Threadlike adult filarial worms reside in lymphatic or subcutaneous tissues. Gravid females produce live offspring (microfilariae) that circulate in blood or migrate through tissues. When ingested by a suitable bloodsucking insect (mosquitoes or flies), microfilariae develop into infective larvae that are inoculated or deposited in the skin of the next host during the insect bite. Life cycles of all filarial worms are similar except for the site of infection. Only a few filarial species infect humans. The major filarial diseases below can be grouped based on the location of adult worms. (See also Approach to Parasitic Infections.)
Subcutaneous filariasis includes
Lymphatic filariasis includes
Bancroftian and Brugian lymphatic filariasis caused by Wuchereria bancrofti, Brugia malayi, and B. timori
Other types of filariasis include
Mansonellosis caused by Mansonella perstans, with adult worms in the pleura, pericardium, or peritoneum, M. ozzardi in subcutaneous tissues, and M. streptocerca in the dermis
Dirofilariasis caused by Dirofilaria immitis, the dog heartworm, with larvae in lungs or rarely in eyes, brain or testes; larvae do not develop to adult worms in humans
Some specialty laboratories have screening serologic tests for filarial infection (including Wuchereria, Brugia, Onchocerca, and Mansonella infections). The tests are sensitive but cannot identify the specific filarial infection or distinguish active from remote infection. This distinction is less important in symptomatic travelers, but limits the usefulness of the tests in people from endemic areas.