Onchocerciasis (River Blindness) 
Onchocerciasis is caused by the prelarval (microfilaria) and adult stages of the nematode Onchocerca volvulus. The disease is transmitted by the bite of certain species of female Simulium flies (black flies) that bite by day and are found near rapidly flowing rivers and streams.
Onchocerciasis is endemic in more than 25 nations located in a broad band across the central part of Africa. Small endemic foci are also present in the Arabian Peninsula (Yemen) and in the Americas (Brazil, Colombia, Ecuador, Guatemala, southern Mexico, and Venezuela).
Risk for Travelers
Those traveling for short periods in onchocerciasis-endemic regions appear to be at low risk for acquiring this condition. However, those who visit or live in endemic regions for >3 months and live or work near black fly habitats are at greater risk for infection. Infections tend to occur in expatriate groups such as missionaries, field scientists, and Peace Corps volunteers.
Infection with O. volvulus can result in dermatitis; subcutaneous nodules; lymphadenitis; and ocular lesions, which can progress to blindness. Symptoms may occur months to years after departure from endemic areas.
No vaccine and no effective chemoprophylaxis are available. Protective measures include avoidance of black fly habitats and the use of personal protection measures against biting insects such as those outlined in the section on Protection against Mosquitoes and Other Arthropods).
Ivermectin is the drug of choice for onchocerciasis. Repeated oral doses are required for up to several years, because the drug kills the microfilaria but not the adult worm. Travelers should be advised to consult with a specialist in infectious diseases or tropical medicine.
Content from CDC, written by— Frank O. Richards, Jr.