Causal Agents:
Filariasis is caused by nematodes (roundworms) that inhabit the lymphatics and subcutaneous tissues.  Eight main species infect humans.  Three of these are responsible for most of the morbidity due to filariasis: Wuchereria bancrofti and Brugia malayi cause lymphatic filariasis, and Onchocerca volvulus causes onchocerciasis (river blindness).  The other five species are Loa loa, Mansonella perstans, M. streptocerca, M. ozzardi, and Brugia timori.  (The last species also causes lymphatic filariasis.)

Life Cycles:
Infective larvae are transmitted by infected biting arthropods during a blood meal.  The larvae migrate to the appropriate site of the host's body, where they develop into microfilariae-producing adults.  The adults dwell in various human tissues where they can live for several years.  The agents of lymphatic filariasis reside in lymphatic vessels and lymph nodes; Onchocerca volvulus in nodules in subcutaneous tissues; Loa loa in subcutaneous tissues, where it migrates actively; Brugia malayi in lymphatics, as with Wuchereria bancrofti; Mansonella streptocerca in the dermis and subcutaneous tissue; Mansonella ozzardi apparently in the subcutaneous tissues; and M. perstans in body cavities and the surrounding tissues.  The female worms produce microfilariae which circulate in the blood, except for those of Onchocerca volvulus and Mansonella streptocerca, which are found in the skin, and O. volvulus which invade the eye.  The microfilariae infect biting arthropods (mosquitoes for the agents of lymphatic filariasis; blackflies [Simulium] for Onchocerca volvulus; midges for Mansonella perstans and M. streptocerca; and both midges and blackflies for Mansonella ozzardi; and deerflies [Chrysops] for Loa loa).  Inside the arthropod, the microfilariae develop in 1 to 2 weeks into infective filariform (third-stage) larvae.  During a subsequent blood meal by the insect, the larvae infect the vertebrate host.  They migrate to the appropriate site of the host's body, where they develop into adults, a slow process than can require up to 18 months in the case of Onchocerca.

Click on genus and species name above to see specific life cycles of each parasite.

Geographic Distribution:
Among the agents of lymphatic filariasis, Wuchereria bancrofti is encountered in tropical areas worldwide; Brugia malayi is limited to Asia; and Brugia timori is restricted to some islands of Indonesia.  The agent of river blindness, Onchocerca volvulus, occurs mainly in Africa, with additional foci in Latin America and the Middle East.  Among the other species, Loa loa and Mansonella streptocerca are found in Africa; Mansonella perstans occurs in both Africa and South America; and Mansonella ozzardi occurs only in the American continent.