Quantitative aspects of the infection of Simulium ochraceum by Onchocerca volvulus: the relation of skin microfilarial density to vector infection.

In Guatemala, 16 wild Simulium ochraceum were permitted to feed on a 15" x 15" grid on the back of each of 13 persons infected with Onchocerca volvulus. Based on 6 skin biopsies, the mean microfilarial density (mfd) in these study participants ranged from 1.256 mf/mg of skin to 111.824 mf/mg. The files were dissected 6 to 10 hours after feeding and the number of microfilariae in the blood meal and escaping the midgut into the thoracic musculature were counted. A nearly linear relationship was found between the mean skin mfd and the mean microfilarial uptake by S. ochraceum (rs = 0.868). An even stronger relation existed between the mean mfd and the mean number of microfilariae escaping the midgut (rs = 0.915). Because the approximate 1:1 relationship observed between the number of microfilariae escaping the midgut and the subsequent number of infective (L3) larvae has been previously demonstrated, it can be concluded that a similar linear relationship exists between microfilarial skin densities and the number of L3 larvae available for transmission of O. volvulus by the Guatemalan vector.