Lymphatic filariasis is caused by infection with nematodes of the family Filarioidea: 90% of infections are caused by Wuchereria bancrofti and most of the remainder by Brugia malayi.
Humans are the exclusive host of infection with W. bancrofti. Although certain strains of B. malayi can also infect some animal species (felines and monkeys), the life cycle in these animals generally remains epidemiologically distinct from that in humans.
The major vectors of W. bancrofti are mosquitoes of the genus Culex (in urban and semi-urban areas), Anopheles (in rural areas of Africa and elsewhere) and Aedes (in islands of the Pacific).
The parasites of B. malayi are transmitted by various species of the genus Mansonia; in some areas, anopheline mosquitoes are responsible for transmitting infection. Brugian parasites are confined to areas of east and south Asia, notably India, Indonesia, Malaysia and the Philippines.
An estimated 120 million people in tropical and subtropical areas of the world are infected with lymphatic filariasis; of these, almost 25 million men have genital disease (most commonly hydrocele) and almost 15 million, mostly women, have lymphoedema or elephantiasis of the leg.
Approximately 66% of those at risk of infection live in the WHO South-East Asia Region and 33% in the African Region.