What are the mechanisms associated with post-ivermectin serious adverse events?

Update TRENDS in Parasitology Vol.22 No.6 June 2006244

database where these data can be archived and accessedby all who need them. In addition, many groups arebeginning to identify genetic markers that may signalresistance to quinine, lumefantrine, amodiaquine and theartemisinins [7–10]. As these other genes are defined, thisinformation should also be added to the common database.For newly introduced drugs, it is imperative that wearchive samples before widespread use of the drugs, evenbefore the markers are defined. If this is done, scientiststhen can examine the DNA from archived samplesretrospectively to reconstruct the genetic changes thatresulted from drug use. Before any archived samples areanalyzed, questions about permission and appropriate usecertainly will need to be addressed by institutional reviewboards. A thoughtful discussion of the appropriate safe-guards and consents required to employ this approach hasbeen published recently [11].

However, if we have only contemporary isolates, thevery important temporal information needed for predic-tions of useful drug life will be lost. To understand in depththe selective forces exerted by these drugs, we need toreconstruct the historical patterns of resistance to theolder drugs as well. Unfortunately, samples collectedearlier than the mid 1990s are not easy to find. BeforePCR, there was no reason to retain slides; they weregenerally washed and reused. We now know that DNAfrom slides with thick or thin films either stained orunstained is remarkably stable, at least for PCR amplifi-cation and analysis [12]. For the historical analysis,samples from periods before the widespread use ofantimalarials (other than quinine!) would be exceptionallyvaluable, but samples from the 1970s or 1980s would alsobe informative. Even samples with only an approximatedate or location of sampling can provide baselineinformation about allele prevalences for molecular mar-kers of relevance to drug resistance.

Casual inquiries at venerable institutions such as theLondon School of Hygiene and Tropical Medicine, theLiverpool School of Tropical Medicine and Walter ReedArmy Institute of Research have not yielded any samplesthat might be available for analysis, but there may bearchives in many other places. I urge all members of themalaria community to examine the archives of theirown institutions.

Corresponding author: Boussinesq, M. (boussinesq@ird.fr).Available online 24 April 2006

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There are many centers that are equipped to carry outthese molecular analyses, so that institutions need notlose access to their valuable samples. I would be happy towork with any groups who find slides that could beanalyzed and connect them with laboratories in theircountry or region that have the expertise necessary toanalyze the isolates. The laboratories involved in thesecollaborative arrangements can then work together todefine the molecular markers to be analyzed and the exactquestions to be addressed. If such samples can be locatedand analyzed, the whole community will benefit from theinformation gained.

References

1 Wootton, J.C. et al. (2002) Genetic diversity and chloroquine selectivesweeps in Plasmodium falciparum. Nature 418, 320–323

2 Cortese, J.F. et al. (2002) Origin and dissemination of Plasmodiumfalciparum drug-resistance mutations in South America. J. Infect.Dis. 186, 999–1006

3 Roper, C. et al. (2004) Intercontinental spread of pyrimethamine-resistant malaria. Science 305, 1124

4 Kublin, J.G. et al. (2002) Molecular markers for failure of sulfadoxine-pyrimethamine and chlorproguanil-dapsone treatment of Plasmo-dium falciparum malaria. J. Infect. Dis. 185, 380–388

5 Djimde, A. et al. (2001) A molecular marker for chloroquine-resistantfalciparum malaria. N. Engl. J. Med. 344, 257–263

6 Plowe, C. (2005) Antimalarial drug resistance in Africa: strategiesfor monitoring and deterrence. Curr. Top. Microbiol. Immunol. 295,55–79

7 Price, R.N. et al. (2004) Mefloquine resistance in Plasmodiumfalciparum and increased pfmdr1 gene copy number. Lancet 364,438–447

8 Nelson, A.L. et al. (2005) pfmdr1 genotyping and in vivo mefloquineresistance on the Thai-myanmar border. Am. J. Trop. Med. Hyg. 72,586–592

9 Sisowath, C. et al. (2005) In vivo selection of Plasmodium falciparumpfmdr1 86N coding alleles by artemether-lumefantrine (Coartem).J. Infect. Dis. 191, 1014–1017

10 Jambou, R. et al. (2005) Resistance of Plasmodium falciparum fieldisolates to in-vitro artemether and point mutations of the SERCA-typePfATPase6. Lancet 366, 1960–1963

11 Kamau, E.M. (2005) Field-derived research materials in the Africanregion: challenges and opportunities for the malaria researcher. ActaTrop. 95, 167–171

12 Kimura, M. et al. (1995) Amplification by polymerase chain reaction ofPlasmodium falciparum DNA from Giemsa-stained thin bloodsmears. Mol. Biochem. Parasitol. 70, 193–197

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doi:10.1016/j.pt.2006.04.005

What are the mechanisms associatedwith post-ivermectin serious adverse events?

