96 Onchocerciasis
Onchocerciasis is caused by a filarial nematode, the roundworm Onchocerca volvulus. Humans are infected by larval microfilariae transmitted by bloodfeeding female flies of the genus Simulium.
Symptoms include skin damage, extreme itching, and ocular lesions, which can lead to permanent blindness. Synonyms include river blindness in West Africa, sowda in Yemen, and enfermedad de Robles in Latin America.Distribution and Incidence
Onchocerciasis is widely distributed in Africa south of the Sahara, especially in the savanna grasslands from Senegal to Sudan. Its range extends southward into Kenya, Zaire, and Malawi. The region encompassing the headwaters of the Volta River system in northern Ghana, northeastern Ivory Coast, southern Burkina Faso (Upper Volta), and adjacent territories has been a major center for the disease. Onchocerciasis was almost certainly indigenous to Africa, but it has been transmitted by the slave trade to the Arabian Peninsula (Saudi Arabia and Yemen) and to the Caribbean basin, where scattered foci exist in Mexico, Guatemala, Colombia, Venezuela, Ecuador, and Brazil. The disease has a patchy distribution within its range; infection rates in particular villages may range from zero to virtually 100 percent.
In the 700,000 square-kilometers of the Volta Basin region alone, the World Health Organization estimated that in the early 1970s, about 1 million of the 10 million inhabitants were infected, with about 70,000 classified as “economically blind.” In northern Ghana alone, surveys in the early 1950s determined that about 30,000 people, roughly 3 percent of the population, were totally blind because of onchocerciasis. In some West African villages, adult blindness rates of from 10 to 30 percent have been observed. Conversely, dermatologic symptoms predominate in Arabia, and ocular involvement is rare.
Clinical Manifestations, Diagnosis, and Treatment
O. volvulus, one of several filarial worms that are important human parasites, lives in the cutaneous and subcutaneous tissues. Humans are the only definitive host; there is no animal reservoir. Numbers of adult worms, the females of which may reach a length of 50 centimeters, live in large coiled masses, which usually become surrounded by fibrotic tissue generated by the host. In these nodules, which may reach the size of a walnut and are often easily visible on the head, trunk, hips, or legs, the adults can live and breed for as long as 16 years. Thousands of larvae, the microfilariae, emerge from the nodules and migrate in the tissues of the skin. Host immune reactions to dead or dying microfilariae cause various forms of dermatologic destruction, including loss of elasticity, depigmentation, and thickening of the skin. These changes are complicated by the host’s reaction to the extreme pruritis caused by the allergic reactions to the worm proteins; the victim may scratch him- or herself incessantly in a vain attempt to relieve the tormenting itch. This condition is sometimes called “craw-craw” in West Africa. Wandering microfilariae can also cause damage to the lymphatic system and inguinal swellings known as “hanging groin.” Microfilariae that reach the eye cause the most damage. Larvae dying in various ocular tissues cause cumulative lesions that, over a period of one to several years, can lead to progressive loss of sight and total blindness. There appear to be distinct geographic strains, which help to explain different pathological pictures in parts of the parasite’s range; for example, in forest regions of Cameroon a smaller percentage of infected persons experience ocular complications than do inhabitants of the savanna.
Diagnosis is by detection of nodules, by microscopic demonstration of microfilariae in skin snips, and, in recent years, by a number of immunologic tests. Therapy includes surgical removal of nodules, which has been widely practiced in Latin America to combat eye damage, and various drugs to kill the wandering microfilariae.
The first drug to be widely used was diethylcarbamiazine (DEC), but in heavily infected people it may cause serious side effects when the immune system reacts to the allergens released by large numbers of dying worms. Dermatologic complications of DEC can be severe, but these are treatable with antihistamines and corticosteroids; ophthalmologic complications are less common but more dangerous. Suramin, a drug used to combat trypanosomiasis, is effective against both microfilariae and adult worms, but it has serious side effects and, like DEC, is too dangerous to use in mass campaigns. Some success has been reported with antihelminthics like mebendazole, but only ivermectin, a Inicrofilaricide introduced in the early 1980s, seems safe and effective enough for widespread use in rural areas of developing countries.Etiology and Epidemiology
The vectors and intermediate hosts of the pathogen are blood-feeding flies of the genus Simulium, especially members of the species complex Simulium damnosum. These annoying and appropriately named insects, sometimes called buffalo gnats, are close relatives of the familiar “black flies” of the northern United States and southern Canada. The females bite people, cattle, goats, wild animals, or birds to obtain blood meals. Flies feeding on infected humans may ingest microfilariae. These migrate from the insect’s stomach to the muscles of the thorax, where they undergo about a 1-week developmental process before migrating to the salivary glands. Here, as infective larvae, they await the opportunity to enter a new host when the fly feeds again. Once this happens, the larvae wander briefly in the skin before settling down in clumps to mature, breed, and produce microfilariae.
Simulium females lay eggs on rocks and vegetation in swiftly flowing, richly oxygenated water. Ripples around rocks, bridge abutments, and dam spillways provide favorable conditions for the larval development of the vector. There is no transovarial transmission, so newly emerged adults must acquire onchocerca in a blood meal.
