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Ivermectin Resistance in Onchocerca volvulus: Towarda Genetic Basis 1 Laboratory of Molecular Parasitology, Lindsley F. Kimball Research Institute, New York Blood Center, New York, New York, United States of America, 2 Divergence Inc., St.
Louis, Missouri, United States of America, 3 Department of Genetics, Washington University School of Medicine, St. Louis, Missouri, United States of America parasite transmission, possibly with IVM-resistant O. volvulusleading to disease recrudescence. Additionally, studies have Onchocerciasis (river blindness) is a human disease caused by the filarial worm Onchocerca volvulus. Adult worms can live for over particularly the b-tubulin gene in human O. volvulus and the a decade in skin nodules of affected humans, releasing millions of livestock nematode parasite Haemonchus contortus [22,26]. However, microfilariae that cause debilitating itching and blindness [1]. An previous O. volvulus genotyping studies were non-longitudinal, estimated 37 million people are infected [2], and there are 46,000 using worms collected from different IVM-naı¨ve and treated new cases of blindness annually (
International programs supported by the World Health Organization and many other groups have worked to control A New Study: IVM Causes Genetic Selection on the impact of onchocerciasis using vector control with insecticides beginning in 1974 and mass drug administration (MDA) withivermectin (IVM, brand name Mectizan) beginning in 1987 A new study by Catherine Bourguinat and colleagues published (Figure 1) [3]. IVM is a highly effective microfilaricide and inhibits in PLoS Neglected Tropical Diseases extends these previous reports and female worm microfilarial production for several months. Annual concludes not only that IVM causes genetic selection on O. volvulus IVM MDA reduces morbidity [4,5] and lowers transmission [6,7].
worms, but that this selection is also associated with a lower From 1974 to 2002, the Onchocerciasis Control Programme reproductive rate of the female parasites [27]. In this study of O.
(OCP) in West Africa greatly decreased O. volvulus transmission in volvulus treatment in a hyperendemic region of central Cameroon, the 11 OCP countries and prevented 600,000 cases of blindness parasite genotypes (b-tubulin gene and two controls) and pheno- [8–10]. IVM without vector control has been the principal tool for types (female fertility) were characterized in worms collected from the Onchocerciasis Elimination Program of the Americas (1992– the same individuals before and after four or 13 IVM treatments present) [9] and the African Programme for Onchocerciasis over three years. Parasites were collected pre- and post-treatment Control (1995–present). In the Americas, where O. volvulus is less from clinical trial patients in four IVM treatment groups: 150 mg/ common, the Onchocerciasis Elimination Program has substan- kg of body weight annually or three-monthly, and 800 mg/kg tially reduced transmission and is on track to eliminate the disease Analyses of the genetic polymorphism in parasites pre- and post- The African Programme for Onchocerciasis Control has treatment clearly showed a significant selection for b-tubulin extended treatment to 19 countries beyond those originally heterozygotes in female worms. The most marked effect was in included in the OCP through sustainable community-directed the three-monthly treated groups, where the frequency of the b- IVM treatment [1,11]. By the end of 2005, 400 million treatments tubulin ‘‘aa’’ homozygotes post-IVM was reduced on average from had been supplied in Africa by Merck’s Mectizan Donation 68.6% to 25.6%, while the ‘‘ab’’ heterozygotes increased from Program, with an estimated 40 million people treated by nearly 20.9% to 69.2% over three years. Moreover, b-tubulin ‘‘aa’’ 300,000 community distributors ( Never- homozygous females were significantly more fertile than hetero- theless, the ecology of the disease in Africa, including the broad zygotes before treatment (67% versus 37%) and 12 months geographic range of O. volvulus and its blackfly vector, leads to theestimation that IVM treatment of at least 65% of the population after the last IVM dose in the groups treated annually (60% for 25 or more years will be necessary to eliminate infection [9,12].
versus 17%). No significant selection was observed in the control There are significant logistical obstacles to achieving such broad-ranging and prolonged treatment, and there is also concernthat O. volvulus resistance to IVM will emerge. IVM resistancehas become widespread in many parasitic nematodes of Citation: Lustigman S, McCarter JP (2007) Ivermectin Resistance in Onchocerca livestock [13,14]. At present there are no alternative drugs for volvulus: Toward a Genetic Basis. PLoS Negl Trop Dis 1(1): e76. doi:10.1371/journal.
pntd.0000076 IVM for use in the Onchocerca MDA programs that reducemicrofilariae or kill adult worms, which can live up to 15 years in Received July 17, 2007; Accepted August 1, 2007; Published August 30, 2007 Copyright: ß 2007 Lustigman, McCarter. This is an open-access article The emergence of drug-resistant O. volvulus has been suggested distributed under the terms of the Creative Commons Attribution License, whichpermits unrestricted use, distribution, and reproduction in any medium, provided by reports of patients failing to respond to IVM treatment [15,16].
the original author and source are credited.
