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Warning Sign: River blindness parasite shows resistance

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8:31pm, June 20, 2007
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The worm that causes river blindness appears to be developing resistance to the sole drug that's effective against it, a study in West Africa shows. The finding is bad news in the fight against this parasite, Onchocerca volvulus, which infects roughly 37 million people worldwide.

The river blindness parasite spreads among people bitten by certain black flies that live near fast-flowing water. The worms can survive inside a person for 15 years. The female worms produce larvae that cause itchy skin and scarring of the cornea, which leads to blindness.

Since 1987, widespread use of the drug ivermectin has stalled river blindness in many countries. The once-a-year tablet doesn't kill adult worms living in a human host, but it wipes out larvae and hampers the females' ability to reproduce.

In Burkina Faso, whole villages had moved from fertile river valleys to avoid the scourge, says parasitologist Roger K. Prichard of McGill University in Montreal. Since the introduction of ivermectin, many villagers have been able to move back, he says.

People in nearby northern Ghana have also benefited from the drug, but Prichard and his colleagues in Canada and Ghana have now found evidence of ivermectin resistance. The drug's effects are waning markedly in several Ghanaian villages that have received ivermectin treatment for 10 to 17 years, the researchers report in the June 16 Lancet.

To gauge whether ivermectin is working, the researchers obtained tiny skin samples from infected people after their annual treatment. In most of the 20 communities sampled in Ghana, these tests showed little sign of O. volvulus larvae.

In four villages, however, many skin samples contained significant numbers of larvae 90 days after ivermectin treatment, and the parasite's presence increased over the ensuing months.

Scientists had seen hints of ivermectin resistance, "but it's always been from anecdotal reports," says immunologist Eric Pearlman of Case Western Reserve University in Cleveland. "This is the first solid documentation."

Despite the evidence, there remains a chance that the parasites may not be developing resistance, says immunologist Thomas B. Nutman of the National Institute of Allergy and Infectious Diseases in Bethesda, Md. Rather, it's possible that some people don't absorb enough of the drug to fully fend off the parasite, he says.

In either case, he says, "there needs to be an additional hit, whether it's a second drug or a vaccine. This study makes the case [for] having an additional weapon."

Certain antibiotics can kill a microbe inside the worm that contributes to eye damage and keeps the worm fertile. Combining an antibiotic with ivermectin may be a useful strategy (SN: 6/17/00, p. 389). Unfortunately, the best available antibiotic is doxycycline, which patients must take daily for 4 to 6 weeks. Mass compliance with such a regimen would be poor, says Prichard.

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