Onchocerca volvulus is transmitted by the Simulium, the blackfly or buffalo gnat. When the female simuliid takes a blood meal the skin is stretched by its apical teach and cut by its mandibles, forming a pool of blood. When an infected individual is bitten, L1 larva in the blood are pumped into the simuliid. After ten days, the larva have developed into infective L3 larvae and are transmitted to a new person via salvia when the simuliid takes its next blood meal. The adult simuliids live about 4 weeks. In order to actually develop onchocerciasis, an individual usually needs hundreds of bites from infected flies. (6, 14, 38, 55)


Simulium Species

There are at least 15 species of simulium that transmit O.volvulus. These are some of the most important and the regions in which they are located. (14)

S damnasum African savanna (one of the most important vectors; travels long distances)
S neavei Uganda, Tanzania, Ethiopia, and the Congo (restricted range)
S albivirgulatum Congo river basin
S sirbanum African savanah
S ochraceum Central and South America
S metallicum Central and South America
S exiguum Central and South America
S guianense Amazon
S incrustatum Amazon
  Vector Habitat  
"Nearness to rivers can eat the eyes"
- African Proverb (38)

Simulium are found near rivers and streams where the rapid flow provides well oxygenated water necessary for development. This obligatory aquatic stage of development usually occurs on rocks or vegetation just below the surface and lasts about 10 days. Different species are limited to specific environments, such as savannah or forest. The adult flies of some species stay relatively localized while others can travel long distances. Deforestation in Africa is altering the ecosystem in many Simulium habitats, thus the vector distribution is changing. (38, 55)

  Agumatsa Falls, Ghana: ideal black fly breeding site (56)  
  There are zoofillic forms of Onchocerciasis, but there are no non-human reservoirs for O. volvulus. (7, 55)