Studies on Human Onchocerciasis and Simulium in Nyanza Province, Kenya. I. Distribution and Incidence of O. volvulus

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Abstract
A survey of the incidence and distribution of Onchocerca volvulus in Nyanza Province was carried out by the “skin-snip” method of determining the presence of microfilariae in the skin. 5,842 Africans were examined.An account is given of the incidence of the infection in five different foci, with some observations on ocular complications, cutaneous nodules, abnormal skin, and age and sex incidence.The survey disclosed one hitherto unrecorded focus of the infection, namely, at Riana in South Kavirondo District and confirmed the existence of another on parts of the Kuja River in the same district. The endemic focus discovered in Lumbwa District by Dry in 1921 was surveyed for the first time.Each focus is fairly well circumscribed and all are. situated at an altitude roughly defined by the 5,000′ and 6,000′ contour lines. Both of these limitations are determined by topography and by the insect vector, Simulium neavei, whose habits confine it to a certaintype of environment. This is characterized by the presence of fastrunning rivers or streams; a hilly or mountainous terrain; well-wooded river banks.

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