A recent report from Ghana has provided the first proof of IVM Funding: The authors received no specific funding for this study.
resistance in O. volvulus: Mike Osei-Atweneboana and colleagues Competing Interests: SL has received research grants for the development of showed that the ability of IVM to suppress skin microfilariae anti-Onchocerca vaccines and macrofilaricides. JPM is a share-holder and repopulation was reduced in some communities that had received employee of Divergence, Inc., a company that works on parasite control in 6–18 years of IVM MDA [17]. The authors predict that a high rate of repopulation of skin with microfilariae will allow continued PLoS Neglected Tropical Diseases | Figure 1. Distribution of Onchocerciasis Showing Current Status of Global Onchocerciasis Control. Red shading represents areasreceiving ivermectin treatment. Yellow shading represents areas requiring further epidemiological surveys. Green shading indicates the area coveredby the OCP in West Africa. Pink zones indicate the special intervention zones, i.e., previous OCP areas receiving ivermectin and some vector control.
Figure from [10].
doi:10.1371/journal.pntd.0000076.g001 important to examine for polymorphisms in these genes in theuniquely collected O. volvulus female worms described in this study A major strength of this study is that the O. volvulus parasites were collected from the same individuals before and after IVM Despite these caveats, the study indicates that IVM causes treatments. Therefore the observed changes in genotype frequen- genetic selection on O. volvulus worms and points to the daunting cies between IVM-naı¨ve and treated O. volvulus populations are not possibility of the spread of IVM-resistant parasites in endemic due to factors such as geographical or sampling effects.
regions that have been treated with IVM.
The main limitation of this study was that some worm samples could not be genotyped, thus reducing the number that could be Implications of the Study for River Blindness analyzed, particularly after treatment. This limitation might also have impeded the genotyping of DNA, ideally prepared fromworm sections instead of just whole females. What is given as The finding that IVM treatment selected for b-tubulin hetero- a single genotype is, in fact, a consensus of multiple genotypes zygotes and that this selection was dependent on dosage raises including the adult female body and progeny (uterine embryos and important concerns for the current river blindness control microfilariae). Furthermore, the samples from unfertile females, programs. These concerns are heightened by the fact that this which probably represent true singletons, were treated the same as gene has been linked with IVM resistance in another parasitic those from females classified as being of low or high fertility.
nematode [26], and by the recent evidence that IVM resistance is Consequently, the study leaves unanswered questions including occurring in O. volvulus [17]. Semiannual or more frequent whether the selection of the b-tubulin heterozygote genotypes is on treatments are ongoing in some endemic areas and are under the females or their progeny. If it is on the progeny, questions consideration in other areas. Such treatment might increase the remain regarding the fitness and susceptibility to IVM treatment selection pressure. Therefore, Bourguinat and colleagues’ study is of the b-tubulin heterozygote microfilariae once they develop into a wake-up call for control programs to select their treatment the infective stage larvae and enter a new human host.
regimens carefully and to develop plans for detecting IVM The authors do not present a hypothesis to explain why IVM resistance and the associated genetic markers (control programs causes selection for b-tubulin heterozygote genotypes. The will require additional funding for these plans). This study presents glutamate-gated chloride channels are thought to be involved in a possible structure of study design that will incorporate the the mode of action of IVM and resistance to the drug [28].
detection and validation of the genetic markers associated with Treatment with IVM is known to cause a loss of polymorphism not only at certain b-tubulin gene loci, but also at certain loci of the Simultaneously, we need to greatly increase our current level of genes encoding the gamma-aminobutyric acid receptor, gluta- effort and support to develop and test a new generation of control mate-gated chloride channel, and ATP-binding cassette trans- tools for onchocerciasis. These tools should include both vaccines porter of IVM-resistant H. contortus [23]. It will therefore be and macrofilaricides (drugs which kill adult worms) that have new PLoS Neglected Tropical Diseases | classes of chemistry with novel modes of action. Recent break- McConnell Clark Foundation [31]. It may be possible to link such throughs now make macrofilaricide development more feasible, a vaccine with drug treatments in a program of vaccine-linked and accordingly such development is now a high-priority goal with chemotherapy [32,33]. These new generations of control tools the World Health Organization’s Special Programme for Re- would complement the present control measure—the establish- search and Training in Tropical Diseases and the Bill and Melinda ment of sustainable community-directed treatment with IVM— Gates Foundation [29,30]. The development of an anti-Onchocerca and ultimately support the long-term goal of eliminating vaccine has been the focus of research supported by the Edna onchocerciasis as a public health problem in Sub-Saharan Africa.
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