Epidemiology and Preventive Measures for Onchocerciasis

Onchocerciasis is a disease caused by a parasitic filaroid worm named Onchocerca volvulus. The disease is characterized by severe skin disease and blindness. An individual may acquire the disease if he is bitten by a black fly under the genus Simulium carrying O. volvulus parasites. This article presents the general epidemiology of the disease and the ways to decrease the number of people being infected.

Epidemiology of Onchocerciasis

Landscape epidemiologists use onchocerciasis as a model system for the study of the origin and spread of the disease in a community. The vector for O. volvulus parasite, Simulium spp., leaves their larval stages only in clear, fast-running streams. Survival of adult flies ultimately depends on high humidity and the presence of plants along the stream. Some indigenous people of South Africa have long known that the disease is associated with the river, but were unsure on the involvement of the black flies. The indigenous people called the disease river blindness, because they observed that people who went to the river and got sick eventually became blind. It was not until 1926 that the flies were proven to be involved in the transmission of O. volvulus parasites. The flies in a river will attack anyone who disturbs them, just like what bees do. Parasitologists have found that wild-caught Simulium spp. flies carry various species of filaroids, most of them are still unidentified, but in places where O. volvulus is endemic, juveniles can usually be recognized as that species. In hyperendemic communities, more done 90% of the population can be microfilaremic, a condition where the blood contains microfilariae (juveniles of Onchocercus. (Somo et al. 1993)

Interestingly, cases of onchocerciasis have been recorded in arid West African savannas and deserts along the Nile River, close to the Sudan-Egypt border. Epidemiology of the disease in these areas has not been comprehensively studied, but it may depend on the adaptations for survival of the Simulium spp. vectors or strain of O. volvulus or both.

Preventive Measures for Onchocerciasis

The best preventive measure for onchocerciasis is eliminating the vectors (Simulium spp.) of the Onchocerca volvulus microfilariae. However, eradicating the insect vectors has not been so easy over the years. Garnham and colleagues (1947) reported the effectiveness of applying DDT pesticide to fast running streams in killing Simulium insects, but the undesirable environmental impacts of the pesticide are well known. Biodegradable pesticides could be used, but are very expensive for poor countries to afford. Nonetheless, chemotherapy campaigns and vector control between 1974 and 1995 have prevented less than 200,000 people from going blind, protected over 30,000,000 people from eye and skin lesions, and protected at least 110 million new born children from the risk of blindness in the eleven African nations in the Onchocerciasis Control Programme (OCP) (Molyneux 1995). Even though the program ended in the year 2002, onchocerciasis is still a significant public health problem in the African and South American continents. (Abraham et al. 2002)

References

Somo RM, Enyong PA, Fobi G, Dnga JS, Lafleur C, Agnamey P, Ngosso A, and EM Ngolle. 1993. A study of onchocerciasis with severe skin and eye lesions in a hyperendemic zone in the forest of southwestern Cameroon: Clinical, parasitologic, and entomologic findings. Am. J. Trop. Med. Hyg. 48:4-19.

Abraham D, Lucius R, and AJ Trees. 2002. Immunity to Onchocerca spp. in animal hosts. Trends Parasitology. 18:164-171.

Molyneux DH. 1995. Onchocerciasis control in West Africa: Current status and future of the Onchocerciasis Control Programme. Parasitology. 11:399-402.

Garnham PCG and JP McMahon. 1947. The eradication of Simulium neavi Raubaud, from an onchocerciasis area in Kenya Colony. Bull. Ent. Res. 37:619-628.