Wuchereria bancrofti and Filariasis

Wuchereria bancrofti and Filariasis

Wuchereria bancrofti is a slender, thread-like nematode parasite that lives in the human lymphatic system and is the principal cause of lymphatic filariasis, popularly known as elephantiasis in its advanced form.

Structure: male and female differences

Adult worms are long, filiform (thread-like) and coiled, living together in the lymphatic vessels and lymph nodes. As in most parasitic nematodes, the two sexes are readily distinguished: the female worm is considerably longer and thicker than the male, and is viviparous, releasing large numbers of live, sheathed, immature larvae called microfilariae directly into the host's lymph and blood rather than laying eggs; the male worm is shorter and thinner, and its posterior end is curved ventrally.

Nocturnal periodicity

A striking feature of Wuchereria microfilariae is nocturnal periodicity: although they are present throughout the day in the deep blood vessels of the internal organs (especially the lungs), they migrate into the peripheral blood circulation in large numbers specifically at night, broadly coinciding with the sleeping and feeding habits of the vector mosquitoes (Culex), which are night-biters. This periodicity is of great practical importance, because blood samples for diagnosing microfilaria must therefore be collected at night to reliably detect the parasite.

Life cycle

1. Infection begins when a female Culex mosquito, biting an infected person at night, ingests blood containing circulating microfilariae.
2. Inside the mosquito, the microfilariae shed their sheath, penetrate the stomach wall, and migrate to the thoracic flight muscles, where they develop through first-, second- and third-stage larvae. The third-stage (infective) larvae migrate to the mosquito's proboscis.
3. When this infective mosquito next bites a human, the third-stage larvae actively escape from the proboscis and enter the human body through the bite wound.
4. Inside the human host, the larvae migrate into the lymphatic vessels and lymph nodes (commonly of the lower limbs and genital region), where they develop and moult further to become adult male and female worms over several months.
5. The mated adult worms live coiled together in the lymphatics; the female begins releasing sheathed microfilariae into the lymph and blood, which show nocturnal periodicity in the peripheral circulation, ready to be picked up by a biting Culex mosquito and complete the cycle.

Pathogenicity …