Early filariasis. The right leg of this 16 year old girl is affected by lymphatic filariasis. She has been infected for one month and the swelling is at an early stage. Filariasis is caused by parasitic worms spread by blood-sucking insects. The worms invade the lymphatic system and can grow up to 50cm long, producing thousands of larvae that spread around the body. Repeated infection causes inflammation of lymph vessels and accumulation of fluid, which can lead to enormous enlargement of parts of the body (elephantiasis). Treatment involves a course of the drug diethylcarbamazine, which is usually effective.