Wuchereria bancrofti

Wuchereria bancrofti

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, Mansonia, and Aedes. 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.

Morphology

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, is ovoviviparous, 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.

Biology

In humans, the adult W. bancrofti reside in the lymphatic ducts and are found mostly in the lymph glands of the afferent lymphatic channels in the lower part of the body. The microfilariae produced by the female worms have a membrane "sheath". This "sheath", along with the area in which the worms reside, makes identification of the type of species of microfilariae in humans easier to determine. The microfilariae are found mainly in the peripheral blood and can be found at peak amounts from 10p.m. to 2 a.m. The cause of this periodicity remains unknown but the advantages of the microfilariae being in the peripheral blood during these hours may ensure that the vector, the nighttime mosquito, will have a higher chance of transmitting them elsewhere. In the South Pacific, where W. bancrofti shows diurnal periodicity it is known as subperiodic.

Pathogenesis

The pathogenesis of W. bancrofti infection is dependent on the host's immune system and inflammatory responses. After infection, the worms will mature within 6-8 months and then the release of the microfilariae will begin. These microfilariae worms can be released for up to ten years.

1. Asymptomatic Phase -Usually consists of high microfilaremia infection and individuals show no symptoms of being infected. This occurs due to the cytokine IL-4 suppressing the activity of TH1 cells in our immune system. This can occur for years until the inflammatory reaction rise again. 2. Inflammatory (Acute) Phase -The antigens from the female adult worms elicit inflammatory responses. The worms in the lymph channels disrupt the flow of the lymph causing lymphedema. The individual will exhibit fever, chills, skin infections, painful lymph nodes, and tender skin of the lymphedematous extremity. These symptoms often lessen after 5-7 days. Other symptoms that may occur include: orchitis-inflammation of the testes, which is accompanied by painful immediate enlargement and epididymitis-which is the inflammation of the spermatic cord. 3. Obstructive (Chronic) Phase -marked by lymph varices, lymph scrotum, hydrocele, chyluria(lymph in urine), and elephantiasis. Microfilariae are not normally present in this phase. A key feature of this phase is scar formation from affected tissue areas. Other features include thickening of the skin and elephantiasis which develops gradually with the attack of the lymphatic system. Elephantiasis affect men mainly on the legs, arms, and scrotum. In women, the legs and arms are affected.

Treatment/Control

The parasite's severe symptoms can be avoided by cleansing the skin, surgery, or the use of therapeutic drugs, such as Diethylcarbamazine(DEC), ivermectin, or albendazole. The drug of choice however, is DEC, which can eliminate the microfilariae from the blood and also kill the adult worms with a dosage of 6 mg/kg semiannually or annually. A treatment that includes ivermectin with DEC or albendazole is more effective than each drug alone. Protection is similar to that of other mosquito spread illnesses; one can use barriers both physical (a mosquito net), chemical (insect repellent), or mass chemotherapy′ as a method to control the spreading of the disease.

Trivia

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.
  • Roberts, Larry S. and Janovy, John Jr., Foundations of Parasitology 7th Edition. 2005. pages 461-466.

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