Onchocerciasis (river blindness) occurs in Africa south of the Sahara and in Central America. It is due to Onchocerca volvulus transmitted by the bite of black flies Simuliidae which breed by fast flowing rivers. The inoculation of microfilariae by the bite of a black fly causes intense local inflammation and is followed by an incubation period of many months. The adult worms live in nodules around the hips and cause no harm in themselves. They produce thousands of microfilaria each day which travel to the skin and eyes. In the skin they produce a very itchy rash which looks like lichenified eczema. On the lower legs there is often spotty depigmentation. Involvement of the eyes causes blindness.
Risk factors for being infected
• Living, working, or playing near fast flowing rivers.
• Not wearing enough clothes so that the skin is exposed to insect bites.
• The construction of dams leads to less breeding of black flies in the dam itself but increased breeding in the dam spillways.
• Demonstrate the microfilaria in the skin by skin snips.
• Remove a skin nodule and see the adult worms inside it.
• Polymerase chain reaction to show parasite DNA—not much use in the field.
• Spray the breeding areas with insecticides.
• Annual dose of ivermectin 400 micrograms/kg body weight for four to six years. This stops the release of microfilaria from the adult worms.
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