Onchocerciasis

End of river blindness in sight?

In the areas of Africa and Latin America nearest the equator, blackflies that breed in river water carry a parasite called Onchocerca volvulus from human to human. An estimated 18 million people suffer from the disease onchocerciasis, and the parasite threatens another 85 million worldwide, according to world health officials. When O. volvulus invades its human host, the results can be devastating. The adult worms, which can reach 2 feet in length, travel under the skin and cause extremely itchy lumps and thick scars. But it is their millions of tiny offspring, called microfilariae, that cause the eye lesions leading to permanent blindness--giving onchocerciasis its more common name of "river blindness.'

The World Health Organization estimates that more than 336,000 people worldwide, and up to 15 percent of the population in heavily infested areas, are blind as a result of the parasite. Local and international agencies spray pesticides in river areas to reduce the number of parasite-carrying blackflies, but the disease persists.

Scientists are looking for ways to stop the spread of river blindness, which has been targeted for global eradication by various health organizations. Adult worms can live in a human for 15 years, shedding millions of microfilariae. So the parasite itself has become the focus of recent research.

The most promising medical tool appears to be ivermectin, a drug already used against other parasites in domestic animals. Subsequent trials in humans with onchocerciasis have shown that ivermectin also can reduce microfilarial counts to under 10 percent of pretreatment levels within days, an effect that lasts for up to a year. It apparently acts by inhibiting parasite reproduction and paralyzing the microfilariae. Developed at Merck Sharp and Dohme Research Laboratories in Rahway, N.J., ivermectin can be given orally, and one or two doses yearly appear to be sufficient. The two drugs presently used as treatment for onchocerciasis are more toxic, and must be administered more frequently.

Last week, Merck officials announced plans, in cooperation with the World Health Organization, to donate ivermectin to interested countries. The announcement follows the approval of ivermectin's use in humans by the Directorate of Pharmacy and Drugs, the French equivalent of the U.S. Food and Drug Administration. Once in place, the program will continue "for the foreseeable future,' a Merck spokesman told SCIENCE NEWS. Company and health officials are hoping the program eventually will eradicate river blindness by reducing the number of microfilariae available for transmission by blackflies to humans, the parasite's only known reservoir.

Despite the promising aspects of ivermectin, it does not eliminate the adult worms from the body, nor does it prevent infection. Also, its use in pregnant women is not recommended. An alternative may be antiparasite vaccines, although developing a vaccine against something accepted so readily by the body is a difficult endeavor. Among those looking for a river blindness vaccine is Alan L. Scott of Johns Hopkins Medical Institutions in Baltimore. By using proteins from the surface of O. volvulus, Scott hopes to find a mixture of antigens that, when injected, cause the production of protective antibodies against the parasite. He told SCIENCE NEWS that he is trying to clone antigens that look promising. Scott and his co-workers plan to isolate the O. volvulus gene that codes for whatever protein elicits antibody production in the host, and then use recombinant DNA technology to make large quantities of the protein. Early vaccines, says Scott, would most likely not be preventive, but rather therapeutic--reducing parasite numbers and the incidence of blindness. A more immediate by-product of his research, says Scott, could be an improved early diagnostic test for river blindness.

COPYRIGHT 1987 Science Service, Inc.
COPYRIGHT 2004 Gale Group