The parasitic worm that causes river blindness may be developing resistance to the drug used to treat it, research has suggested.
River blindness is caused by infection with a parasitic worm
Resistance could lead to breakouts of the infection in communities where it has been brought under control, a Canadian study in the Lancet reports.
Ivermectin, used since the late 1980s, is the only drug available for the mass treatment of river blindness.
Experts warned the findings highlighted the urgent need for new treatments.
River blindness (onchocerciasis) is caused by a nematode worm which is transmitted by a black fly.
The worm burrows into the skin before releasing many hundreds of offspring which spread throughout the body.
Over decades, they cause inflammation in the eye which eventually destroys sight.
Around 37 million people worldwide are suspected to be infected.
An annual dose of ivermectin, a drug originally used in farm animals, has been widely used to treat river blindness since the late1980s.
It works by killing the larvae, preventing them from becoming adult worms.
Study leader Professor Roger Prichard said there had been recent indications that the drug was not working as well as it should.
He carried out a study of the West African Onchocerciasis Control Programme among Ghanaian communities that had received between six and 18 years of annual ivermectin treatment and other communities that had never received treatment.
The team from the Institute of Parasitology at McGill University in Montreal, Canada, found the prevalence of the worm larvae in the skin of individuals doubled between 2000 and 2005 in two communities who had been treated.
Overall, in treated communities the prevalence varied from 2.2% to 51.8%.
Although the drug clears 100% of larvae in 99% of those treated, after 90 days four out of 10 communities had significant repopulation.
Professor Prichard said the results showed ivermectin was still an effective treatment but suggested resistant adult parasite populations were emerging.
"We need new treatments and this makes it more urgent, we also need more monitoring of any resistance."
He added that isolated resistance could be controlled by using insecticides or an antibiotic called doxycycline, which is effective but needs to be given every day for a long time.
Professor Mark Taylor, professor of parisitology at the Liverpool School of Tropical Medicine said any parasite was likely to develop resistance to a drug which places it under selection pressure.
"The main problem and concern is that this resistance will manifest in an increase in transmission of the parasite.
"This would increase the proportion of resistance parasites in a community and lead to the spread of resistant parasites - including the spread of resistant parasites into populations, which are currently responding normally to the drug."
He added there could also be a resurgence in the prevalence and severity of skin disease and blindness.
"We have some unpublished data from a recent trial using doxycycline in Cameroon that shows a course kills the adult worms.
"This treatment would appear to be the most suitable and only option currently available for treating populations with evidence of resistance."
Professor Taylor's team have recently received a large grant from the Bill and Melinda Gates Foundation to search for new drugs or combinations that can have the same effect as a course of doxycycline but delivered in shorter time frame.