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Introduction Wuchereria bancrofti
is a nematode causing lymphatic filariasis throughout the tropics and subtropics. There are two strains of W. bancrofti; the nocturnal periodic strain which is widely distributed in endemic regions, the
microfilariae being in their highest concentrations between the hours of 10pm and 2am, and the sub-periodic strain which is found in the Pacific region, and has a microfilaraemia all the time with the highest numbers
being detected between noon and 8pm. Humans are the only known reservoir host of W. bancrofti. Life cycle and morphology
The adult worm inhabits the lymphatics and the female produces sheathed microfilariae which circulate in the peripheral blood. The mosquito acquires the infection by ingestion of the microfilaria in the blood meal. The microfilariae lose their sheath on arrival in the stomach of the mosquito. The larvae migrate to the thoracic muscles and develop into infective larvae over a period of 6 - 14 days. The larvae then migrate to the mouth parts of the mosquito and enter the skin of the definitive host through the puncture wound when a blood meal is taken. The infective larvae enter the peripheral lymphatics where they grow to mature male and female worms. After mating, the gravid females release sheathed microfilariae which can be detected in the peripheral blood 8 - 12 months after initiation of infection. The tail of the microfilariae of W. bancrofti tapers to a delicate point and exhibits no terminal nuclei and can thus be easily distinguished from the microfilariae of Brugia malayi and Loa loa, the other sheathed microfilariae of clinical importance. The sheath the microfilariae of W. bancrofti stains pink with Giemsa. The microfilariae are 230 - 275 m in length.
Microfilaria of W. bancrofti exhibiting a sheath (Giemsa stain)
Tail of W. bancrofti free from nuclei (Giemsa stain) Clinical disease
Many patients are asymptomatic. Patients may present with fever. Lymphangitis and lymphadenitis develop in the lower extremities and there may also be genital and breast involvement. An inflammatory
reaction occurs in the lymphatic vessels that harbour the adult worms. Oedema develops which may resolve after the first few attacks. However, in long standing disease after several episodes of
lymphangitis, thickening and verrucous changes in the skin known as elephantiasis develop. Secondary bacterial and fungal infections may occur in patients with long-standing elephantiasis.
Obstruction of the genital organs may result in hydrocoele formation and scrotal lymphoedema. Obstruction of the retroperitoneal lymphatics may cause the renal lymphatics to rupture into the urinary
tract producing chyluria. Some patients with filariasis do not exhibit microfilaraemia. but develop tropical pulmonary eosinophilia
which is characterised by peripheral eosinophilia, wheeze and cough. High eosinophilia, high IgE level and high anti-filarial antibody titres are features of this syndrome. |
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| [UKNEQAS Parasitology] [Teaching Information] [Intended results] [UKNEQAS Study Day] |