Wuchereria bancrofti is a parasitic filarial nematode (roundworm) spread by a mosquito vector. It is one of the three parasites that cause lymphatic filariasis, an infection of the lymphatic system by filarial worms. If the infection is left untreated, it can develop into a chronic disease called elephantiasis.
Habit and habitat
- Wuchereria bancrofti is a dreaded human endoparasite of human blood and lymph.
- It is a digenetic parasite completing its life cycle in two hosts. Final host is man, harboring the adult worms, while the intermediate host is blood-sucking mosquito.
- Adult worms live coiled up in the lymph glands and lymph passages of man, where they often obstruct the flow of lymph.
- Adult worms are filiform and cylindrical in shape and both body ends terminate bluntly. They are creamy white in colour.
- Sexes are separate and there is a distinct sexual dimorphism.
- Mouth aperture is simple, without lips.
- Pharynx or oesophagus is divisible into an anterior muscular portion and a posterior glandular portion.
- Intestine is simple, as in other nematodes.
- Posterior end of male is sharply curved ventrally, containing a number of genital papillae, two unequal copulatory spicules.
- Vulva or genital pore of female is located ventrally in the pharyngeal region, and provided with pyriform ejector mechanism or ovijector.
- Female measures 65 to 100 mm in length and 0-25 mm in diameter, while male measures 40 to 50 mm in length and 0-1 mm in diameter.
- Thus males are smaller than females.
W. bancrofti completes its life cycle in two hosts. The definitive host is man and vector (female mosquito) is intermediate host. The major vector in India and most other parts of Asia is Culex fatigans, Aedes and anopheles species of mosquito also serve as intermediate host.
Development In man:
In man adult worms live in the lymphatic vessels and glands. Most often, males and females remain so coiled together that it becomes difficult to separate them. Copulation takes place when individuals of both sexes are present in the same lymph gland.
- In human body, the female worm gives birth to embryos, called microfilariae.
- Thus females are viviparous (probably ovoviviparous).
- Microfilariae are born in a very immature stage. They are microscopic, surrounded by a delicate cuticular sheath and containing rudiments of various adult structures.
- Body of a microfilaria consists of a surface covering of flattened epidermal cells and an inner column of cytoplasm containing nuclei. Important structures from anterior end backwards are: future mouth, nerve ring band, nephridiopore, dark staining inner mass, 4 large cells and future anus.
- Microfilariae discharged into lymph vessels, soon enter blood vessels and circulate with blood showing active movements. They migrate to reside ultimately in deeper blood vessels of thorax, but they do not undergo further development until sucked by the intermediate host. i.e., mosquito.
- Further, the microfilariae migrate through the peripheral blood and skin, from where they are taken up by blood-sucking mosquitoes.
- In blood of man, microfilariae show day and night periodicity.
- During day they live in large deep-located blood vessels, but at night or during sleep they come into peripheral vessels of skin, to be sucked by nocturnal feeding mosquitoes, which serve as intermediate host.
- In places where the mosquitoes diurnal in feeding habit, microfilariae reverse this periodicity. Micro-filariae at this stage in human blood eventually die unless ingested by blood-sucking mosquitoes from infected humans.
Development In mosquito:
- In the stomach of mosquito, microfilariae lose their sheaths, penetrate the stomach wall and migrate to thoracic muscles, where they moult twice and become ineffective.
- They change first to a plump sausage-shaped organism, later to a more elongated form, and finally to a long slender juvenile of third infective stage.
- Microfilariae undergo two moults in about 10 days to reach the third stage larvae which are about 1-5 mm long.
- Infective third stage microfilariae now migrate into mosquito’s labium (proboscis).
Infection to new human host:
- When this mosquito takes another meal of blood of another potential human host, the infective third-stage larvae are injected into the blood of the human host through the mosquito’s proboscis.
- This probably penetrates through the wound made by mosquito and enters the blood of human host.
- The final two moults take place as the larvae enter the lymphatic vessels.
- In new human host, juveniles pass into lymph glands and lymph passages, where they coil up and develop into adult worms.
- Adult worms copulate and the female deliver microfilariae.
Filariasis is caused by blockage of lymph channels by Wuchereria bancrofti, the young accumulating in blood vessels near the skin. These elongate, threadlike wormslive in the lymphatic system, where they block the vessels. Because lymphatic vessels return tissue fluids to the circulatory system, when the filarial worms block these
vessels, fluid tend to accumulate in peripheral tissues. This fluid accumulation causes the enlargement of various appendages, a condition called elephantiasis.
The outcome of filarial infection varies in different persons. In some patients, especially from endemic areas, infection may be entirely asymptomatic even with high microfilarial density in peripheral blood (20000 per ml.). In other patients, particularly coming from non-endemic areas, infection may cause early clinical manifestations characterized by fever lymphangitis with chills and recurrent febrile attacks. Clinical incubation period usually varies from 8 to 16 months.
A. Lymphatic filariasis
(a) Early infection: Patients experience fever, lymphangitis, headaches, nausea and urticaria.
(b) Chronic infection: Repeated attacks cause lymphatic damage.
1. Inflamation: Masses of adult worms result in allergic lymphangitis.
2. Dilation of lymphatics: The presence of adult worms cause lymphadenitis and duct dilation, leading to lymph retention.
3. Elephantasis: Lymphatic obstruction leads to swelling, fibrosis and eventually elephantiasis.
B. Occult filariasis:
The term occult filariasis refers to a condition which is caused by a hypersensitivity reaction to microfilarial antigens and is characterized by massive eosinophilia and absence of microfilaraemia (The presence of microfilariae in the host bloodstream is called “microfilaraemia”).
Prevention and control
1. Eradication of vector mosquito is one of the major measures of prevention.
2. Education in respect of filarial infection and its vector is essential.
3. Protection from bites of insect by the use of screening, mosquito net, insect repellants, along with treatment of infected cases and carrier is critical during the incidence of infection
1. The most effective therapeutic drugs are diethylcarbamazine (DEC) and sodium caparsolate, which kill the adult worms and microfilariae. However, killing of parasites may induce severe allergic reactions that require treatment with corticosteroids.
2. Ivermectin destroys microfilaria, but not adults.
3. Antibiotic soaps and topical creams may be helpful in reducing elephantiasis.