Onchocerciasis
ONCHOCERCIASIS
Onchocerciasis, or river blindness, is a filarial infection caused by the parasitic nematode, Onchocerca volvulus, carried by black flies of the genus Simulium. It is estimated that 17.7 million people are infected with O. volvulus, with 95 percent of those infected found in Africa. The infection is also found in areas of Central and South America and in the Arabian peninsula. Onchocerciasis is a significant cause of blindness in endemic countries. Of those people currently infected, approximately 270,000 are blind and a further 500,000 have severe visual impairment.
Simulium spp. are biting flies that are active during the day, breeding in fast flowing, well-oxygenated waters. Infection prevalence is highest in areas adjacent to such rivers, hence the term "river blindness." Transmission occurs when an infected fly takes a blood meal from a human and injects O. volvulus larvae (called microfilariae) into the host. The microfilariae develop into adult worms over the next one to two years. When mature, the adult worms produce new microfilariae that migrate throughout the skin of the host. A female adult worm may live for as long as ten years and will produce 1,300 to 1,900 microfilariae per day. These new microfilariae can be taken up by a biting fly, and thus the cycle of infection is completed.
Adult worms seldom cause any symptoms except as subcutaneous nodules around bony prominences. As microfilariae migrate throughout the body the following symptoms occur: severe itching, rashes, depigmentation of the skin, especially of the lower limbs ("leopard skin"); and destruction of skin elasticity, resulting in loose, hanging folds ("hanging groin"). The most devastating effect of onchocerciasis results when micrifilariae enter the eye, leading to visual impairment and eventually to blindness. These clinical manifestations are predominantly due to the body's inflammatory response to dead or dying microfilariae and vary depending on the number of worms.
Diagnosis is made by microscopic examination of skin snips for microfilariae. Adult worms may be found in subcutaneous nodules. There are several immunodiagnostic tests that have been developed for antigen detection with varying sensitivities and specificities. A small dose of diethylcarbamazine (DEC) when given to a patient (the "Mazzotti test") results in death of microfilariae and intense itching. This reaction is an indirect, sensitive method for diagnosing very light infections.
Treatment is with a single dose of ivermectin. As this drug kills only microfilariae, and not adult worms, treatment must be repeated every six to twelve months until the adult worms die of old age. Although DEC is also an effective treatment, it tends to induce more severe side effects than does ivermectin. Visible nodules can be removed to decrease the number of adult worms.
In 1974, the World Health Organization established the Onchocerciasis Control Program (OCP) to try to eliminate the Simulium fly by spraying the river systems in West Africa. With the discovery of ivermectin, a second strategy was added to the OCP. Populations in affected regions are given an annual dose of ivermectin to decrease the effects of the infection and to reduce further spreading by eliminating microfilariae. It is estimated that since the initiation of the program, 300,000 cases of blindness have been prevented. However, in large areas of Africa onchocerciasis continues to cause significant morbidity and mortality.
Martha Fulford
Jay Keystone
(see also: Vector-Borne Diseases )
Bibliography
Burnham, G. (1998). "Onchocerciasis." The Lancet 351:1341–1346.
Hall, L. R., and Pearlman, E. (1999). "Pathogenesis of Onchocercal Keratitis (River Blindness)." Clinical Microbiology Reviews 12(3):445–453.
Molyneux, D. H., and Morel, C. (1998). "Onchocerciasis and Chagas' Disease Control: The Evolution of Control via Applied Research through Changing Development Scenarios." British Medical Bulletin 54(2):327–339.
Ottesen, E. A. (1993). "Filarial Infections." Infectious Disease Clinics of North America 7(3):619–633.
World Health Organization (2000). Onchocerciasis (River Blindness). Fact Sheet No. 95. Geneva: Author.
River Blindness
River blindness
River blindness is a disease responsible for a high incidence of partial or total blindness in parts of tropical Africa and Central America. Also called onchocerciasis, the disease is caused by infection with Onchocerca volvulus,a thread-shaped round worm (a nematode), which is transmitted between people by the biting blackfly Simulium. The larvae of Onchocerca develop into the infective stage, called L4, inside the blackfly and are introduced into humans by the bite of an infected blackfly.
Adult Onchocerca (2.5 ft or 0.76 m in length) develop in the connective tissue under the skin of humans. The adult worms lie coiled within subcutaneous nodules several inches in diameter. The nodules are painless and cause little damage, but can be cosmetically unattractive; fortunately, they are easily removed by simple surgery. The more serious health problems associated with onchocerciasis are caused by the release of masses of early-stage larvae, known as microfilaria, into the host's connective tissue under the skin. The mobile larvae spread throughout the body, including the surface tissues of the eyes. It is the burrowing activities of these larvae that cause the symptoms associated with onchocerciasis—either severe dermatitis or blindness. Onchocerciasis can now be treated with drugs such as ivermectin, which kill the larvae, but the blindness is usually permanent.
Avoiding blackfly bites with protective clothing and skin repellents is not a practical control measure on a large scale. The major method of prevention of onchocerciasis in humans is the control of the blackfly intermediate hosts and vectors. The larvae and pupae of blackfly are strictly aquatic and are found only in fast-running water. In Kenya, the larval stages of S. neavei have been killed by releasing DDT into streams where the blackflies breed and evolve. However, the waterways must be treated frequently to prevent the reestablishment of blackfly populations, and there have been serious questions raised over the safety of DDT. In West Africa, S. damnosum is more difficult to control, since the adults can fly over considerable distances, and can easily reinfect cleared sites from up to 60 mi (97 km) away.
Onchocerciasis has long been socially debilitating in an 11-country area of West Africa where both the flies and parasites are abundant. Here, more than one-fifth of all males over the age of 30 may be blind, turning productive people into long-term dependents. The presence of the disease often results in the migration of people away from rivers to higher ground, which they then clear for cultivation. Clearing vegetation in Africa frequently results in soil erosion and the formation of gullies that channel water during heavy rains. The moving water allows blackflies to breed, spreading the disease to the new area. The reappearance of river blindness results in still further human migration, until large areas of badly eroded land are left unpopulated and unproductive, and entire villages are abandoned. The disease has now been brought largely under control by a World Health Organization program begun in 1974, in which 63 million acres (25 million ha) of land have been made safe for resettlement.
[Neil Cumberlidge ]
RESOURCES
BOOKS
Bullock, W. A. People, Parasites, and Pestilence. An introduction to the Natural History of Infectious Disease. Minnesota: Burgess Publishing Company, 1982.
Crosskey, R. W. The Natural History of Blackflies. New York: John Wiley and Sons, 1990.
Markell, E. K., M. Voge, and D. T. John. Medical Parasitology. 7th ed. Philadelphia: Saunders, 1992.
Rodger, F. C., ed. Onchocerciasis in Zaire: a New Approach to the Problem of River Blindness. New York: Pergamon Press, 1977.