HOW LONG DO THE EFFECTS OF IVERMECTIN ON ADULT ONCHOCERCA-VOLVULUS PERSIST

The persistence of the effects of ivermectin on the viability, morphol ogy and reproductivity of adult Onchocerca volvulus was examined eight een 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 o f ivermectin in Sierra Leone. Adult filariae, 545 females and 348 male s, were isolated by collagenase digestion. The nodules were significan tly smaller, contained fewer young worms and supported lower microfila rial production in those treated with five doses of ivermectin. The pr oductivity index, a measure of the reproductive potential of a worm po pulation, was still reduced by 83 % eighteen months after five doses a nd by 63 % after a single dose compared to controls. These results sho w that worms recover their fertility even after multiple doses of iver mectin, but do so slowly compared to standard dosage intervals. In add ition ivermectin may have a partial chemoprophylactic effect which con tributes to the maintenance of low microfilarial production in conditi ons of on-going transmission.