Observations on filarial infections and parous rates of anthropophilic blackflies in Guatemala, with reference to the transmission of Onchocerca volvulus.

In Guatemala, in areas of different onchocerciasis endemicity and outside the endemic zone, the main anthropophilic blackfly species Simulium ochraceum, S. metallicum, S. callidum and S. gonzalezi were examined for infections with Onchocerca volvulus, other filariae, and non-filarial parasites. Third stage larvae, indistinguishable from those of O. volvulus were found only in 3 females of S. ochraceum out of 3,513 examined. In S. metallicum, 2 females out of 3,121 dissected harboured infective larvae of another filaria species, morphologically different from O. volvulus. Thoracic infections were encountered in all four Simulium species. Only in S. ochraceum was there a correlation between the infection rates in the human and in the fly populations. In the endemic zone, up to 15% of the parous flies were infected, against none in the non-endemic area. Parous S. metallicum showed low over-all infection rates of 0.6 to 2.6%, and had infections also outside the endemic zone, but these could not have been of human origin. Non-filarial infections, including fungi of the ovaries, ciliates, mermithids, nematodes, were commonly observed in Guatemalan blackflies. Parous rates were 40% in S. ochraceum, 23% in S. metallicum, 39% in S. callidum, and 49% in S. gonzalezi. The size of the follicular relics showed a daily cycle in S. ochraceum. They were usually large in flies biting man in the afternoon, but small in flies biting in the morning. This suggests that the flies found a new host the same day, or the morning after they had deposited their eggs.