Long-term effect of three different strategies for mass diethylcarbamazine administration in bancroftian filariasis: follow-up at 10 years after treatment.

The long-term effect of three different strategies for mass diethylcarbamazine (DEC) administration in bancroftian filariasis was assessed 10 years after start of treatment in three endemic communities in Tanzania. The strategies were the standard 12 day treatment (strategy I); a semi-annual single-dose treatment (strategy II); and a monthly low-dose treatment (strategy III). Treatment was given only during the first year. Following reductions immediately after treatment, overall community microfilaraemia levels were approaching pre-treatment levels in all three communities, 10 years later. In individuals who were microfilaria-positive and treated at baseline, the treatment had a long-term effect on microfilarial intensities, with geometric mean intensities being only 11%, 13% and 2% of pre-treatment levels 10 years later for strategies I, II and III, respectively. This suppressive effect was most pronounced for strategy III, which also cleared microfilaraemia and circulating filarial antigenaemia in a larger proportion of treated individuals than the other strategies. Most of the follow-up individuals who developed microfilaraemia between 2 and 10 years after start of treatment had also been microfilaraemic before treatment, suggesting that reappearance of microfilaraemia may be due to surviving female worms and/or that previously microfilaraemic individuals have a higher chance of reinfection than previously amicrofilaraemic individuals.