Figure 1.

Transmission cycle of lymphatic filariasis with density-dependent mechanisms. This figure shows the life cycle of Wuchereria bancrofti, the main parasitic cause of lymphatic filariasis. The adult worms (macrofilariae) are located in the lymphatic system of the human host, where they live for 5–10 years [24, 37]. After mating with male worms, female worms can produce millions of microfilariae (mf), which can be found in the bloodstream and have a lifespan of 6–24 months [32]. A mosquito that takes a blood meal may engorge some mf. Inside the mosquito, mf develop in about 12 days into L3 stage larvae (L3), which are infectious to humans. When the mosquito takes another blood meal, the L3 can enter the human body and some will migrate to the lymphatic system and will develop into mature adult worms. The immature period lasts about 6–12 months [38]. Mf cannot develop into adult worms without passing through the developmental stages in the mosquito. Larval development and mosquito survival are density-dependent [17, 18]. Two possible mechanisms of acquired immunity are shown [20].