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Filariasis a chronic disease caused by the presence of filarial worm (Nematoda) in the blood and lymph channels, lymph glands, and other tissues. Adult worms live in vertebrates. Typically, the female produces larvae called microfilariae, which reach the peripheral blood or lymphatic vessels, where they may be ingested by a blood-sucking arthropod (mosquito). In the intermediate host, they transform into rhabditoid larvae that metamorphose into infective filariform larvae. These migrate to the proboscis and are deposited in or on the skin of the vertebrate host. The patient may have larvae in his blood for years, but may not have had any adult worms.

Adult worms produce lymphatic obstruction, especially if they are present in masses, and if they die and disintegrate. The obstruction causes varicosity of the superficial vessel and lymphatic edema. When the lymph nodes are enlarged, their sinuses become distended with lymph and elephantiasis may develop. Elephantiasis is caused by infection of the lymphatics by one of the three filarial parasites of humans, Wuchereria bancrofti, Brugia malayi, or B. timori. This is a condition in which the tissue become enormously thickened and indurated. The legs and scrotum are parts commonly affected. In most severe forms the legs of the sufferer become swollen. In Bangladesh the disease occurs mostly in coastal parts areas. [M Sayedur Rahman]