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Geographic Distribution

The WHO Expert Committee on Filariasis (October, 1991) estimated that 3287 million persons lived in countries in which lymphatic filariasis is endemic. Of these, some 79 million people were infected. Recognizing that statistical information may be unreliable, particularly from many African countries, these figures likely underestimate the extent of the disease. In China, in the Sahandong Province before the antifilarial campaign was started, there were 50 million people at risk and over 2.5 million who were affected. The prevalence of elephantiasis and hydrocele was 6.1%. About 14% of mosquitoes were infected and over half of these carried the significant larval stage. Although the campaign has changed these figures locally, they are an indication of the extent of the disease and reflect the situation in many other parts of the world.

Wuchereria bancrofti is found in East, Central, and West Africa, on the coast of Central and South America, in some of the Caribbean islands, in India, Bangladesh, Myanmar (Burma), Thailand, Malaysia, and Indonesia, as well as other parts of Asia, Melanesia, Samoa, and the neighboring islands. It is spread in particular among urban populations.

Brugia malayi is found in southern China, South Korea, Japan, Vietnam, Thailand, Malaysia, Borneo, Sumatra, Indonesia, New Guinea, Myanmar (Burma), the eastern states of India, and Sri Lanka.

Brugia timori is only found in the lesser Sunda islands of Indonesia.

Epidemiology and Pathology

Each species has a similar, complex life cycle. Lymphatic filariasis, caused by Wuchereria bancrofti and Brugia spp., starts when a mosquito infected with filariform third-stage larvae bites a human host (Fig. 26.1). Further molting and maturation occurs in humans, with the male and female adult worms mating and ultimately residing in lymphatic vessels. This process of development in humans is still little understood, but probably takes months to complete. Significant illness in the human host may then take years to develop.

Fig. 26.1. The life cycle of human filariasis

Each fertilised adult female worm releases a large number of microfilariae into the lymphatics over a period of years. These embryonic first-stage larvae circulate in a nocturnal or subperiodic pattern that corresponds to the biting habits of the mosquito vector, surviving and circulating for 1-2 years until ingested by a suitable mosquito (Fig. 26.2). Within the mosquito vector, further development and molting produces infective third-stage larvae. When another human is bitten, the life cycle is complete.There are three interesting and clinically important patterns of microfilarial periodicity. The microfilarial parasitemia in W. bancrofti is nocturnal, increasing in the evening to reach a maximum between midnight and 4 a. m. before declining from the peripheral circulation (perhaps an evolutionary adaptation of the parasite to facilitate infection of the nocturnal host mosquito and propagation of the life cycle). In contrast, B. malayi has a subperiodic microfilaremia in some regions that matches the diurnal or crepuscular biting habits of Mansonia spp. mosquitoes. The periodicity is important because it determines the time when blood samples from patients will most likely show microfilariae. Although the basis of the periodicity is unknown, the pattern can be altered by reversing the patient's sleep/wake cycle.

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Fig. 26.2. A Peripheral blood smear showing numerous microfilariae that can be recognized easily in a low-power field. B Sheathed microfilaria of B. malayi. The tail is tapered and there is a constriction which separates the terminal nuclei. x630. (From Marty and Anderson 1995) C,D The microfilaria of W. bancrofti. The tail is tapered and there is no constriction.

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