Etiology and Epidemiology

The threadlike, white, adult W. bancrofti (males: 40 mm long x 0.1 mm in diameter; females: 65-100 mm x 0.2-0.3 mm) lie coiled together in human lymphatic vessels and lymph glands, where they can live for 10 to 18 years. Within 6 months to 1 year of infection, tiny larvae called microfilariae leave the adult female and enter the host's peripheral blood and lymph channels. Microfilariae move freely through the lymph or blood and, depending on the strain, show nocturnal or diurnal periodicity in the blood. Nocturnal microfilariae (the most common infective form) reside in the arterioles of the lungs during the day, whereas the diurnal (also called subperiodic) strain appears in the peripheral blood continuously, although in reduced numbers at night. Geographically, nocturnally periodic microfilariae are generally found west of 140° east longitude, and diurnal microfilariae are present east of 180° east longitude. Both types may be found between these two meridians. The largest concentrations of diurnal microfilariae exist in the Polynesian and New Caledonian regions of the Pacific Ocean (Sasa 1976; Beaver et al. 1984; Manson-Bahr and Bell 1987).

Bancroftian filariasis is transmitted only by mosquito. There is no known animal reservoir of W. bancrofti. Microfilariae may be transmitted to another human through blood transfusion, and from the maternal to the fetal circulation, but in both situations the microfilariae never develop into adults (Beaver et al. 1984; Manson-Bahr and Bell 1987).

Microfilariae have adapted their daily cycles to either day- or night-feeding mosquitoes, depending on the species and activity of these insects in a particular geographic area. The mosquito becomes an intermediate host of microfilariae after taking a blood meal. Microfilariae develop into infective larvae within the insect host in less than 2 weeks, and escape from its proboscis onto the skin of the mosquito's next human host during feeding. The larvae burrow into the human's skin through the tiny puncture wound and find their way to lymph vessels where they mature within a year and mate, producing more microfilariae (Beaver et al. 1984; Manson-Bahr and Bell 1987).

In a given geographic area, the microfilaria rate (percentage of a given population carrying mi crofilariae in the peripheral blood), the density of the intermediate host mosquito population, and the presence of a susceptible human population available for repeated bites by infected mosquitoes are key factors in the epidemiology of bancroftian filariasis. The significance of filariasis in an area may be measured by either the microfilaria rate or the actual disease rate (percentage of the population displaying symptoms of filarial infection) (Beaver et al. 1984; Manson-Bahr and Bell 1987).

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