the avirulent organism E. dispar may erode thecolon wall and cause symptoms such as bloating and cramps(15).

PREVALENCE OF BALANTIDIUM COLI

Infections in Nonhuman Mammals

Although pigs are a major source of balantidiosis, a numberof other mammalian species have been found to harbor theciliate. In a study done in Japan with fecal samples from 56mammalian species, 6 were found to harbor the ciliate (49).The species were mainly nonhuman primates but also includedwild boars. Rodents and carnivores (cats and dogs) testednegative. From the small number of infections found, Nakau-

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chi (49) concluded that the disease was not a veterinary con-cern in Japan.

Swine. Studies that have examined the prevalence of Balan-tidium in pigs in the United States are few. Morris et al. (47)studied intestinal parasites of pigs, including B. coli, fromOklahoma hog farms. Balantidium was detected in 55.1% ofpigs examined, with adult swine having a higher prevalence(18.6%) than shoats (14.6%) and nursing pigs (5%). Pigs onpasture land or dirt lots had a somewhat higher percentage(16.4%) of balantidia than pigs kept on wood slats (13.2%) oron cement surfaces (7.4%) (36). A study of hogs in southernGeorgia found a higher prevalence of B. suis in gestating hogsthan in lactating animals, but differences were attributed tolocations (different units) where hogs were confined and to agedifferences (45). Weaned pigs were negative for balantidia butsoon became infected either from the mother or from caproph-agy of residual fecal material in the holding pens. In general,however, prevalence increased in pigs with age, as also shownin the Oklahoma survey.

In two reports from Europe, the prevalence of Balantidiumin pigs from a Danish research farm increased from 57% forsuckling piglets to 100% for most other age groups (31). In asurvey of 15 of 20 pig-raising farms (n � 514 fecal samples) inGermany, the prevalence of infection was 60% for sucklingpigs (13).

Wild boars in rural Western Iran were a reservoir for Bal-antidium, and since wild boars scavenge farms, Solaymani-Mohammadi et al. (63) examined boars to assess the potentialfor spread of ciliates to livestock and humans. Sixty-sevenpercent of the animals were infected, with females and olderanimals carrying a heavier parasite load than males andyounger boars. Because pork is proscribed for Moslems, rais-ing pigs in Iran is not an important factor in zoonotic infec-tions, and cases that occur are assumed to be the result ofhuman-to-human transmission. A study of fecal material from292 feral pigs in water catchment areas in southwest Australiaidentified 10 positive cases (3%) in genetically distinct popu-lations (29). Infections were not uniformly distributed amongall genetic groups; most of the infected pigs (nine animals)were from a single genetic subpopulation. Balantidium wasuncommon or absent from other subgroups. It was concludedthat feral pigs pose a public health threat to drinking watersupplies, as the animals wallow in creeks feeding into reser-voirs providing water to local municipalities.

Simians and apes. A study of 910 fecal samples from 222nonhuman primates confined in groups was carried out at fourzoological gardens in Belgium (40). Entamoeba spp. and Giar-dia were the most common endocommensals, with infectionrates of 44% and 41%, respectively, and Balantidium was de-tected in 13% of the fecal samples, mostly from Old Worldmonkeys (Celebes crested macaques, mandrills, and hamadr-yas baboons); prosimians (e.g., lemurs) and New World mon-keys were uninfected. Thus, though infection with the ciliatewas found in some groups, it was not a pervasive problem inzoo populations. Its incidence in Old World monkeys mightrelate to these simians spending more time on the ground thanother animals and being more likely to come into contact withfeces containing Balantidium cysts (40).

Nursing rhesus macaque monkeys at a research center werestudied for milk production (30). Animals with B. coli infec-

tions produced milk with a lower fat content than that inanimals without B. coli. Furthermore, the heavier the infection,the less fat was in the milk (about 6.5% in “clean” animalsversus 4.2% in animals with heavy infections). The lower fatconcentration, however, did not affect infants’ weights.

