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The development of drug resistance in the parasite that causes river blindness could lead to outbreaks in communities where it has been under control, according to research published last week (16 June) in The Lancet.

River blindness (onchocerciasis) is caused by a parasitic worm, Onchocerca volvulus, and is transmitted by black flies breeding along fast-flowing streams.

It causes blindness and skin disease in sub-Saharan Africa and some tropical regions of the Americas. Around 37 million people worldwide may be infected.

Only one drug — ivermectin — is available to control the parasite, and has been widely used since 1987. It is designed to kill the larval parasite, and prevent the adult parasite from reproducing and releasing new larvae into the skin.

Due to the drug's long-term use, a study of drug resistance levels is timely, say the authors.

Researchers from McGill University in Canada and the Onchocerciasis Control Programme of Ghana took skin samples from 2,501 people from 20 communities in Ghana to test for resistance to ivermectin.

People in 19 of the communities had received the drug annually for 6–18 years while one community had never been given ivermectin.

They found that the drug killed the larval stage of the parasite, but the adult parasites in some communities did not respond as expected to the drug. They were still able to reproduce, suggesting they have developed resistance to the drug.

Lead researcher Roger Prichard, of the Institute of Parasitology, McGill University in Canada says the research shows the fight to control river blindness is far from being successfully completed. It highlights the importance of monitoring for resistance, otherwise the disease can re-emerge, he says.

"We need better tools to monitor for resistance to [ivermectin]. This will involve research and the involvement of the pharmaceutical industry as well as international agencies such as the World Health Organization."

He said the discovery of resistance shows the problems of relying on a single drug for mass treatment for onchocerciasis.

There is a need for more and better means to control onchocerciasis, such as new pharmaceuticals for mass chemotherapy or a vaccine, Prichard says.

Policy makers in infected areas "need to encourage donor agencies, foundations and governments to urgently invest in new tools to monitor for drug resistance and for new drugs or vaccines for parasitic diseases," he added.

Link to summary in The Lancet*

Reference: The Lancet 369, 2021 (2007)

*requires free registration

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