ScienceDaily (Sep. 5, 2007) — Ivermectin, the standard drug for treating river blindness (onchocerciasis), is causing genetic changes in the parasite that causes the disease, according to a new study by Roger Prichard (McGill University, Canada) and colleagues. These genetic changes have previously been linked with parasites becoming resistant to ivermectin.

Recent reports of patients failing to respond to ivermectin treatment have suggested the emergence of drug-resistant Onchocerca volvulus (the parasite that causes river blindness), and recent studies have associated ivermectin resistance with certain genetic markers, particularly the â-tubulin gene. In Prichard and colleagues' study, genetic changes in â-tubulin were seen in parasites obtained from patients exposed to repeated ivermectin treatment when compared with parasites obtained from the same patients before any exposure to ivermectin.

Furthermore, the researchers found that the extent of the genetic changes was dependent on the level of ivermectin treatment exposure.

Ivermectin has been used for mass treatment of river blindness for up to 18 years and is currently the only safe drug available for mass treatment of the estimated 37 million people infected with the O. volvulus parasite. According to this study, the genetic selection shown "could have implications for the development of ivermectin resistance in O. volvulus and for the ongoing onchocerciasis control programmes."

In a related commentary article, publishing the same day in PLoS Neglected Tropical Diseases, Sara Lustigman (New York Blood Center) and James McCarter (Washington University School of Medicine), who were not involved in Prichard and colleagues' study, say that the study is "a wake-up call for onchocerciasis control programs to select their treatment regimens carefully and to develop plans for detecting ivermectin resistance and the associated genetic markers."

Bourguinat C, Pion SDS, Kamgno J, Gardon J, Duke BOL, et al (2007) Genetic Selection of Low Fertile Onchocerca volvulus by Ivermectin Treatment. PLoS Negl Trop Dis 1(1): e72. doi:10.1371/journal.pntd.0000072 Lustigman S, McCarter JP (2007) Ivermectin Resistance in Onchocerca volvulus: Toward a Genetic Basis. PLoS Negl Trop Dis 1(1): e76. doi:10.1371/journal.pntd.0000076

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As an adult, this Simulium sp. larva, or black fly, is a vector of the disease, onchocerciasis, or river blindness. (Credit: World Health Organization)

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