Effects of temperature on development of Onchocerca volvulus in Simulium ochraceum, and longevity of the simuliid vector.

The effects of temperature on the development of Onchocerca volvulus in Simulium ochraceum, and on the vector's longevity were studied under simulated environmental conditions in the laboratory. When S. ochraceum (which had fed on a person infected with O. volvulus) were maintained at different, constant temperatures (10-30 C), larvae developed to the infective stage between 20 and 28 C. The time required for development to the infective stage depended on the relationship of y = -0.3760 + 0.0222x (y = velocity in development; x = rearing temperature). However, larval development to the infective stage occurred in all groups kept at different temperatures by day and by night (25 C during the day and 10-18 C during the night), although the developmental period was prolonged (10 days in the group held at 25/18 C, and 13 days in other groups). The percentage of females harboring infective larva(e) among those maintained was highest at a constant temperature of 22 C (21.9%), followed by 15.0% at 25 C. When S. ochraceum females were held at day/night differing temperatures, much higher rates were observed at 25/16 C (30.0%) and at 25/14 C (23.0%). Our results suggest that the distribution of onchocerciasis in Guatemala may be related to ambient temperature and to day/night temperature cycles.