Microfilaria worms in blood. Computer illustration showing immature filarial worms (microfilaria), the cause of filariasis. The worm larvae are spread from person to person by blood-sucking insects, and cause inflammation of the lymph vessels resulting in localised oedema (swelling). Repeated infection leads to elephantiasis, where the affected area becomes greatly enlarged and the skin thick and fissured. Drugs may affect a cure if the infection is caught early.