W. bancrofti is most widely spread and is responsible for more than 90% of the infections These are long, thread-like worms, also known as filariae.
•Bancroftian filariasis, caused by W. bancrofti, is mainly transmitted by Culex quinquefasciatus and by some Anopheles and Aedes species. It was estimated that some 107 million people were infected in parts of China, India, other parts of south-east Asia, the Pacific Islands, tropical Africa, and South and Central America.
•Brugian filariasis, caused by Brugia malayi and B. timori, was estimated to infect some 13 million people, mainly in south-east Asia. Its main vectors are the Mansonia species. B. timori occurs on the islands of Flores, Timor and Alor, to the east of Java, and is transmitted by Anopheles barbirostris.
Bancroftian filariasis occurs in two forms: in the most common form the microfilariae circulate in the blood at night, whereas in the second form they occur continuously in the blood but increase in number during the day.
Filariae are responsible directly or indirectly for many of the manifestations associated with elephantiasis. These include fluid retention and tissue swelling (also called lymphoedema) in the lower limbs, genital diseases (hydrocele, chylocele and swelling of the scrotum and penis) and frequent acute disease episodes.
Lymphatic filariasis infection is chronic in nature due to the long life span of the worms and accumulation of infection over time. Many people may be infected without even knowing it, but on the long-term some people may develop severe chronic manifestations, including hydrocele and lymphoedema.
Infection is usually acquired in childhood, but the painful and profoundly disfiguring visible manifestations of the disease occur later in life.
Lymphatic filariasis: One of the oldest diseases known to humanity