Baboons (Papio doguera) captured in the wild in Kenya werethe basis for a study of intestinal protozoa (48). At the time ofcapture, 63% of the animals harbored Balantidium, the mostfrequently found protozoon. After transfer and captivity in theUnited States for a year and a half, none of the animals wereinfected, although most other intestinal protozoa (e.g., Ent-amoeba spp.) did not diminish in numbers.

A comparative survey of parasites of semicaptive (at a re-habilitation center) versus free-ranging orangutans in Sabah,Malaysia, found that the prevalence of B. coli was 14% in thefree-ranging group, while it was 42% among semicaptive younganimals (34).

Amphibia. A number of species of Balantidium have beendescribed from amphibia (e.g., Rana, Xenopus, and Bufo spe-cies). A recent finding by Li et al. showed that Balantidiumoccurred in the feces of the giant Chinese salamander (42). Anew species, Balantidium andianusis, was described from asingle animal; a second species from the salamander, Balan-tidium sinensis, had previously been described from the frog.The basis of identification to the species level was detailedmeasurements of the oral apparatus, length, width, etc.

Infections in Humans

The prevalence of human balantidiosis is higher in popula-tions in regions of endemicity having close contact with pigs orpig feces, such as farmers and workers in abattoirs (51, 58).Contacts between humans and pigs are necessary but not suf-ficient to cause disease. Other factors must be taken into ac-count, such as (i) host factors, including resistance and/or pos-sible immunity; and (ii) the etiologic agent itself and its abilityto invade host tissues.

Balantidiosis is an uncommon human disease mostly re-stricted to tropical and subtropical regions because of sanitarystandards, climatic conditions, and cultural mores. The majorfactors in spread of the disease to humans are the presence ofinfected swine and little or no means of disposal of animal andhuman waste. It is a disease of poor, rural areas where peopleare likely to live in close proximity to their livestock, with theirhomes offering protection not only for themselves but also fortheir domestic animals. It is the close association of people andpigs that leads to infection. Pigs pass Balantidium cysts in theirfeces, which can contaminate wells and groundwaters, servingas a vehicle for transmission of parasites.

In a survey of 325 waterborne diseases in North America andEurope, Balantidium infections accounted for 0.3% (n � 1) ofthe outbreaks (33).

A comprehensive study of stool samples from �2,000 Ay-mara Indian children from the Altiplano region of Boliviafound widespread infection with balantidia but a low level offulminant disease among the children (19). The overall prev-alence of B. coli was 1.2% (range, 1.0 to 5.3%). More than halfof the pigs (n � 50) in the same communities were infectedwith balantidia, as determined by examination of stool sam-ples. One-third of the children in the survey showed stunted

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growth as a result of chronic malnutrition. The authors of thestudy concluded that the children were asymptomatic carriersof balantidia but showed the consequences of long-term infec-tions.

Areas of endemicity. Areas of endemicity are regions wherebalantidiosis is a present and constant threat. Included amongthese are the Philippines, parts of Papua New Guinea andWest Irian (Irian Jaya) in the western Pacific, and rural areasof South America. As described above, conditions for spreadof disease are close contact with pigs and water contaminatedby human and porcine feces. Tropical temperatures and highhumidity favor survival of excreted Balantidium cysts in pig orhuman feces. The disease is also in found in highland areas ofPapua New Guinea and Irian Jaya, where temperatures arecooler than in the lowlands. Because of the highland temper-atures, pigs often seek shelter and warmth in human habitats.Prevalence studies based on surveys carried out in the 1950sand onward have put the numbers at 28%, 20%, 11%, and inmore recent times (1970s), 1.7%, in only 3 of a total of 60villages (53). Other studies found prevalence rates of 1% to20% among people in the Central Highlands of Papua NewGuinea (5). Infections among women were twice as common asthose among men because women tend to the pigs.

Other possible pathways for transmission. Balantidiosis is acosmopolitan disease with potential for developing almost any-where. The absence of pigs in strict Moslem societies makeshuman-to-human transmission more likely. Rats may be carri-ers of Balantidium, but it is not known if the rat Balantidiumspecies can infect humans. The cockroach, which has its ownspecies of Balantidium, may serve as a mechanical agent oftransmission from feces to food (67).

