The disease is transmitted by the bite of certain species of female Simulium flies (also known as blackflies, or buffalo gnats). Females require a blood meal for ovulation. This is the point at which the fly either receives the parasite or transmits it.
See the section on Life Cycle for further details on Transmission and how it relates to the life cycle of O. volvulus.
There are several species of the Simulium flies that are capable of transmitting onchocerciasis. These vectors occur in 28 sub-Saharan African countries, Yemen, Saudi Arabia, and six countries in Central and South America (Mexico, Guatemala, Brazil, Venezuela, Ecuador, Colombia). Different species of Simulium flies are found in different regions:
S. sirbanum, S. damnosum and S. neavei: African savanna
S. neavei: African forest regions
S. yahense and S. squamosum: rainforest regions
S. leonense: lowlands of Sierra Leone
S. anctipauli: large coastal rivers of West Africa
S. ochraceum: densely forested areas of Latin America
S. metallicum and S. exiguum: hilly, sparsely wooded areas of Latin America.
See the section on Global Distribution of Clinical Manifestations for more details on the various symptoms O. volvulus causes depending on which Simulium species it is associated with.
Simulium flies breed around fast-flowing streams and rivers, as they prefer highly oxygenated areas. This is because their larvae have an obligatory aquatic stage, which requires high oxygen tension. In the aquatic stage, the larvae and pupae attach to submerged rocks or vegetation, certain African species attache themselves to fresh-water crabs. Adult forms of the fly emerge after 8-12 days.
The Simulium flies live for 4 weeks, and can travel several hundred kilometers to reach areas that are suitable for mating and reproduction. This allows the disease to spread prolifically. However, once they find their breeding site, blackflies tend to stay within 2 km of the site.
Female blackflies require a blood meal for ovulation. The female flies bite by day. If the bitten person is infected with onchocerciasis, microfilariae in the skin are ingested along with blood, penetrate the midgut of the fly, and move into its flight muscles, where they mature into third stage infective larvae.
Blackly on human skin
Courtesy of Liz Webb (Leeds University)
Courtesy of Martin Eichner (Eberhard-Karls-University of Tubingen, Dept. of Medical Biometry)
O. volvulus relies on the blackfly for the key stage of larval development, a central part of the parasite’s life cycle. Simulium flies are therefore obligate intermediate hosts.
Humans are the only definitive host for O. Volvulus
(Note: A definitive host is one in which the parasites have an adult stage and carry out sexual reproduction)
None – Although there are zoophilic forms of onchocerciasis, there are no known reservoirs for the strains of O. volvulus that affect humans
Onchocerciasis ia a chronic infection with clinical manifestations that develop months or years after infection. The initial bite of the blackfly tends to go unnoticed. After a latent period of about 1-2 years after exposure, clinical symptoms start to appear, as the infective larvae have developed into adult worms .