Parasitological and clinical characterization of Simulium neavei-transmitted onchocerciasis in western Uganda.

In the forests of western Uganda onchocerciasis is transmitted by Simulium neavei s.s. Since little is known about the infection transmitted by this vector, a survey was made in 1991 with special regard to adult persons of 15 years and older in the 13 villages of the parish Kigoyera 40 km northeast of Fort Portal. 3268 (52%) of the 6271 registered inhabitants were examined clinically and parasitologically. The highest microfilaria (mf) densities were found on the buttocks, lower ones on the shoulders and the lowest on the ankles. In the parish the standardised prevalence of mf carriers was 62%. The crude prevalences of adult mf carriers ranged from 80% to 95% in the 13 villages. Densities of 100 mf/snip or more were found in 25% of male persons. The community microfilarial load (CMFL) in skin snips from the buttocks was 49 mf/snip, ranging from 22 to 93 in the 13 villages. The standardised prevalence of nodule carriers was 25% and the mean nodule load was 1.9 nodules per nodule carrier. Among 3420 nodules 90% were found on the pelvic girdle. The standardised prevalence of onchocercal dermatitis was 19%. The crude rates ranged within the age groups in males from 20% to 45% and in females from 16% to 41%. The standardised prevalence of persons presenting mf in the anterior chamber of the eye was 24% and the CMFL in the anterior chamber ranged between 1.2 and 3.3 mf/chamber in six villages. Standardised rates were 1.6% for sclerosing keratitis and 0.9% for reduced vision of 3/60 or less. These prevalences of eye lesions are comparable to those observed in West African forest areas. The CMFLs and the prevalences of mf and nodule carriers represent suitable criteria for community diagnosis of S. neavei-transmitted onchocerciasis in Uganda to guide ivermectin treatment, whereas the prevalence of "leopard skin" is not useful. Immigrants living less than five years in the endemic focus should be excluded from the assessment of mf carrier rates and those living there less than ten years from rapid assessment of nodule carrier rates.