Mass treatment to eliminate filariasis

Filariasis is an infection caused by a parasitic worm and is transmitted by insect-bites. In India, it is common in eastern Uttar Pradesh and Bihar. Bancroftian filariasis causes elephantiasis, hydrocoele, and economic loss throughout the tropics and subtropics. A global plan for its eradication currently relies on mass treatment with four to six annual doses of antifilarial drugs. The goal is to reduce the reservoir of microfilariae in the blood to a level that is insufficient to maintain transmission by the mosquito vector. The feasibility of the strategy is supported by observations that drugs given only once per year decrease the reservoir of microfilariae. The threshold below which transmission will be interrupted is not known, but it is assumed that annual mass treatment for four to six years will be required. Researchers from the Papua New Guinea Institute of Medical Research assessed nearly 2500 residents of Papua New Guinea, for the effects of four annual treatments with a single dose of diethylcarbamazine plus ivermectin or diethylcarbamazine alone on the incidence of microfilariae-positive infections, the severity of lymphatic disease, and the rate of transmission by mosquitoes. Random assignment to treatment regimens was carried out according to the village of residence, and villages were categorised as having moderate or high rates of transmission. The four annual treatments with either drug regimen were taken by 77 to 86 percent of the members of the population who were at least five years old; treatments were well tolerated. The proportion with microfilariae-positive infections decreased by 86 to 98 percent, with a greater reduction in areas with a moderate rate of transmission than in those with a high rate. The respective aggregate frequencies of hydrocoele and leg lymphedema were 15 percent and 5 percent before the trial began, and 5 percent and 4 percent after five years. Hydrocoele and leg lymphedema were eliminated in 87 percent and 69 percent, respectively, of those who had these conditions at the outset. The rate of transmission by mosquitoes decreased substantially, and new microfilariae-positive infections in children were almost completely prevented over the five-year study period. It can be concluded that annual mass treatment with drugs such as diethylcarbamazine can virtually eliminate the reservoir of microfilariae and greatly reduce the frequency of clinical lymphatic abnormalities due to bancroftian filariasis. Eradication may be possible in areas with moderate rates of transmission, but longer periods of treatment or additional control measures may be necessary in areas with high rates of transmission.