How long do the effects of ivermectin on adult Onchocerca volvulus persist?

The persistence of the effects of ivermectin on the viability, morphology and reproductivity of adult Onchocerca volvulus was examined eighteen months after treatment with a single or five six-monthly doses of ivermectin and compared with untreated controls. Treated nodules were removed from patients participating in a randomised controlled trial of ivermectin in Sierra Leone. Adult filariae, 545 females and 348 males, were isolated by collagenase digestion. The nodules were significantly smaller, contained fewer young worms and supported lower microfilarial production in those treated with five doses of ivermectin. The productivity index, a measure of the reproductive potential of a worm population, was still reduced by 83% eighteen months after five doses and by 63% after a single dose compared to controls. These results show that worms recover their fertility even after multiple doses of ivermectin, but do so slowly compared to standard dosage intervals. In addition ivermectin may have a partial chemoprophylactic effect which contributes to the maintenance of low microfilarial production in conditions of on-going transmission.