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 kilo meters 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.

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