Wuchereria Bancrofti – The Filarial Worm


The generic name of this worm is named after Wucherer who first reported it in1866. The species name is after Bancroft who discovered the adult females in human lymph in 1876. This worm causes filariasis or elephantiasis in human beings. Another related genus Brugia malayi causes filariasis in Malayan Region. The life cycle of this worm involves a secondary host, which is female of a species of mosquitoes of the genus Culex, Aedes or Anopheles depending on the region it is found. The adult parasite is found in lymph vessels and lymph nodes while the larvae, called microfilariae, occur in blood and develop in mosquito females.

MORPHOLOGY

Adults are long, slender, hair-like and creamy-white in colour. The male measures 2.5-4.0 cm in length and female measures 8-10 cm in length. Posterior end of male has a curved tail containing rows of genital papillae, caudal alae on the lateral side and two unequal copulatory spicules projecting from cloaca. Female has a narrow and abruptly pointed tail and an anus before the tip of tail. The female genital pore or vulva is located ventrally on the anterior one-third of the body.

ANATOMY

Mouth is located in the middle of the anterior blunt end, which does not show prominent papillae and sense organs. Pharynx is distinguished into an anterior muscular and a posterior glandular region. The intestine is straight like other nematodes. Excretory organ is a renette cell that opens on the anterior region of the body on ventral side. Circulatory function is performed by the pseudocoelomic fluid that fills the pseudocoelom. There are no particular respiratory organs and respiration is anaerobic. Nervous system includes a nerve ring around the pharynx and longitudinal nerves emerging from it. Female possesses two ovaries and oviducts that join for form a large sac-like ovijector that opens to the exterior by a genital opening. Female is viviparous. Male has a single testis which is a thread-like coiled tube, opening posteriorly into a seminal vesicle which is connected to cloaca by an ejaculatory duct. Male coils around female and copulates.

LIFE CYCLE

The female is viviparous and larvae are called microfilariae, which have semitransparent bodies with blunt anterior ends and pointed tails. Larvae are very active and can move both with and against the blood stream. A microfilaria measures about 0.2 to 0.3 mm in length. Body of microfilariae is covered by a cuticular sheath. Future mouth is present as oral stylets and other structures visible through the semitransparent skin are: nerve ring, excretory pore, renette cells, a dark coloured inner mass, germinal cells and four large cells and future anus.

The microfilariae remain dormant in human body and if not sucked by mosquito, they die in about 70 days. The microfilariae exhibit day-night periodicity as they appear in peripheral blood circulation at night and go to deeper vessels during the day. This facilitates their sucking by the nocturnal female mosquitoes.

In the stomach of mosquito microfilariae lose their sheaths, penetrate the stomach wall and migrate to thoracic muscles where they undergo metamorphosis and grow. The first instar is slender and the second instar becomes plump, while the final instar is long and slender infective stage which is about 1.5 mm long. Infective juveniles now migrate into mosquito’s proboscis and wait for the mosquito to bite man. 

When an infected mosquito bites man the infective juveniles are released into the wound made by mosquito and enter the blood stream. The juveniles then pass from blood to lymph glands and lymph vessels, where they coil up and develop into adult forms.

FILARIASIS OR ELEPHANTIASIS

Filarial worms live in lymphatic system where they obstruct the flow of lymph, causing a severe disease known as elephantiasis, in which the limbs, scrotal sacs, breasts or other body parts grow to enormous size. Presence of microfilariae in the blood causes filarial fever, depression, headache etc. In heavy infection, accumulation of living or dead worms eventually blocks the lymphatic vessels and glands, resulting in various pathological conditions. In conjunctival filariasis the worm larvae migrate to eyes and if untreated can cause blindness.

The disease is diagnosed by screening blood samples for specific proteins produced by the immune system in response to infection. Inflammation and signs of lymphatic obstruction are also diagnostic features. The microfilariae can also be found in the blood but samples should be collected in the night.

No satisfactory treatment is available, although injections of antimony compounds eradicate the worms in a prolonged treatment.