infections
Onchocerciasis
Transmission of onchocerciasis
Onchocerciasis is spread though the bite of female blackflies that breed in swiftly flowing streams (hence, the term river blindness). The cycle of infection begins when a blackfly bites an infected person and the fly is infected with immature forms of the worm called microfilariae. The microfilariae develop into larvae in the fly. When the fly bites another person, larvae are passed into that person’s skin. The larvae move under the skin and form lumps (nodules), where they develop into adult worms in 12 to 18 months. Adult female worms may live up to 15 years in these nodules. After mating, mature female worms produce eggs, which develop into microfilariae that leave the worm. A worm may produce 1,000 microfilariae each day. Thousands of microfilariae move through the tissues of the skin and eyes and are responsible for the disease. Usually, many bites are necessary before the infection causes symptoms. Thus, the infection is much less likely to develop in visitors to affected areas. Because the infection is transmitted near rivers, many people avoid those areas. Not being able to live or to work near a river affects their ability to raise crops. Thus, onchocerciasis can contribute to food shortages in some areas. 