Sewage sludge may be another source of infection. Activatedsludge, a by-product of sewage treatment, can contain bacte-rial, protozoan, and metazoan parasites and is a potentialthreat to health if it is applied as a fertilizer. Such was the casein Bahrain (Arabian Gulf), where sludge was found to containbalantidia (range of 66 to 528 ml�1, with a mean of 234 ml�1).The origin of the balantidia remained uncertain, since neitherpigs nor monkeys, both possible sources, are found in thecountry. The occurrence of the ciliate appeared to have been aone-time event, since ciliates were not found in subsequentsludge samples (2).

Risk factors. The major risk factor for humans is close con-tact with pigs. This is particularly so in areas of endemicity(e.g., Papua New Guinea), where swine are kept in dirt lots; pigand human feces are scattered indiscriminately, allowing con-tamination of water sources; and residents may suffer fromchronic malnutrition or other predisposing factors, such asparasitic infections. Crowding in dwellings can facilitate thespread of infection. Others at risk are workers in abattoirswhere pig intestines are handled. Farmers working with pigfeces are at risk of contracting the infection. Given the num-bers of simians carrying balantidia, zookeepers are anothersuch group, but at low or containable risk. Likewise, veterinar-ians and veterinary students working with sick hogs are at riskof infection.

Institutional balantidiosis. Both residents and workers inasylums, orphanages, and prisons are potential candidates forbalantidiosis. A study of four mental institutions in Italy exam-ined the prevalence of parasites in stool samples from 238

residents (24). About 13 different, mostly protozoan parasites,including B. coli, were found in stool samples. B. coli and themore common organism Cryptosporidium parvum were de-tected in 9.2% of the residents. A study done in the UnitedStates found a 5% incidence of B. coli infections at a mentalhospital, and this appeared to increase with length of residence(77). Poor hygiene among residents of the mental institutionwas associated with spread of parasites on hands, tableware,and dishes and with the practices of pica, coprophagy, andgeophagy. The conclusion is that hygienic surveillance andantimicrobial therapy are necessary in such facilities to limitthe spread of parasites among institutional residents.

LABORATORY DIAGNOSIS

Because of their large size and spiraling motility, balantidiacan readily be recognized in wet mount slide preparations,even at a low magnification (�100). This is the case withfreshly collected diarrheic stool samples, which are likely tocontain actively swimming trophic ciliates, as well as bron-choalveolar wash fluid. Stool samples for examination shouldbe collected over several days because excretion of parasitescan be erratic. Cyst stages are more common in formed stools.To search for cysts, a portion of formed stool is broken up inphosphate-buffered saline or fixative (10% phosphate-bufferedformalin or polyvinyl alcohol) and coarsely filtered throughgauze or a sieve to remove large pieces of debris. The resultingfluid can be examined microscopically for cysts in formedstools or for trophozoites in diarrheic stools. A phase-contrastmicroscope is helpful for viewing internal structures of un-stained living or fixed ciliates. Staining can be done usingLugol’s iodine (1:5 to 1:100 dilutions), but the stain concen-trates progressively in the cytoplasm, obscuring details such asthe macronucleus. The same is true for permanent stains, suchas hematoxylin-eosin, as cells can take up excess stain, obscur-ing all internal detail. Heavily stained cysts can be mistaken forhelminth ova, leading to misdiagnosis. Biopsy of the colon, ifperformed, followed by hematoxylin-eosin staining of sectionsmay be useful in evaluating the extent of damage to the wall(Fig. 4). Methods for concentration of parasites from stoolsamples, making them easier to find, include sedimentationand flotation (21). Since balantidiosis is a rare disease in de-veloped countries, most technicians would not normally belooking for trophozoites or cysts of Balantidium in examiningstool samples. Thus, it is particularly important that balantid-iosis be considered a possibility for patients from areas ofendemicity and travelers returning from such areas. The num-ber of balantidia in a stool sample may be high; 1,230 organ-isms g�1 feces was reported for the stool of a chimpanzee inJapan (49). A Danish study of pigs at a research farm found anaverage of 865 cysts g�1 feces from pigs of 28 to �52 weeks ofage (31).

