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

BACKGROUND

The current global aim for onchocerciasis is to eliminate transmission of infection by administering ivermectin once a year or biannually. However, when ivermectin treatment is given annually for 9-10 y, some individuals still carry low microfilariae (mf) levels, which may allow continued transmission.

METHODS

At four different times (0, 8, 90, 365 days) after treatment with ivermectin, 1608 newly emerged Simulium sanctipauli (Pra form) flies were fed on 35 volunteers, 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 mf and become infected; 29% of the 1608 fed flies picked up mf. 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 < .000).

CONCLUSIONS

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.