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The degree of susceptibility of Simulium metallicum to infection with Onchocerca volvulus was studied in Venezuela. Wild flies which had fed on the infected volunteer were maintained at a temperature varying between 22°C and 28°C, and all flies dying every 24 hours were dissected for larval development. Third-stage larvae were first seen in the head of flies dying between five and six days. Few abnormal and deformed larvae were observed. The larval development was asynchronous; only 41 of 117 larvae recovered from 32 flies examined on days 6-11 were in the third stage, the remaining larvae being still in earlier stages. However, the proportion of third-stage larvae among all larvae recovered increased from 6 to 69 per cent, with the passage of time from days 6-8 to 9-11. Likewise, the percentage of third-stage larvae found in the head during the same period rose from 25 to 54 per cent. These results indicate that despite the asynchronous and retarded development, many larvae can develop to the infective stage in S. metallicum and the possibility of infective larvae capable of being eventually inoculated into man is high, when flies could survive more. It is suggested that the susceptibility of S. metallicum in Venezuela to the indigenous strain of O. volvulus is relatively high, as compared with that of the same species in Guatemala or Colombia.

Susceptibility of Simulium metallicum to infection with Onchocerca volvulus in Venezuela.