Pulmonary Infections

Diagnosing lung infections with B. coli can present a prob-lem because of possible confusion between ciliated epithelialcells (CEC) and trophic balantidia. Bronchoalveolar wash fluidcontaining Balantidium has been reported (3, 58, 70) but mayalso contain motile CEC from the trachea that can be mistaken

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for ciliates in wet mounts. CEC have relatively few cilia ontheir surfaces compared to the uniformly ciliated surfaces ofbalantidia, and the cilia may be clumped in the case of colum-nar epithelial cells; CEC are smaller (30 �m) than balantidia(150 to 200 �m), elongated rather than ovoid, and more likelyto swim aimlessly in circles, unlike the pronounced spiralingmovements of ciliates (14, 28). The use of a phase-contrastmicroscope can help to visualize Balantidium features, such asthe oral apparatus, uniform somatic ciliation, and the macro-nucleus. During a search for respiratory syncytial virus in thenasopharynx of an infant, unusual ciliated cells were seen inwet mounts and initially thought to be parasites, most likely B.coli. Subsequent examination after staining showed ciliary dis-tribution along one edge of the cell and identified the cells asciliocytophthoria, degenerative fragments of epithelial cells(28). CEC can also be confused with other motile pathogens, inthis case the flagellate stage of the ameboflagellate Naegleriafowleri in cerebrospinal fluid samples (14).

Balantidiosis versus “Dysentery”

Most cases of dysentery, regardless of the causal agent, in-cluding balantidial, amebic, and bacterial dysenteries, presentwith similar clinical profiles, including abdominal pain anddiarrhea leading to dehydration and bloody stools. Balantidi-osis outside areas of endemicity is relatively rare; amebiosis ismore likely to be encountered, particularly in travelers fromdeveloped countries visiting areas of the world with poor san-itation and contaminated drinking water. A travel history ofthe patient can be helpful in making a preliminary diagnosis.Bacillary dysentery is a constant and major public health men-ace in developed and underdeveloped countries, with contam-inated water and food and asymptomatic food handlers beinginvolved in disease transmission. Diagnosis is made using dif-ferential or selective agars or manual or automated identifica-tion systems. Among differential diagnoses of dysenteric dis-eases are ulcerative colitis, diverticulosis, and inflammatorybowel disease.

The cystic organism E. histolytica, found in formed stools,measures 10 to 20 �m and has a nucleus with a small centralendosome and peripheral chromatin connected to the endo-some by a delicate fibrillar network. Four nuclei are typical ofthe mature cyst. RNA-containing club-shaped chromatoidbodies are seen in newly formed cysts but disappear as the cystsage. There may be cysts of other amebae, such as E. dispar andE. coli, both of which are harmless commensals that can bemistaken for E. histolytica, in the stool. Balantidium cysts arelarger (40 to 60 �m) than ameba cysts and are binucleate(macro- and micronuclei), and at times the trophic organismcan be seen spinning within the cyst wall due to ciliary activity.

The two organisms are found in the trophic state in diarrheicstools. Balantidia in wet mounts are active swimmers withuniform ciliation and a spiraling swimming pattern. TrophicEntamoeba can be seen moving on the slide surface by meansof an anterior ectoplasmic pseudopod and is smaller (�25 �min diameter) than Balantidium. Food vacuoles containingerythrocytes differentiate trophic E. histolytica from other ame-bae in stools. Both trophic organisms are seen optimally infreshly collected feces that have not been refrigerated or al-lowed to sit on a laboratory bench for hours.

Balantidium and Laboratory Infection

In developed countries, fecal samples containing balantidiaare unusual, and the risk of laboratory infections is very low.No laboratory-acquired infections have ever been reported.Because the cysts pose more of a risk than do trophic ciliates,precautions should be taken in handling formed porcine orhuman fecal matter that might contain cysts. The numbers ofcysts in stool samples may be such that small amounts ofmaterial can be highly infective. Cysts may also survive dryingon bench tops, instruments, and other laboratory surfaces.Sodium hypochlorite (1%) is an effective disinfectant (55).Procedures that produce aerosols should be avoided. Glovesand a laboratory coat are appropriate protective clothing. Bio-safety level 2 precautions are recommended.

