This paper describes the morbidity, intensity and prevalence of onchoc
erciasis in an endemic area of Ethiopia, using a cross-sectional study
(n=425) and a longitudinal cohort study (n=98). During the study peri
od (February - October 1987), the increase in intensity and in number
of cases with dermal lesions was highly significant (p < 0. 005) when
compared with the increase in the microfilarial positivity rate (p < 0
. 1). Since the serial increase in dermatologic problems was wt direct
ly correlated with an increase in 0. volvulus microfilarial prevalence
or intensity, it is suggested that hyper-sensitivity responses to rep
eated Simulium bites might exacerbate onchocercal dermal lesions. The
controversy concerning Simulium bites as a cause of lesions mimicking
those due to O. volvulus parasites is discussed.