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Lymphatic filariasis |
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It has been suggested that this article or section be merged with elephantiasis. (Discuss) |
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This article may require cleanup to meet Wikipedia's quality standards. Please improve this article if you can. (March 2008) |
| Filariasis Classification and external resources |
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| ICD-10 | B74. |
| ICD-9 | 125.0-125.9 |
| MeSH | D005368 |
Filariasis (Philariasis) is a parasitic and infectious tropical disease, that is caused by thread-like filarial nematode worms. There are 9 known filarial nematodes which use humans as the definitive host. These are divided into 3 groups according to the niche within the body that they occupy: Lymphatic Filariasis, Subcutaneous Filariasis, and Serous Cavity Filariasis. Lymphatic Filariasis is caused by the worms Wuchereria bancrofti, Brugia malayi, and Brugia timori. These worms occupy the lymphatic system, including the lymph nodes, and in chronic cases these worms lead to the disease Elephantiasis. Subcutaneous Filariasis is caused by Loa Loa (the African eye worm), Mansonella streptocerca, Onchocerca volvulus, and Dracunculus medinensis (the guinea worm). These worms occupy the subcutaneous layer of our skin, our fat layer. Serous Cavity Filariasis is caused by the worms Mansonella perstans and Mansonella ozzardi, which occupy the serous cavity of the abdomen. In all cases, the transmitting vectors are either blood sucking insects (fly or mosquito) or Copepod crustaceans in the case of Dracunculus medinensis.
Human filarial nematode worms have a complicated life cycle, which primarily consists of five stages. After the male and female worm mate, the female gives birth to live microfilariae by the thousands. The microfilariae are taken up by the vector insect (intermediate host) during a blood meal. In the intermediate host, the microfilariae molt and develop into 3rd stage (infective) larvae. Upon taking another blood meal the vector insect injects the infectious larvae into the dermis layer of our skin. After approximately one year the larvae molt through 2 more stages, maturing into to the adult worm.
Individuals infected by filarial worms may be described as either "microfilaraemic" or "amicrofilaraemic," depending on whether or not microfilaria are found in their peripheral blood. Filariasis is diagnosed in microfilaraemic cases primarily through direct observation of microfilaria in the peripheral blood. Occult filariasis is diagnosed in amicrofilaraemic cases based on clinical observations and, in some cases, by finding a circulating antigen in the blood.
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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 results when the parasites lodge in the lymphatic system.
Elephantiasis affects mainly the lower extremities, while the ears, mucus membranes, and amputation stumps are affected less frequently. However, different species of filarial worms tend to effect different parts of the body: Wuchereria bancrofti can affect the legs, arms, vulva, and breasts, while Brugia timori rarely affects the genitals. Interestingly, those who develop the chronic stages of elephantiasis are usually amicrofilaraemic, and often have adverse immunlogical reactions to the microfilaria as well as the adult worm.
The subcutaneous worms present with skin rashes, urticarial papules, and arthritis, as well as hyper- and hypopigmentation macules. Onchocerca volvulus manifests itself in the eyes causing "river blindness" (onchocerciasis), the 2nd leading cause of blindness in the world. Serous cavity filariasis presents with symptoms similar to subcutaneous filariasis, in addition to abdominal pain because these worms are also deep tissue dwellers.
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.1
In communities where lymphatic filariasis is endemic, as many as 10 percent of women can be afflicted with swollen limbs, and 50 percent of men can suffer from mutilating genital symptoms.2
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.3
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.
The recommended treatment for killing adult filarial worm for patients outside the United States is albendazole (a broad spectrum anti-helminthic) combined with ivermectin.42 A combination of diethylcarbamazine (DEC) and albendazole is also effective.2
In 2003 it was suggested that the common antibiotic doxycycline might be effective in treating elephantiasis.5 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.67
In 1993, the International Task Force for Disease Eradication declared lymphatic filariaisis one of six potentially eradicable diseases.2 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.8 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.8
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.8 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.2 Using a combination of treatments better reduces the number of microfilariae in blood.8 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.89
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."10
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