filarial worm

filarial worm

Any of a group of parasitic nematodes that usually require two hosts to complete the life cycle: an arthropod and another animal, which is bitten by the arthropod. The female worm produces large numbers of microscopic, active embryos, called microfilariae, that pass into the bloodstream of the primary host. These enter the body of an insect when it bites the infected animal; within the insect the microfilariae grow into larvae, which are passed to an animal the insect bites, where they complete their growth. Seealso heartworm.

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See special page for Filariasis in domestic animals

Lymphatic Filariasis (Philariasis) is a parasitic and infectious tropical disease, that is caused by thread-like parasitic worms, of the type filarial nematode. Examples are Wuchereria bancrofti, Brugia malayi, and Brugia timori. The parasites are transmitted by insect bites, usually mosquitoes. Lymphatic filariasis is extremely rare in Western countries.

Presentation

The most spectacular symptom of lymphatic filariasis is elephantiasis—thickening of the skin and underlying tissues—which was the first disease discovered to be transmitted by a mosquito bite. Elephantiasis is caused when the parasites lodge in the lymphatic system.

Elephantiasis affects mainly the lower extremities, whereas ears, mucus membranes, and amputation stumps are rarely affected; however, it depends on the species of filaria. W. bancrofti can affect the legs, arms, vulva, breasts, while Brugia timori rarely affects the genitals.

Incidence/prevalence

Filariasis is endemic in tropical regions of Asia, Africa, Central and South America with more than 120 million people infected and one billion people at risk for infection.

In communities endemic with lymphatic filariasis, as many as 10 percent of women can be affected with swollen limbs and 50 percent of men can suffer from mutilating genital disease.

History

Lymphatic Filariasis is thought to have affected humans since approximately 1500-4000 years ago. The first clear reference to the disease occurs in ancient Greek literature, where scholars discuss diagnosis of lymphatic filariasis vs. diagnosis of similar symptoms that can result from leprosy.

The first documentation of symptoms occurred in the 16th century, when Jan Huygen Linschoten wrote about the disease during the exploration of Goa. Soon after, exploration of other parts of Asia and Africa turned up further reports of disease symptoms. It was not until centuries later than an understanding of the disease began to develop.

In 1866, Timothy Lewis, building on the work of Jean-Nicolas Demarquay and Otto Henry Wucherer, made the connection between microfilariae and elephantiasis, establishing the course of research that would ultimately explain the disease. Not long after, in 1876, Joseph Bancroft discovered the adult form of the worm, and finally in 1877 the life cycle involving an arthropod vector was theorized by Patrick Manson, who proceeded to demonstrate the presence of the worms in mosquitoes. Manson incorrectly hypothesized that the disease was transmitted through skin contact with water in which the mosquitoes had laid eggs. In 1900, George Carmichael Low determined the actual transmission method by discovering the presence of the worm in the proboscis of the mosquito vector.

Diagnosis

The diagnosis is made by identifying microfilariae on a Giemsa stained thick blood film. Blood must be drawn at night, since the microfilaria circulate at night, when their vector, the mosquito, is most likely to bite.

There are also PCR assays available for making the diagnosis.

Treatment

The recommended treatment for killing adult filarial worm for patients outside the United States is albendazole (a broad spectrum anti-helminthic) combined with ivermectin. A combination of diethylcarbamazine (DEC) and albendazole is also effective.

Antibiotics as a possible treatment

In 2003 it was suggested that the common antibiotic doxycycline might be effective in treating elephantiasis. The parasites responsible for filariasis have a population of symbiotic bacteria, Wolbachia, that live inside the worm. When the symbiotic bacteria are killed by the antibiotic, the worms themselves also die. Clinical trials in June 2005 by the Liverpool School of Tropical Medicine reported that an 8 week course almost completely eliminated microfilariaemia.

Eradication efforts

In 1993, the International Task Force for Disease Eradication declared lymphatic filariaisis one of six potentially eradicable diseases. Studies have demonstrated that transmission of the infection can be broken when a single dose of combined oral medicines is consistently maintained annually for approximately seven years. With consistent treatment, the reduction of microfilariae means the disease will not be transmitted, the adult worms will die out, and the cycle will be broken.

The strategy for eliminating transmission of lymphatic filariasis is mass distribution of medicines that kill the microfilariae and stop transmission of the parasite by mosquitoes in endemic communities. In sub-Saharan Africa, albendazole (donated by GlaxoSmithKline) is being used with ivermectin (donated by Merck & Co.) to treat the disease, whereas elsewhere in the world albendazole is used with diethylcarbamazine. Using a combination of treatments better reduces the number of microfilariae in blood. The use of insecticide-treated mosquito bed nets can also be used to help stop transmission of lymphatic filariasis as well as help control malaria, which is prevalent in many of the same communities in Africa.

Because of the efforts of the Global Programme to Eliminate LF, it is estimated that 6.6 million children have been kept from catching the condition, and stopped it from progressing in another 9.5 million people who already have it. Dr Mwele Malecela, who chairs the programme, said: "We are on track to accomplish our goal of elimination by 2020."

See also

*Elephantiasis
*Neglected diseases
*Eradication of infectious disease

References

External links

* (2005). "Special issue". Indian Journal of Urology 21 (1):
* BBC News article on Lymphatic filariasis
* Filariasis Research at the University of Tuebingen
* The Carter Center Lymphatic Filariasis Elimination Program

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