The disease is caused by three species of thread-like nematode worms, known as filariae – Wuchereria bancrofti, Brugia malayi and Brugia timori. Male worms are about 3–4 centimetres in length, and female worms 8–10 centimetres. The male and female worms together form “nests” in the human lymphatic system, the network of nodes and vessels that maintain the delicate fluid balance between blood and body tissues. The lymphatic system is an essential component of the body’s immune system.
Filarial infection can cause a variety of clinical manifestations, including lymphoedema of the limbs, genital disease (hydrocele, chylocele, and swelling of the scrotum and penis) and recurrent acute attacks, which are extremely painful and are accompanied by fever. The vast majority of infected people are asymptomatic, but virtually all of them have subclinical lymphatic damage and as many as 40% have kidney damage, with proteinuria and haematuria.
Filariae are transmitted by mosquitoes. When a mosquito with infective stage larvae takes a blood meal, the parasites are deposited on the person’s skin, from where they enter the body through the skin. These larvae then migrate to the lymphatic vessels and develop into adult worms over a period of 6–12 months, causing damage to and dilatation of the lymphatic vessels. The adult filariae live for several years in the human host. During this time, they produce millions of immature microfilariae that circulate in the peripheral blood and are ingested by mosquitoes that bite the infected human. The larval forms further develop inside the mosquito before becoming infectious to man. Thus, a cycle of transmission is established.