How LF Spreads

The lymphatic filariasis parasite is transmitted in its larval stage by the mosquito vector Culex quinquifasciatus. Following the bite from an infected mosquito, larvae are given a chance to enter a human host. The larvae make their way to the lymphatic system. In its early stages, LF larval infection of the lymphatic system often causes swelling and fever. In the lymph nodes, larvae grow into adult worms, which in turn produce progeny worms called microfilariae. While the adult worms remain in the lymphatic system, microfilariae circulate through the bloodstream at night, so mosquitoes can ingest them along with the blood and further transmit the disease.

Because the microfilariae are only present in the bloodstream at night, classical tests for W. bancrofti infection such as thick blood films require nocturnal sampling and laboratory facilities. However, a much simpler diagnostic tool, the immunochromatographic card test (ICT), can be administered in the field at any time of day. An ICT-positive result indicates presence of filarial antigen (adult and/or juvenile worms) but not necessarily the transmission stage of the worm, the microfilariae. We have conducted significant operational research to determine whether antigen presence in a region reflects transmission or simply results from movement of people in and out of areas that are actually free of and non-conducive for transmission.

Culex mosquito-transmitted LF does not exist where there is proper sewage infrastructure. Moreover, when proper sanitary drainage has been installed, the disease has disappeared; in these areas, the latrine-breeding vector cannot reach the density required to maintain transmission. This is one significant reason LF is a disease almost exclusively found amid desperate poverty in the tropics.