Effect of two successive annual treatments with single doses of ivermectin on microfilaraemia due to Wuchereria bancrofti var. pacifica

Between 1986 and 1988 a single-blind, dose-ranging study was carried out in French Polynesia to determine the efficacy and tolerability of single 50, 100, 150 and 200 μg/kg doses of ivermectin in Wuchereria bancrofti carriers. Forty male microfilariae (mf) carriers between 18 and 50 years of age, in whom mf density was ⩾20 mf/ml, were treated twice at a one-year interval. Twelve months after the second treatment, in carriers who were given a dose ⩾100 ng/kg, mean mf density was 4–7% of the initial pretreatment mf density. Therefore, several successive annual treatments with single doses ⩾100 μg/kg of ivermectin should result in reducing mf densities to a very low level. Nevertheless, at 9 months after the second treatment, residual parasitaemia ranged from 1 to 2182 mf/ml (median 85) in 30 patients. Finally, in patients with pretreatment mf counts ⩽150 mf/ml, mean mf density was 2·8 and 8·9 mf/ml, respectively, during the 2 six-month periods following treatment, while in patients with pretreatment mf densities >150 mf/ml (median 1500) it was 92·3 and 334·1 mf/ ml during the same periods. These results suggest that, when implementing filariasis control programmes, the best strategy might be administration of several treatments with a single dose of ivermectin every 6 months to the entire population, at least in French Polynesia. Afterwards, when mf densities had been reduced to a relatively low level (100–150 mf/ ml), annual treatments could be considered.