Onchocerciasis in the forest-savannah mosaic region of Sierra Leone. Prevalence, intensity of infection, and endemicity levels.

The present study was carried out between December 1986 and January 1988. A series of 3,048 individuals (46% males and 54% females) were examined in 56 villages; onchocercal dermatitis and lymphadenitis were seen. The crude prevalence of nodules and of microfilariae amounted to 14% and 43%, respectively. The prevalence of nodules was significantly higher at the femoral trochanter than at other anatomical sites. The overall prevalence of nodules, and of microfilariae, was significantly higher in males than in females. The age-adjusted prevalence and the geometric mean microfilarial load were significantly higher in the cohort of males older than 15 years than in their female counterparts. The overall geometric mean microfilarial load was 18 microfilariae per skin snip. The correlation between prevalence and intensity of infection was poor, but was perfect between mean microfilarial density per skin snip and mean microfilarial density per milligram skin. The ratio of mean microfilarial density per skin snip to mean microfilarial density per milligram skin was 1:6. The correlation between prevalence of onchocercomata and microfilarial prevalence or microfilarial density was weak. Both of the latter criteria identified the Rokel/Seli as the river with the highest proportion of hyperendemic villages. The implications of the present findings for the epidemiology of onchocerciasis are discussed.