PREVENTION OF INFECTION

The best means of protecting human populations from bal-antidiosis is by providing a source of clean, uncontaminatedwater for drinking and other purposes. Chlorine, at the con-centrations normally used for ensuring water safety, is noteffective against cysts of Balantidium. Pigs should not be al-lowed to roam in and around feeder streams or rivers thatempty into reservoirs that are used for providing municipalwater supplies (29). Likewise, spreading of sludge from sewageprocessing as fertilizer can lead to contamination of produce orwater sources with cysts of balantidia (2). Pigs should not haveaccess to areas where crops are being raised. Judging from theoccurrence of balantidiosis in immunosuppressed individualsliving in urban settings, there are additional sources of infec-tion besides pig-to-human transmission. Raising Balantidium-free pigs is an unrealistic goal. Piglets become infected fromtheir mother or, if not that, through coprophagy.

ANTIMICROBIAL THERAPY

Tetracyclines and metronidazole are treatments of choicefor human Balantidium infection. A number of different dos-age regimens and treatment durations exist in the literature (3,20, 23, 75). For metronidazole (Flagyl), the treatment is typi-cally 5 days (adult dosage, 750 mg three times a day; pediatricdosage, 35 to 50 mg kg of body weight�1 day�1 in three doses[maximum dosage, 2 g]), in contrast to tetracycline (adult dos-age, 500 mg four times a day; pediatric dosage, 40 mg kg�1

dose�1 in four doses) treatment over 10 days. Iodoquinol, fora 20-day treatment course (adult dosage, 650 mg three times aday; pediatric dosage, 40 mg kg�1 dose�1 in three doses), anddoxycycline are alternatives (3, 58). There is some evidencethat nitazoxanide (Alinia), a broad-spectrum antiparasitic andantihelminthic drug, may be another treatment for balantidi-osis (52). The reader may wish to consult The Medical Letter onDrugs and Therapeutics, Drugs for Parasitic Infections (46). Dos-ages given here are from the 2004 edition but are unchanged inthe latest edition (2007).

CONCLUSIONS

Balantidium coli is a cosmopolitan parasitic-opportunisticpathogen that can be found throughout the world. Its reservoirhost is the pig, and humans become infected through direct or

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indirect contact with pigs. In rural areas and in some develop-ing countries where pig and human fecal matter contaminatesthe water supply, there is a greater likelihood that balantidiosismay develop in humans. The infection may be subclinical inhumans, as it mostly is in pigs, or may develop as a fulminantinfection with bloody and mucus-containing diarrhea; this canlead to perforation of the colon. The disease responds to treat-ment with tetracycline or metronidazole.

Balantidiosis is a disease that need never exist given accessto clean water and a public health infrastructure that monitorsthe water supply and tracks infections. Its spread can be limitedby sanitary measures and personal hygiene, but it is a diseasethat will be around as long as there are pigs. Immunocompro-mised individuals have developed balantidiosis without anydirect contact with pigs, perhaps with rats or contaminatedproduce as a possible source of infection. For the clinician,balanatidiosis should be included in the differential diagnosisfor persistent diarrhea in travelers to or from Southeast Asia,the Western Pacific islands, rural South America, or commu-nities where close contact with domestic swine occurs.

Warming of the earth’s surface may provide a more favor-able environment, even in the now temperate areas of theworld, for survival of trophic and cystic stages of Balantidium,and its prevalence may increase. Effective sanitation and un-contaminated water are the most useful weapons against in-fection. Fortunately, balantidiosis responds to antimicrobialtherapy, and there have been no reports of resistance to thedrugs of choice.

ACKNOWLEDGMENTS

We thank Carol Glaser (Viral and Rickettsial Disease Laboratory)for her enthusiastic support of this project. We also thank BlaineMathison and the CDC-DPDx Parasite Image Library (http://www.dpd.cdc.gov/dpdx/), which was the source of illustrations used in this paper,for parasite identification. F.L.S. is grateful to Govinda S. Visvesvara(Division of Parasitic Diseases, CDC) for informative discussionsabout Balantidium.

Neither author of this paper has any conflict of interest or financialrelationship relevant to the study.

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