Lymphatic filariasis is a disease caused by roundworms of several species, mostly occurring in humid tropical areas and spread by many different species of mosquitoes.
The main species of worms (filariae) that cause the disease are Wuchereria bancrofti and Brugia malayi, both belonging to the family Filariidae. The third species with more local distribution is Brugia timori.
The adult stages of these worms are white, threadlike. Females (80-100 mm in length, 0.2-0.3 mm in width) are usually twice the size of male while larval stage (microfilaria) are only 250-300 µm long and 7-9 µm in width.
Internal structure of a Wuchereria bancrofti microfilaria using Giemsa stain (Picture from CDC.gov)
The parasites are found in tropical or subtropical (41N. latitude to 31S. latitude) Africa, the Americas, Asia, Australia, Spain, Turkey and numerous South Pacific Islands.
Lymphatic filariasis is transmitted by mosquitoes that bite infected humans and pick up the microfilariae. Usually the mosquito vectors are those from genus Anopheles together with different culicine mosquitos from Aedes, Culex and Mansonia genus (same as malaria). Culex quinquefasciatus is the main vector in urban areas in tropical countries while Culex pipiens in temperate countries.
Wuchereria bancrofti life cycle (Picture taken from cdc.gov)
The parasites develop inside mosquito for about 7-21 days until they become infective and migrate to mosquito mouth parts where they wait until mosquito bites and so they enter the human body (picture 2).
The most spectacular symptom of lymphatic filariasis is elephantiasis—thickening of the skin and underlying tissues. Symptoms can appear 5-18 months after a mosquito bite. In about 5% of infected persons legs become grossly swollen. Enlargement of the entire leg or arm, the genitals, vulva and breasts may occur. In endemic communities, 10-50% of men and up to 10% of women can be affected. Lymphatic filariasis is rarely fatal, but it can also cause recurring infections, fevers, severe inflammation of the lymph system, and a lung condition called tropical pulmonary eosinophilia (TPE).
Because most infections are asymptomatic, many go unrecognized. On average, 16 cases are diagnosed, reported to CDC, and treated annually in the United States.
The parasitic worms responsible for lymphatic filariasis have a population of symbiotic bacteria, Wolbachia, that live inside the worm. These bacteria can be killed by some antibiotics, also killing the worms themselves.
At least 120 million people in 73 countries worldwide are estimated to be infected with filariasis parasites with around 100 million people worldwide infected by Wuchereria bancrofti and about 12 million people in Southeast Asia infected by Brugia malayi and Brugia timori.