What Is River Blindness?
River blindness is an eye and skin disease caused by a tiny worm called onchocerca volvulus, which is spread by the bite of an infected blackfly. These flies breed in fast-flowing streams and rivers, hence the name river blindness. People with severe infections will usually have dermatitis, eye lesions, and/or subcutaneous nodules. Blindness is almost always inevitable for those with severe infections.
The disease spreads from person to person by the bite of a blackfly. When a blackfly bites a person who has river blindness, microscopic worms (called microfilariae) in the infected person's skin infect the blackfly. The microfilariae develop over two weeks to a stage where they are infectious to humans. When an infectious blackfly goes on to bite another person, it will typically inject one or two larvae along with its bite, thus transmitting the disease to the person. Since the worms can only increase their numbers in humans by first mating and then cycling their microfilariae through the black fly, the intensity of human infection (number of worms in an individual) is related to the number of infectious bites sustained by an individual.
According to the World Health Organization (WHO), an estimated 17.7 million around the world are affected by river blindness. Of this number, about 270,000 are blind and another 500,000 have visual impairment. About 99% of infected persons live in Africa, with the remainder in Yemen, Mexico, Guatemala, Ecuador, Colombia, Venezuela and Brazil. Visitors to villages in have reported that all of the adult members of a village can be blind from river blindness and are led around by children who have not yet become blind.
Those at greatest risk, aside from the inhabitants of the region, are adventure travelers, missionaries, and Peace Corps volunteers who are exposed to blackfly bites in endemic areas. The disease is most prevalent in remote African rural agricultural villages, located near rapidly flowing streams.
Blackflies bite during the day. The best prevention is to avoid infective bites of the blackfly by using insecticides such as DEET, and wearing long sleeved shirts and pants. Neither a vaccine nor preventive drugs are available.
Infected persons may be without symptoms. Those with symptoms will usually experience one or more of the following: intense itching, skin rash and discoloration, eye lesions, and/or subcutaneous bumps (nodules) under the skin. The skin becomes depigmented after the lesions heal. The most serious manifestation consists of lesions in the eye that can progress to blindness. The microfilaria often can be seen swimming in the anterior chamber of the eye with a special magnifying instrument (slit lamp). When these worms reach the end of their life cycle and die in the eye, the result is an inflammatory reaction that causes uveitis (inflammation of the uveal tract), vitritis (inflammation of the vitreous), retinitis (inflammation of the retina) and ultimately blindness. The cornea is also commonly affected, resulting in severe keratitis, scarring and blindness. These inflammatory reactions can result in secondary glaucoma, which can also lead to blindness.
The normal incubation period of river blindness ranges from nine to 24 months after the bite of an infected blackfly. Each female worm can reproduce millions of microfilariae during her lifetime. Left untreated, worms can live for 10 to 15 years.
If you have been in an endemic area and think you might have contracted river blindness, it is important to see your health care provider who may order skin biopsies of the nodules ("snips") to diagnose the presence of the worms. However, examination of skin does not always show the parasites. Serologic testing for antibodies is available but a positive result does not necessarily indicate an active case of river blindness. An eye exam can usually determine the presence of microfilariae in the eye. The important message is that there is treatment available.
River blindness is commonly treated with an oral medicine called Ivermectin (Mectizan), which is administered with a dose of 150 micrograms per kilogram (maximum 12 mg) every six to 12 months. The drug should probably not be given to pregnant women or children under five years of age. Ivermectin does not kill the adult parasites, but slowly reduces the numbers of microfilariae in the skin and eye, so that there is a significant reduction in the severity of systemic and ocular reactions that persists for a relatively long time.