Mass treatment with ivermectin for filariasis control in Papua New Guinea:impact on mosquito survival

Field studies were carried out to determine the impact of mass human treatm ent with ivermectin on the survival of anthropophagic mosquitoes of the Ano pheles punctulatus complex (Diptera: Culicidae), the vectors of lymphatic f ilariasis and malaria in Papua New Guinea. In a village where mass treatmen t had been given, using 400 mu g/kg ivermectin plus 6 mg/kg diethylcarbamaz ine citrate (DEC), we performed pre- and post-treatment collections of fres hly blood-engorged mosquitoes from the same nine bedrooms. All blood-fed mo squitoes collected less than 4 days after mass treatment died within 9 days , whereas 67% of those collected before treatment survived for > 9 days. Co mparison (using the log-rank test) of the survival curves for mosquitoes co llected (i) before treatment, (ii) < 4 days after treatment, and (iii) 28 d ays after treatment, showed the survival rate of group (ii) to be significa ntly lower than the other two (chi(2) = 176, df = 2, P < 0.0001). Pre- and post-treatment all-night landing catches showed no reduction in human bitin g rates in the experimental village. In another village, where people were mass treated with ivermectin (400 mu g/kg) only, the survival rates of fres hly blood-engorged An.punctulatus collected from bedroom resting-sites less than 1 day after treatment, were compared to similar collections carried o ut at the same time in a nearby village where people were not treated with ivermectin, The 48-h survival rate for the ivermectin-treated village was 3 1% compared to 94% for the other; this difference was highly significant (c hi(2) = 32.42, df = 1, P < 0.0001). Mosquitoes fed 2 months post-treatment with DEC or collected 38 days post-treatment with ivermectin had normal sur vival rates. We conclude that the duration of the systemic lethal effect of ivermectin on mosquitoes is insufficient to be of epidemiological signific ance in filariasis control programmes that are based on biannual and annual single-dose treatments, but might reduce vectorial capacity sufficiently t o block epidemics of dengue or even malaria.