Onchocerciasis in the Usambara Mountains, Tanzania: The disease, its epidemiology and its relationship to ocular complications

Abstract

A focus of onchocercal infection has been found in the Usambara mountains in the region of the Zigi valley and Amani. The incidence appears to increase with age, and by middle age approximately three-quarters of the population sampled were infected. A detailed clinical, parasitological and ophthalmic study was carried out on 88 patients, examined during the earlier survey and found to be infected. Among 84 patients shown during the study to have microfilariae in their skin, corneal lesions were present in 23·8%, iritis in 7·14%, fundal lesions similar to those of senile choroidal sclerosis in 10·85%, and microfilariae in the anterior chamber of the eye in 16·5%. Pruritus was the commonest symptom complained of, being present in 83·3% of patients. Thickening of the skin was present in 28·6%, an excoriated rash with or without depigmented patches in 61·9%, presbydermia in 14·3%. Laxity of the skin of the groin, constituting either incipient or actual “hanging groin,” was present in 9·5%. Onchocercal nodules were encountered in 48·8%. The microfilarial densities in the series were high, the mean number of microfilariae per milligramme of skin being at the shoulders 29·6, at the buttocks 51·3, at the ankles 26·9. It is notable that the highest density of microfilariae was encountered at the buttocks instead of at the ankles as in a number of other reported studies. No statistically significant differences between microfilarial counts from the left and right sides of the body were encountered. Within the range of size of skin snips examined in this series, distribution in depth of microfilariae was uniform. No difference was found between density of microfilariae in skin taken from over bony prominences and from nearby soft tissue. This and the previous two points are clearly of practical importance in survey work. A statistically weak correlation was found between density of microfilariae in the skin and age of the patient. There is a tendency for microfilariae to increase very slowly if at all beyond the age of 20 years, suggesting the development of partial immunity to infection, or a change in personal habits or environment leading to an infection rate low enough to maintain but not increase microfilarial density. Counts from neither shoulders, buttocks nor ankles showed any statistically significant correlation with either anterior or posterior ocular lesions or freedom from ocular lesions, but patients with posterior lesions had heavier counts and were older than those with anterior. These observations, though they could result from chance, suggest that posterior segment lesions in the eye are associated with heavy infections borne for many years. Heavy and prolonged onchocercal infection may be one of many factors which can accelerate senile ophthalmic change.