By Julie Steenhuysen

CHICAGO, June 14 (Reuters) - The parasite that causes river blindness appears to be developing resistance to the only drug widely available to treat it, researchers said on Thursday.

They said their findings raise concerns of a resurgence of a disease that has been effectively controlled in many parts of Africa.

River blindness or onchocerciasis is an eye and skin disease caused by the filaria worm. It is transmitted to humans by blackflies breeding along fast-flowing tropical rivers and streams.

In humans, the adult female worm produces thousands of baby or larval worms that spread throughout the body, causing skin lesions and blindness. The disease affects 18 million people worldwide.

It has been controlled for nearly two decades through annual doses of a drug developed by Merck & Co. <MRK.N> called ivermectin, or Mectizan, which also treats the severe skin itching caused by the disease.

Researchers at McGill University in Montreal, Canada, writing in the journal The Lancet, report that the parasite is starting to resist treatment with ivermectin.

"Because it doesn't eliminate the parasite, it requires treatment at least every year, and that doesn't get rid of it. It just reduces transmission," said Roger Prichard of the Institute of Parasitology, who led the study.

"With resistance, that breaks down and we expect to see the disease becoming a problem again," he said.

Prichard and colleagues studied 2,501 people in 20 communities in Ghana, West Africa. They found infection remained common despite annual treatments.

The researchers also studied skin samples from 342 people in 10 communities who had tested positive before treatment and at 90 and 180 days after treatment.

Although the drug cleared 100 percent of the worm larvae in 99 percent of those treated, they found significant repopulation of the larvae in four out of 10 communities after 90 days.

"In some communities, we were seeing relatively rapid repopulation, which is not what you should see," Prichard said in a telephone interview.

"Just knowing there is the development of resistance is important because it gives a bit of a spur to efforts to fast-track alternative approaches," he said.

A Wyeth <WYE.N> drug for heartworm in dogs called moxidectin is in Phase II clinical trials in humans, but Prichard said the drug is in the same chemical family as ivermectin, which may hurt its chances at treating resistant parasites.

Dr. Peter Hotez of George Washington University said the development of resistance highlights the risks of having only one drug in the "toolbox."

"This finding represents a wake-up call that any parasite control program that relies on a single antimicrobial agent is always at risk of derailment," Hotez wrote in a commentary.

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