Michel Boussinesq1, Joseph Kamgno2, Sebastien D. Pion3 and Jacques Gardon4

1Institut de Recherche pour le Developpement (IRD), UR 024, Departement Societes et Sante,

213 rue La Fayette, 75480 Paris Cedex 10, France2Centre Pasteur du Cameroun, BP 1274, Yaounde, Cameroon3Department of Infectious Disease Epidemiology, Imperial College, St Mary’s Campus, Norfolk Place, London, W2 1PG, UK4Institut de Recherche pour le Developpement (IRD), UR 024, CP 9214 Obrajes, La Paz, Bolivia

Update TRENDS in Parasitology Vol.22 No.6 June 2006 245

We welcome the article in which Geary presents the wayin which the development of ivermectin has revolutionizedthe animal health pharmaceutical industry and enabledmajor advances in pharmacology and molecular biology[1]. However, we would contest several of the author’sstatements about the rare but serious adverse events(SAEs) that can occur after ivermectin treatment in somepatients living in Central Africa. First, contrary to whatGeary says, these accidents are not ‘all or nothing’. Allthose who have had to manage such cases know well thatthere is a continuum between what we have empiricallydefined as ‘serious non-neurological’, and ‘serious neuro-logical’ adverse reactions [2]. For example, in some cases,such as patients who present unable to move without help,and with mutism or mild confusion, but without objectiveneurological signs, it is difficult to decide to which categorythe condition corresponds.

Geary expresses some doubts about the role of thefilarial nematode Loa loa in the occurrence of these SAEs.Apart from the fact that all patients who developed aneurological SAE had considerable Loa microfilaraemias,a detailed analysis of results collected from thousands ofindividuals enabled us to demonstrate the clear and closecorrelation between the pre-treatment Loa microfilarialdensity and the risk of developing an SAE [3]. A modeldescribing this relationship has even been proposed [4].Not all patients with very high Loa microfilaraemiasdevelop an SAE, and we acknowledge that other co-factorscould have a role in the occurrence of SAE – indeed, we aretrying to identify them. However, it is very clear that theLoa microfilarial load is a major factor associated withthe post-ivermectin SAEs. It was also the case for theencephalopathies described after treatment with diethyl-carbamazine (DEC), and it is interesting to note that,although the modes of action of ivermectin and DEC aredifferent, the risk threshold was similar for both drugs:30 000–50 000 Loa microfilariae per millilitre of blood [5].

We do not think that the fact that these events are veryrare constitutes an argument in favour of an idiosyncraticphenomenon. The distribution ofLoamicrofilaraemia is, asis usual for helminths, very overdispersed – that is, aminority of hosts harbours the majority of the parasites [6].As a consequence, the proportion of patients who harbourhigh microfilaraemia levels, and who thus are at risk ofdeveloping a neurological SAE, is fairly low: when theprevalence of Loa microfilaraemia in adults reaches theexceptional value of 40%, the proportion of individualsharbouring more than 30 000 microfilariae per ml is about5% [6]. Once the role of high Loa microfilaraemias in thedevelopment of SAEs is accepted, one could easily raisehypotheses about the mechanisms that would lead to theobserved clinical conditions. From what is known about theeffects of ivermectin on other parasites [7], one might thinkthat treatment paralyses theLoa microfilariae. In patientsharbouring high microfilarial loads, the passive drainage ofthe parasites in the blood stream would lead to a massiveand diffuse embolisation of microfilariae in the brain micro-circulation. The appearance of the haemorrhages and otherlesions found in the retina of patients who develop an SAEstrongly supports this hypothesis of an obstructive process[8]. Pathological examinations of brain tissue collected

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from monkeys experimentally infected with Loa loa andthen treated with ivermectin confirmed the accumulationof Loa microfilariae in the brain vessels [9].

Geary [1] suggests that the post-ivermectin neurologicalSAEs are related, ‘at least in part’, to human geneticfactors, that is, to the fact that some individuals have amutation in the MDR1 gene. According to this hypothesis,the SAEs reported in humans would be similar to theaccidents seen in some dog breeds, in which this mutationpermits the passage of ivermectin into the brain, leading totoxic manifestations. We feel that this hypothesis isunlikely for two main reasons. First, tens of millions ofivermectin treatments have been distributed to controlonchocerciasis and lymphatic filariasis in areas that aremostly non-endemic for Loa infection (loiasis); however, allcases of neurological SAE have been reported from areaswhere loiasis is endemic (its distribution is now fairly wellknown [10,11]). Thus, Geary’s hypothesis [1] implies thatthe mutation in the MDR1 gene would be present only inthose areas where loiasis is endemic, a link that is difficultto conceive. Second, the clinical condition observed inpatients who develop an SAE does not support thehypothesis of a toxic origin. The several reports of over-dosage of ivermectin and other avermectins in humans (i.e.a situation in which the drug can pass into the brain byovercoming the P-glycoprotein pump), and the obser-vations made in animals, show that the main signs andsymptoms of ivermectin toxicity are mydriasis, apparentblindness, tremor, hypersalivation, vomiting and hypoten-sion [12,13]. We have never seen such signs in the tens ofpatients that we had to examine for a post-ivermectin SAE,and thus we believe that there is no toxic component.However, it would be easy to clarify this point byinvestigating the genetic profile of the patients who surviveafter a post-ivermectin neurological SAE.

