Decline in onchocerciasis in the eastern Usambara mountains, north eastern Tanzania, and its possible relationship to deforestation.

A survey to establish the extent and possible impact of deforestation and increased farming activities on the prevalence of onchocerciasis was carried out in all the 17 villages in the eastern Usambara mountains. Results revealed that the disease is widespread. Out of 2,968 inhabitants of all ages skin-snipped in 1984 in the area, 22.7% were positive for Onchocerca volvulus microfilariae. The infection rates in the under nineteen and under ten age groups were 10.3% and 6.6% respectively. The overall prevalence rose steadily with age, with the mean microfilarial density reaching a plateau at age 24. Of the population studied 35.4% were found to present with at least one clinical manifestation commonly associated with onchocerciasis. Visual impairment (9.4%) and blindness (0.1%) rates suggest that ocular onchocerciasis is not a health problem in the area at present. Comparison with results obtained in the area during the last survey show that during the past 25 years there has been a significant decline in prevalence of infection. This drop is thought to be due to the effects of extensive deforestation on the vector population. Control of transmission, which is likely to rise as a result of a re-afforestation programme in the area is discussed.