Programmatic implications of the efficiency of uptake of Onchocerca volvulus microfilaria by Simulium sp . vectors

Background The current global aim for onchocerciasis is to eliminate transmission of infection by administering ivermectin once or biannually. However, when ivermectin treatment is given annually for 9-10 years, some individuals still carry low mf levels, which may allow continued transmission. Methods 1608 newly emerged Simulium sanctipauli (Pra form) flies were fed on 35 volunteers at four different times (0, 8, 90, 365 days) after treatment with ivermectin, with initial dermal microfilarial loads ranging from 0 to >30 Onchocerca volvulus mf/mg skin. Ten days after feeding, the number and load of mf in these flies were assessed by dissection. Results Notably, even at the lowest skin snip mf densities—including those showing zero infection flies can still ingest O. volvulus microfilariae and become infected; 29% of the 1608 fed flies picked up microfilariae. The lowest skin mf levels were recorded on Day 90 after treatment, at which time vector mf uptake increased by 21% with every unit increase in skin mf density (p<0.000). Conclusion Very low dermal mf levels can still support parasite transmission. Consequently, widespread low O. volvulus infection in human populations with highly productive vector-breeding sites could significantly affect efforts to eliminate transmission.