Filarial worms are nematodes that remain in subcutaneous tissues and lymphatics. Eight species infect human of which 4 are responsible for serious filarial infection. They are W. bancrofti, B. malayi, O.volvulus, L. loa. About 170 millions are infected world wide, transmitted by specific mosquitoes or arthropodes, a complex life cycle of larva in insects and adult worm in humans. The adult worms produce microfilaria which vary in size (200 – 250 m long and
5-7 m wide), may have loose sheath, when ingested by insect develop in 1-2 weeks. Adult worm survives for many years but microfilaria survives for 3-36 months. A rickettsia like bacteria Wolbachia has been seen intracellularly in cases of Brugia, Wuchereria, Mansonella and Onchocerca.

Infection is established by repeated exposure to infective larvae. In natives of endemic areas the presentation is chronic debilitating whereas in travelers and recent exposure it presents as acute manifestation.