Wuchereria bancrofti
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| Microfilaria of Wuchereria bancrofti, from a patient seen in Haiti. Thick blood smears stained with hematoxylin. Microfilaria of Wuchereria bancrofti, from a patient seen in Haiti. Thick blood smears stained with hematoxylin.
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]
Overview
Wuchereria bancrofti is a parasitic filarial nematode worm spread by a mosquito vector. It is one of the three parasites that cause lymphatic filariasis. Named for Otto Wucherer and Joseph Bancroft, it affects over 120 million people, primarily in Africa, South America, and other tropical and sub-tropical countries. Elephantiasis can result if the infection is left untreated. Limited treatment modalities exist and no vaccines have been developed.
W. bancrofti carry out their life cycle in two hosts. Human beings serve as the definitive host and mosquitoes as their intermediate hosts. The adult parasites reside in the lymphatics. They are viviparous. The first stage larvae are known as microfilariae. The microfilaria are present in the circulation. The microfilaria migrate between the deep and the peripheral circulation. During the day they are present in the deep veins and during the night the migrate to the peripheral circulation. Next, the worm is transferred into a vector; the most common vectors are the mosquito species: Culex, Anopheles, Aedes, and Mansonia. Inside their second host, it matures into motile larvae. When its current host feeds, and it is egested into the blood stream of its new human host. The larvae moves to the lymph nodes, predominantly in the legs and genital area, and develops into adult worm over the course of a year. By this time, an adult female can produce microfilariae itself.
W. bancrofti displays a large size gap between the male and female—a difference known as sexual dimorphism. The adult male worm is long and slender, between four and five centimeters in length, a tenth of a centimeter in diameter, and features a curved tail. The female, in contrast, is six to ten centimeters long, and three times larger in diameter than the male. This size deviation can be attributed to the vast numbers of microfilariae that the female produces each day.
The onset of symptoms is slow, but the effects are very apparent after several years. During the initial inflammatory stage, a host can exhibit swelling, granulation lesions, and impaired circulation. Following, the lymph nodes are enlarged and dilated. They become hardened and clogged with fibrous tissue, and this prevents the lymphatic system from operating correctly. The microfilariae also cause swelling, thickening, and discolouration of the skin. Without the proper drainage of fluids, the affected tissue will expand and elephantiasis, a gross expansion of body, will result, followed sometimes by death.
The parasite's severe symptoms can be avoided by the use of therapeutic drugs. Both diethylcarbamazine and sodium caparsolate are used to kill the worms and their microfilariae. Diethylcarbamazine is most commonly used and is administered orally. Protection is similar to that of other mosquito spread illnesses; one can use barriers both physical (a mosquito net) and chemical (insect repellent).
Trivia
- Joseph Merrick, "The Elephant Man", was not afflicted by a parasitic worm, but by Proteus syndrome.
References
- Oetinger, David. "Filaria." The World Book Encyclopedia. Chicago: World Book Inc. 2000.
- "Elephantiasis." Human Diseases and Conditions. New York: Charles Scribner's Sons. 2000.
- David F. Oetinger, "Filaria." World Book Online Reference Center. http://www.worldbookonline.com/ar?/na/ar/co/ar196440.htm, November 28, 2003.
- "Lymphatic filariasis." World Health Organization. http://www.who.int/ctd/filariasis/home/, November 28, 2003.