Lymphatic Filariasis: Transmission, Treatment and Elimination

Lymphatic filariasis (LF) is a mosquito-borne, tropical disease caused by filarial worms. Infection can lead to disabling chronic disease, characterized by swelling of extremities or external genitalia (lymphoedema, elephantiasis and hydrocele). Mass treatment with antifilarial drugs is used to reduce the parasite load in the population, in order to reduce transmission and prevent disease. This is so effective that elimination of LF seems possible. Using a mathematical for lymphatic filariasis transmission in Pondicherry, India, we predicted how long yearly mass treatment would have to be continued to achieve elimination. Six annual treatments with the recommended combination of diethylcarbamazine (DEC) and albendazole would be sufficient if population coverage is 65% per round. Only four rounds would do if coverage is 80%. The required duration of mass treatment increases with endemicity level. In a comparison of transmission efficiency for different mosquito species, we found that the relationship between infection intensity in humans and the number of infective larvae developing in mosquitoes differed markedly. Mosquito characteristics may largely influence elimination prospects and predictions for Pondicherry cannot be generalized. A further study concentrated on the role of acquired immunity. We found that existing models for such immunity, which predict a decline in infection intensity in older humans, are not valid for lymphatic filariasis. We also quantified the effects of drugs treatment on different parasite stages: DEC and ivermectin were found to affect a very large proportion of microfilariae and adult worms; these drugs were even more effective when given in combination with albendazole. In conclusion, prospects for LF elimination by mass treatment are good if population coverage is sufficiently high, but the required duration depends on local transmission dynamics, endemicity level, and the efficacy of employed treatment regimens.

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