Adult flies have extensive flight ranges: Infected females aided by winds and weather fronts can move hundreds of kilometers to establish new foci of disease. However, because of the vector’s breeding preferences, most flies and hence most onchocerciasis cases are found within a few kilometers of a stream with suitable breeding sites. The term “river blindness” accurately reflects the geographic distribution of the disease.Onchocerciasis often has dramatic effects on human activities and settlement patterns. In many heavily infested areas, notably in the headwaters of the Volta River, swarming flies, tormenting skin infestation, and progressive blindness among a significant proportion of the population have resulted in progressive abandonment of rich, well-watered farmlands near the rivers. Depopulation of river valleys due to onchocerciasis has been going on for decades in much of northern Ghana, with people being forced to cultivate crowded and eroding lands away from the streams. In many areas, land-hungry people were settling the river valleys in the early twentieth century, but, as John Hunter has shown in his classic study of Nangodi on the Red Volta, in recent decades the line of settlement has been retreating from the rivers. It is possible that a cycle of colonization and retreat, with farmers caught between malnutrition and land shortages on the one hand and the perils of onchocerciasis on the other, has been going on for centuries in parts of the Volta basin.
History
As stated above, onchocerciasis is almost certainly a disease that originated in Africa and has spread to Arabia and the New World as an unintended byproduct of the slave trade. Skin lesions caused by onchocerciasis were first described by J. O’Neill, who detected microfilariae from Africans in the Gold Coast (modern Ghana) suffering from the tormenting itch of “craw-craw.” The organism was first described in 1893 by the eminent German parasitologist K. G. Friedrich RudolfLeuckart, who studied adults in nodules extracted from Gold Coast Africans by a missionary.
In 1916 the disease was first recognized in the Americas when the Guatemalan investigator Rodolfo Robles discovered onchocerciasis in the highlands of his country. Robles linked nodules to eye disease and suggested that the distribution of infection implicated two species of Simulium as vectors. D. B. Blacklock, working in Sierra Leone, showed in 1926 that S. damnosum was the vector. In 1931 J. Hissette, working in the Belgian Congo, linked onchocerciasis with blindness for the first time in Africa, but despite confirmation in the Sudan a year later, colonial doctors generally considered onchocerciasis only a skin disease. Just before World War II, French doctors in what is now Burkina Faso began to link the disease with mass blindness and river valley abandonment. Their colleagues across the frontier in the British Gold Coast did not make a similar discovery until 1949. Although British physicians and administrators were aware of river valley depopulation, onchocerciasis, and substantial blindness in the northern part of the colony, they did not link these phenomena, partly because doctors who became interested in the problem in the 1930s were repeatedly distracted by other duties or transferred. After the war, the association was finally made, especially in a crucial 1949 report by B. B. Waddy. A series of investigations in the 1950s confirmed the widespread incidence and serious consequences of the disease in a number of African countries, and Latin American foci were delimited.Control
In 1975 the World Health Organization, supported by a number of donor and afflicted countries, began the Onchocerciasis Control Programme, an ambitious and expensive 20-year effort to eliminate onchocerciasis in the entire Volta Basin. The basic strategy was to kill fly larvae by aerial spraying of an organophosphate called Abate (generic name: temephos) over breeding sites scattered over a huge and repeatedly extended portion of West Africa. The absence of an effective agent to kill adult flies has not prevented tremendous success in reducing and sometimes eliminating Simulium populations.
Although treatment of victims was less successful until very recently because existing drugs were too dangerous for mass use, the vector control program, though costly and constantly faced with the problem of reintroduction of adult flies from places beyond the limits of the control program, has resulted in dramatic declines in biting rates, infection, and blindness in most of the region. Recently, treatment of thousands of victims with ivermectin, a safe and effective microfilaricide, has helped to reduce blindness among infected persons and has reduced the chances that a feeding fly would ingest infective microfilariae. The absence of an agent to kill adult worms and the logistical and financial difficulties of the massive Iarviciding campaign make total eradication unlikely, but vector control and Hiicrofilarici- dal treatment can reduce the number of infected persons and lessen or eliminate severe clinical symptoms. Success at this level could help make thousands of square kilometers of valuable farmland safe for use in many regions of Africa. It is, however, clear that there must be a long-term commitment to the campaign for many years into the future, or river blindness will reconquer its old haunts.K. David Patterson
Bibliography
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1981. Progress and concerns in the World Health Organization Onchocerciasis Control Program in West Africa. Social Science and Medicine 15D: 261-75.
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Patterson, K. David. 1978. River blindness in Northern Ghana, 1900-1950. In Disease in African history: An introductory survey and case studies, ed. Gerald W. Hartwig and K. David Patterson, 88—117. Durham, N.C.
Puyelo, R., and M. M. Holstein. 1950. L’Onchocercose humaine en Afrique noire franςaise: Maladie sociale. Medicine Tropicale 10: 397—510.
Waddy, B. B. 1969. Prospects for the control of onchocerciasis in Africa. Bulletin of the World Health Organization 40: 843—58.