Onchocerciasis Health Article

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

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World Health Organization (2000). Onchocerciasis (River Blindness). Fact Sheet No. 95. Geneva: Author.