Development of Onchocerca volvulus Larvae in Simulium pintoi in the Amazonas Region of Venezuela

The intake and development of Onchocerca volvulus in Simulium pintoi from the Parima mountain region of the Federal Territory of Amazonas in Venezuela, were studied experimentally. When wild females fed on the lower half of the legs and lower third of the back of an onchocerciasis patient harboring 23 and 264 microfilariae per skin snip, respectively, at each site, an average (median) of 14 (range, 1–77) and 245 (range, 58–495) microfilariae was ingested. However, within 24 hours of microfilarial ingestion a mortality of 47% (16/34 flies) was observed in the group of flies which fed on the back, as compared with 2% (2/101 flies) in the other group which fed on the legs. At a temperature varying between 16°C and 24°C, the development of O. volvulus larvae in S. pintoi was synchronous and orderly; no abnormal nor deformed larvae were observed. Third-stage larvae were first seen in the head of flies dying between 8 and 9 days after microfilarial ingestion, and 98 of 100 larvae recovered from days 10–16 were in the third stage. The proportions of females harboring third-stage larvae among flies which lived through day 8 in the two groups which fed on the legs and back, respectively, were 55% (21/38 flies) and 63% (5/8 flies). Although only two of five positive flies in the latter group contained third-stage larvae in the head (1 and 12, respectively), 71% (15/21 positive flies) of the former group had an average of 2.7 third-stage larvae in the head (range, 1–10). In conclusion, it is suggested that S. pintoi is an efficient vector of O. volvulus due to its high susceptibility, in spite of the high mortality caused by an excessive intake of microfilariae