File: <onchocerciasis.htm> <Medical Index> <General Index> Site Description Glossary <Navigate to Home>

ONCHOCERCIASIS

(River Blindness)

(Contact)

Please CLICK on image & underlined links for details:

This disease, often called "River Blindness" is an infection by species of the filarial genus Onchocerca and of a more limited distribution Mansonella. Onchocerca volvulus Leuck. produces tumors below the skin that vary from small nodules to large swellings (Matheson 1950). Sometimes infection produces no swellings. The parasite was originally found on the African Gold Coast in 1893, but it is now known to be widely spread in central Africa and tropical portions of America. Service (2008) estimated that over 17 million people are infected and among whom around 770-thousand suffer partial or total blindness. Black flies of the family Simuliidae are the vectors, and only humans serve as reservoir hosts. The activities of rasping and tearing skin make blackflies ideal vectors of the disease. The flies ingest microfilariae of O. volvulus when they bite. Then the microfilariae penetrate the insect's stomach wall and move to the thoracic muscles, where they develop into the larval stage and begin to molt. Third-stage worms eventually enter the proboscis from which they can penetrate the host when the insect feeds.

In Africa the Simulium damnosum complex are the most important vectors of onchocerciasis. Adult flies of this complex are black and are recognized by broad and flat front tarsi with a dorsal crest of tiny hairs, and by a broad white area on the first segment of the hind tarsus (Service 2008). Important vector species are Simulium damnosum, S. sirbanum, S. sanctipauli and S. leonense while S. neavei, S. .

In America Simulium ochraceum (shown as similar S. ochraceus) vectors the disease in Mexico, Central and South America and S. metallicum in Mexico and Central America to parts of northern South America. Service (2008) noted that Simulium exiguum is the only vector in Colombia and it is also quite active in Ecuador. In Brazil Simulium guianense and Simulium oyapockense are vectors. In South America Mansonella ozzardi is also a filarial parasite that may be regarded as non pathogenic (Service 2008). However there are reports of it causing morbidity in Colombia and Brazil. The disease is vectored by Culicoides species (e.g., C. furens and C. phlebotomus) in the Caribbean area, Trinidad and Surinam and to a lesser extent in Argentina. Simulium amazonicum vectors M. ozzardi in various parts of Panama and South America.

Control involves using repellants and applying insecticides to black fly breeding areas. The World Health Organization and local health agencies are active in implementing other control measures in Africa especially. This includes the widespread governmental use of insecticides, which ultimately will become less effective as pesticide resistance develops. Therefore, education in avoidance through use of repellants should reduce the number of people infected and the spread of disease.

Control of Vectors & Disease

Repellents are the common means for reducing annoyance caused by blackfly vectors. Although Service (2008) suggested that the application of insecticides to developmental sites of blackfly larvae is the only practical control method, the development of insecticide resistance is rapid and the degree of control drops significantly after only a few years. There are drugs containing ivermectin to control the microfilaria for those who have become infected. It is recommended that entire communities be treated with such drugs to reduce the incidence of microfilaria in the area, which in turn greatly reduces infection. Then the requirement for insecticidal control of vectors is also reduced, which prolongs development of resistance.

= = = = = = = = = = = = = = = = = = = =

Key References: <medvet.ref.htm> <Hexapoda>

Adler, P. H., D. C. Currie & D. M. Wood. 2004. The Black Flies (Simuliidae) of North America. Comstock Publ. NY & London

Boatin, B. A. & F. O. Richards. 2006. Control of onchocerciasis. Adv. in Parasitol. 61: 349-54.

Crosskey, R. W. 1990. The Natural History of Blackflies. Wiley Publ., Chichester, England.

Davies, J. B. 1994. Sixty years of onchocerciasis vector control: a chronological summary with comments on eradication, reinvasion and

insecticide resistance. Ann. Rev. Ent. 39: 23-45.

De Villiers, P. C. 1987. Simulium dermatitis in man: clinical and biological features in South Africa. So. Afr. Med. J. 71-523-525

Hougard, J. M., L. Yameogo, A. Seketeli, H. Boatin & K. Y. Dadzie. 1997. Twenty-two years of flack-fly control in the onchocerciasis control programme in West Africa. Parasitology Today 13: 425-28.

Matheson, R. 1950. Medical Entomology. Comstock Publ. Co, Inc. 610 p.

Legner, E. F. 1995. Biological control of Diptera of medical and veterinary importance. J. Vector Ecology 20(1): 59_120.

Legner, E. F. 2000. Biological control of aquatic Diptera. p. 847_870. Contributions to a Manual of Palaearctic Diptera, Vol. 1, Science Herald,

Budapest. 978 p.

Molyneux, D. H. 2005. Onchocerciasis control & elimination: coming of age in resource-constrained health systems. Trends Parasitol. 21:

525-29.

Raybould, J. N. & G. B. White. 1979. The distribution, bionomics and control of onchocerciasis vectors (Diptera: Simuliidae) in easter Africa

and the Yemen. Tropenmedizin u. Parasitol. 30: 505-547.

Service, M. W. 1977. Methods for sampling adult Simuliidae, with special reference to the Simulium damnosum complex. Trop. Pest Bull. 5: 1-

48.

Thylefors, B. & M. Allman. 2006. Towards the elimination of onchocerciasis. Ann. trop. Med. & Parasitol. 100: 733-46.

World Health Organization. 2002. Success in Africa: the Onchocerciasis Control Programme in West Africa, 1974-2002. WHO, Geneva.

World Health Organization. 2004. Onchocerciasis (river blindness): report from the 13th Inter-American Conf. on Onchocerciasis, Cartagena

de Indias, Colombia. Weekly Epidemiological Record 79: 310-12.