In conclusion, we agree that the exact mechanismsassociated with the development of the Loa-related post-ivermectin accidents should be clarified, to determine themost appropriate treatment to be administered to thepatients. Possible co-factors should also be identified.However, there is clear evidence supporting the role of Loaload as the main risk factor for post-treatment SAEs. To raisedoubts about its role is therefore unfounded and could evenhave negative consequences for the African Programme forOnchocerciasis Control (APOC). In particular, it might bringabout a relaxation of the research being conducted to preventthese SAEs and to improve their prognosis, and of the effortswhich are made to limit their impact on the participation ofthe onchocerciasis-endemic populations in the large-scaleivermectin treatments.

References

1 Geary, T.G. (2005) Ivermectin 20 years on: maturation of a wonderdrug. Trends Parasitol. 21, 530–532

2 Boussinesq, M. et al. (1998) Three probable cases of Loa loaencephalopathy following ivermectin treatment for onchocerciasis.Am. J. Trop. Med. Hyg. 58, 461–469

3 Gardon, J. et al. (1997) Serious reactions after mass treatment ofonchocerciasis with ivermectin in an area endemic for Loa loainfection. Lancet 350, 18–22

4 Boussinesq, M. and Gardon, J. (1998) Challenges for the future:loiasis. Ann. Trop. Med. Parasitol. 92(Suppl. 1), S147–S151

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5 Fain, A. (1978) Les problemes actuels de la loase. Bull. WHO 56,155–167

6 Pion, S.D.S. et al. Microfilarial distribution of Loa loa in the humanhost: population dynamics and epidemiological implications. Para-

sitology (in press)7 Vuong, P.N. et al. (1992) Ivermectin in human onchocerciasis: a

clinical-pathological study of skin lesions before and three days after

treatment. Ann. Parasitol. Hum. Comp. 67, 194–1968 Fobi, G. et al. (2000) Ocular findings after ivermectin treatment of

patients with high Loa loa microfilaremia. Ophthalmic Epidemiol. 7,27–39

9 Kamgno, J. et al. (2005) Lutte contre l’onchocercose en Afrique: lesnouveaux defis. Bull. Soc. Pathol. Exot. 98, 143

Table 1. Seropositivity rates in Europe, the Americas and Southea

Continents and countries Year

Western EuropeAustria 1998Belgium 1997France 2001Germany 2004Italy 2001

The Netherlands 2004Spain 2004Switzerland 1995UK 1992ScandinaviaDenmark 1999Finland 1995Norway 1998

Sweden 2001Central and Eastern EuropeCroatia 2000Poland 2001Slovenia 2002Yugoslavia 1998The AmericasUSA 2004

Central AmericaCosta Rica 1996Cuba 1993Mexico 2001Panama 1988South AmericaArgentina 2001

Brazil 2001West Indies 1991Southeast AsiaIndonesia 2000Malaysia 2004Thailand 1992, 1997, 2000, 2001

aMale:female ratioZ63:52.

DOI of original article: 10.1016/j.pt.2006.01.007Available online 27 April 2006

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10 Boussinesq, M. and Gardon, J. (1997) Prevalences of Loa loamicrofilaraemia throughout the area endemic for the infection. Ann.Trop. Med. Parasitol. 91, 573–589

11 Thomson, M.C. et al. (2004) Mapping the distribution of Loa loa inCameroon in support of the African Programme for OnchocerciasisControl. Filaria J. 3, 7

12 Lovell, R.A. (1990) Ivermectin and piperazine toxicoses in dogs andcats. Vet. Clin. North Am. Small Anim. Pract. 20, 453–468

13 Chung, K. et al. (1999) Agricultural avermectins: an uncommon butpotentially fatal cause of pesticide poisoning. Ann. Emerg. Med. 34,51–57

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doi:10.1016/j.pt.2006.04.006

Erratum

Erratum: Toxoplasmosis: beyond animals to humans[Trends Parasitol. 22 (2006) 137–142]

In the article ‘Toxoplasmosis: beyond animals to humans’by Yaowalark Sukthana, which was published in theMarch 2006 issue of Trends in Parasitology, there wereerrors in Table 1. Data from Yugoslavia were displayed inthe UK entry and vice versa. The correct version of thetable is shown here.

Because of these changes, Ref. [58] in the originalarticle should have been cited as Ref. [52] and,

consequently, Refs [52–57] in the original article shouldhave been cited as Refs [53–58] (see corrected Table 1).The author apologizes to readers for any confusion causedby these errors.

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doi:10.1016/j.pt.2006.03.009

st Asia

Seropositivity (%) Refs

43 [46]50 [47]Up to 75 [39]26–54 [48]18–60 [39]

40.5 [49]28.6 [50]46 [51]23–33 [52]

27.8 [53]20.3 [54]10.9 [55]

14.0–29.4 [22]

38.1 [56]46.4–58.5 [57]34 [58]57–93 [59]

16–40 [60]

76 [61]60 [62]35 [39]90 (at 60 years of age) [63]

72 [39]

59 [39]29.7 [64]

58a [65]44.8 [66]2.3–21.9 [67–70]