ESSENTIALS OF DIAGNOSIS
Episodic attacks of lymphangitis, lymphadenitis, and fever.
Chronic progressive swelling of extremities and genitals; hydrocele; chyluria; lymphedema.
Microfilariae in blood, chyluria, or hydrocele fluid; positive serologic tests.
Lymphatic filariasis is caused by three filarial nematodes: Wuchereria bancrofti, Brugia malayi, and Brugia timori, and is among the most important parasitic diseases of man. Approximately 120 million people are infected with these organisms in tropical and subtropical countries, about a third of these suffer clinical consequences of the infections, and many are seriously disfigured. W bancrofti causes about 90% of episodes of lymphatic filariasis. It is transmitted by Culex, Aedes, and Anopheles mosquitoes and is widely distributed in the tropics and subtropics, including sub-Saharan Africa, Southeast Asia, the western Pacific, India, South America, and the Caribbean. B malayi is transmitted by Mansonia and Anopheles mosquitoes and is endemic in parts of China, India, Southeast Asia, and the Pacific. B timori is found only in islands of southeastern Indonesia. Mansonella are filarial worms transmitted by midges and other insects in Africa and South America.
Humans are infected by the bites of infected mosquitoes. Larvae then move to the lymphatics and lymph nodes, where they mature over months to thread-like adult worms, and then can persist for many years (eFigure 35–47). The adult worms produce large numbers of microfilariae, which are released into the circulation, and infective to mosquitoes, particularly at night (except for the South Pacific, where microfilaremia peaks during daylight hours).
Filariasis (elephantiasis). Life cycles of Wuchereria bancrofti and Brugia malayi. A: The typical vector for B malayi filariasis are mosquito species in the genera Mansonia and Aedes. 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 into adults that commonly reside in the lymphatics
. The adult worms resemble those of W bancrofti
but are smaller. Female worms measure 43–55 mm in length by 130–170 mcm in width, and males measure 13–23 mm in length by 70–80 mcm in width. Adults produce microfilariae, measuring 177–230 mcm in length and 5–7 mcm in width, which are sheathed and have nocturnal periodicity. The microfilariae migrate into lymph and enter the bloodstream 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
Different species of the following genera of mosquitoes are vectors of W bancrofti
filariasis depending on geographic distribution. Among them are ...