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Overview of Filarial Nematode Infections

by Richard D. Pearson, MD

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. They can be grouped based on the location of adult worms.

Subcutaneous filariasis is caused by Loa loa (the African eye worm—see Loiasis) and Onchocerca volvulus (see Onchocerciasis (River Blindness)).

Lymphatic filariasis is caused by Wuchereria bancrofti, Brugia malayi, and B. timori (see Bancroftian and Brugian Lymphatic Filariasis).

Rarely, Dirofilaria immitis, the dog heartworm, causes infection in humans (see Dirofilariasis).

Some specialty laboratories have a general screening serologic test for filarial infection (including Wuchereria, Brugia, Onchocerca, and Mansonella infections). The test is highly sensitive but cannot identify the specific filarial infection and cannot distinguish active from remote infection. This distinction is less important in symptomatic travelers, but limits the usefulness of the test in people from endemic areas.

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