As Ghana commemorates 50 years of independence this year, the fight against river blindness there also marks its half century. The really bad news is that the parasite that causes the crippling disease endemic in Africa may be developing resistance to the one drug used to treat it.
River blindness is caused by a thin worm that's transmitted to humans by black flies. The worms can cause intense itching, elephantiasis of the genitals and blindness if they reach the eyes. Millions of persons are still infected with onchocerciasis (the cause of river blindness) in 30 countries in Africa. Eleven countries are located in the Onchocerciasis Control Programme Zone in West Africa (OCP) and the disease is still a major cause of blindness.
Out of some 120 million people world-wide who are at risk of onchocerciasis, 96% are in Africa. A total of 18 million people are infected with the disease of whom 99% are in Africa. Now ivermectin used to be a most effective way of tackling this disease. In the mid-1980's, it was introduced as probably the most broad-spectrum anti-parasite medication ever. It is effective against most common intestinal worms (except tapeworms), most mites, and some lice, though it is not effective against fleas, ticks, flies, or flukes.
It is effective against larval heartworms (the "microfilariae" that circulate in the blood) but not against adult heartworms (that live in the heart and pulmonary arteries). But when researchers tested 2,501 people in disease-endemic regions of Ghana from 2004 to 2005, they found 19.5 percent had the worms that cause river blindness. Ivermectin only acts to kill young worms, so any adult worms in patients would still be present, even after treatment.
In four of the 10 communities, people were shown carrying a higher number of immature worms - the opposite result expected from the drug - suggesting the parasite was developing immunity. ``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,'' wrote Dr. Peter J. Hotez, president of the Sabin Vaccine Institute, in an accompanying commentary in The Lancet. Hotez was not linked to the study.
Officials said there were currently there are 18 million cases in 36 countries worldwide, including in tropical regions of the Americas.
Every year, public health officials give out more than 20 million doses of ivermectin, used to treat river blindness since 1987. Experts estimate the drug has prevented nearly 40,000 cases of blindness a year. To date, nearly $600 million has been spent trying to eliminate river blindness from the world.
``It's not surprising that we're seeing some drug resistance,'' Hotez said. ``You can't rely on a single tool without any backup.'' The emerging problem of resistance to this drug underlines the need to find new drugs and possibly even vaccines, he said.
Experts have acknowledged that relying exclusively on ivermectin was not the best strategy. ``It's been taken for granted for years that the river blindness problem has been solved because we have ivermectin,'' Prichard said. ``But the problem has not been solved. We need another drug.''