The causative agents of lymphatic filariasis (LF) include the mosquito-borne filarial nematodes Wuchereria bancrofti, Brugia malayi, B. timori An estimated 90% of LF cases are caused by W. bancrofti (Bancroftian filariasis).
Brugia Malayi Life Cycle
During a blood meal, an infected mosquito (typically Mansonia spp. and Aedes spp.) introduces third-stage filarial larvae onto the skin of the human host, where they penetrate into the bite wound . They develop into adults that commonly reside in the lymphatics . The adult worms outwardly resemble those of Wuchereria bancrofti but are smaller. Female worms measure 43 to 55 mm in length by 130 to 170 μm in width, and males measure 13 to 23 mm in length by 70 to 80 μm in width. Adults produce microfilariae, measuring 177 to 230 μm in length and 5 to 7 μm in width, which are sheathed and have nocturnal periodicity (in some regions B. malayi may be sub-periodic, and note that microfilariae are usually not produced in B. pahangi infections). The microfilariae migrate into lymph and enter the blood stream reaching the peripheral blood . A mosquito ingests the microfilariae during a blood meal . After ingestion, the microfilariae lose their sheaths and work their way through the wall of the proventriculus and cardiac portion of the midgut to reach the thoracic muscles . There the microfilariae develop into first-stage larvae and subsequently into third-stage larvae . The third-stage larvae migrate through the hemocoel to the mosquito’s proboscis and can infect another human when the mosquito takes a blood meal .
Wuchereria bancrofti Life Cycle
During a blood meal, an infected mosquito introduces third-stage filarial larvae onto the skin of the human host, where they penetrate into the bite wound . They develop in adults that commonly reside in the lymphatics . The female worms measure 80 to 100 mm in length and 0.24 to 0.30 mm in diameter, while the males measure about 40 mm by 1 mm. Adults produce microfilariae measuring 244 to 296 μm by 7.5 to 10 μm, which are sheathed and have nocturnal periodicity, except the South Pacific microfilariae which have the absence of marked periodicity. The microfilariae migrate into lymph and blood channels moving actively through lymph and blood . A mosquito ingests the microfilariae during a blood meal . After ingestion, the microfilariae lose their sheaths and some of them work their way through the wall of the proventriculus and cardiac portion of the mosquito’s midgut and reach the thoracic muscles . There the microfilariae develop into first-stage larvae and subsequently into third-stage infective larvae . The third-stage infective larvae migrate through the hemocoel to the mosquito’s prosbocis and can infect another human when the mosquito takes a blood meal .
Hosts and Vectors
Wuchereria bancrofti, Brugia malayi, and B. timori are considered human parasites as animal reservoirs are of minor epidemiologic importance or absent; felid species and some primates are the primary reservoir hosts of zoonotic B. pahangi.
The typical vector for Brugia spp. filariasis are mosquito species in the genera Mansonia and Aedes. W. bancrofti is transmitted by many different mosquito genera/species, depending on geographical distribution. Among them are Aedes spp., Anopheles spp., Culex spp., Mansonia spp., and Coquillettida juxtamansonia.
W. bancrofti was once widespread in tropical regions globally but control measures have reduced its geographic range. It is currently endemic throughout Sub-Saharan Africa (excluding the southern portion of the continent), Madagascar, several Western Pacific Island nations and territories and parts of the Caribbean. Bancroftian filariasis also occurs sporadically in South America, India, and Southeast Asia.
Brugia spp. associated with LF are more geographically limited and occur only in Southeast Asia. Like W. bancrofti, control measures have reduced the occurrence and endemic range considerably. Brugia timori is restricted to the Lesser Sunda Islands of Indonesia.
While severe manifestations do not develop in the majority of infections, LF is a potentially highly disfiguring and disabling disease. The most prominent clinical feature is the development of severe lymphedema of the limbs (“elephantiasis”) and occasionally genitalia (hydrocele) due to dysfunction of lymphatic vessels. Affected limbs become grossly swollen; the skin may become thick and pitted, and secondary infection are frequent due to lymphatic dysfunction. Scrotal hydrocele is also seen in some infected males. Lymphangitis, lymphadenopathy, and eosinophilia may accompany infection in the early stages.
A chronic syndrome called “tropical pulmonary eosinophilia” has been associated with W. bancrofti and B. malayi infections, involving eosinophilic pulmonary infiltrate, peripheral hypereosinophilia, wheezing, chest pain, splenomegaly, and bloody sputum. This has most frequently been documented in South and Southeast Asia.