Thin worms varying in length from a few cm to 60 cm. They live in tissue, blood vessels and lymph vessels, and occur in the tropics and sub tropics. The female worms bear larvae, which move into the bloodstream at certain periods of the day or night, then return to the tissues; they can be transmitted from one human being to another by bloodsucking insects, which thus function only as intermediate host or vector. In man the larvae can grow into adult worms. The most significant disorders caused by filariae are filariasis (caused by loiasis (caused by and onchocerciasis (caused by Filariasis is caused by worms that infest lymph vessels and nodes, especially in the legs and the pelvic organs, causing extremely painful inflammation of the lymph vessels and the reproductive organs, fever, and loss of appetite (filariasis attacks). In the long run these attacks lead to blockage of the lymph vessels, causing in particular swelling of the legs and thickening of the skin (elephantiasis). Filiariasis attacks are treated by rest and pain-killing drugs. Elephantiasis is usually treated with surgical stockings, sometimes by operation. Drugs can be used to prevent attacks. Loiasis is caused by infestation under the skin by loa loa worms, causing skin swelling, particularly around joints, leading to pain and restricted movement. The worm is sometimes visible in the conjunctiva of the eye. A worm directly under the skin can be removed by making a small incision. Treatment with drugs is usually effective.

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