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- See special page for Filariasis in domestic animals
Filariasis (Philariasis) is a parasitic and infectious tropical disease, that is caused by thread-like filarial nematode worms in the superfamily Filarioidea,[1] also known as "filariae".[2] 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 the skin, the 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.
Presentation
The most spectacular symptom of lymphatic filariasis is elephantiasis—edema with 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 affect 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.
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.[3]
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.[4]
History
Lymphatic Filariasis is thought to have affected humans since approximately 4000 years ago [5]. Artifacts from ancient Egypt (2000 BC) and the Nok civilization in West Africa (500 BC) show possible elephantiasis symptoms. The first clear reference to the disease occurs in ancient Greek literature, where scholars differentiated the often similar symptoms of lymphatic filariasis from those of leprosy.
The first documentation of symptoms occurred in the 16th century, when Jan Huyghen van Linschoten wrote about the disease during the exploration of Goa. Similar symptoms were reported by subsequent explorers in areas of Asia and Africa, though an understanding of the disease did not began to develop until centuries later.
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. In 1876, Joseph Bancroft discovered the adult form of the worm. 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.[5]
Diagnosis
Filariasis is usually diagnosed by identifying microfilariae on a Giemsa stained thick blood film. Blood must be drawn at night, since the microfilaria circulate at night(nocturnal periodicity), when their mosquito vector is most likely to bite. Also,decreased peripheral temperature may attract more microfilariae.
Concentration Methods
Various concentration methods are applied:
- i. Membrane filter
- ii. Knott's concentration method
- iii. Sedimentation technique
Polymerase chain reaction (PCR) and antigenic assays are also available for making the diagnosis. The latter are particularly useful in amicrofilaraemic cases. Lymph Node aspirrate,Chylus fluid may also yield Microfilriae. Imaging like CT,MRI may reveal "Filarial Dance Sign" in Chylus fluid. X-ray can show calcified adult worm in lymphatics. DEC provokation test is performed to obtain satisfying number of parasite in day-time samples. Circulating Filarial Antigen (CFA) may be detected by PCR. Xenodiagnosis is now obsolete EOsinophilia is a non-specific primary sign.
Treatment
The recommended treatment for killing adult filarial worms in patients outside the United States is albendazole (a broad spectrum anthelmintic) combined with ivermectin.[6][4] A combination of diethylcarbamazine (DEC) and albendazole is also effective.[4]
In 2003 the common antibiotic doxycycline was suggested for treating elephantiasis.[7] Filarial parasites have symbiotic bacteria in the genus Wolbachia, which 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 microfilaraemia.[8][9]
Eradication efforts
In 1993, the International Task Force for Disease Eradication declared lymphatic filariaisis to be one of six potentially eradicable diseases.[4] 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.[10] 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.[10]
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.[10] 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.[4] Using a combination of treatments better reduces the number of microfilariae in blood.[10] The use of insecticide-treated mosquito bed nets also reduces the transmission of lymphatic filariasis as well as malaria, which is prevalent in many of the same communities in Africa.[10][11]
The efforts of the Global Programme to Eliminate LF are estimated to have already prevented 6.6 million new filariasis cases from developing in children, and to have stopped the progression of the disease in another 9.5 million people who have already contracted it. Dr Mwele Malecela, who chairs the programme, said: "We are on track to accomplish our goal of elimination by 2020."[12]
See also
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References
- ^ MeSH Filariasis
- ^ filariasis at Dorland's Medical Dictionary
- ^ The Carter Center (2002-10) (PDF), "Summary of the Third Meeting of the International Task Force for Disease Eradication", http://www.cartercenter.org/documents/1181.pdf, retrieved on 2008-07-17
- ^ a b c d e The Carter Center, "Lymphatic Filariasis Elimination Program", http://www.cartercenter.org/health/lf/index.html, retrieved on 2008-07-17
- ^ a b "Lymphatic Filariasis Discovery". http://www.stanford.edu/class/humbio103/ParaSites2006/Lymphatic_filariasis/Discovery.htm. Retrieved on 2008-11-21.
- ^ U.S. Centers for Disease Control, "Lymphatic Filariasis Treatment", http://www.cdc.gov/ncidod/dpd/parasites/lymphaticfilariasis/treatment_lymphatic_filar.htm, retrieved on 2008-07-17
- ^ Hoerauf A, Mand S, Fischer K, Kruppa T, Marfo-Debrekyei Y, Debrah AY, Pfarr KM, Adjei O, Buttner DW (2003). "Doxycycline as a novel strategy against bancroftian filariasis-depletion of Wolbachia endosymbionts from Wuchereria bancrofti and stop of microfilaria production". Med Microbiol Immunol (Berl) 192 (4): 211–6. doi:10.1007/s00430-002-0174-6. PMID 12684759.
- ^ Taylor MJ, Makunde WH, McGarry HF, Turner JD, Mand S, Hoerauf A (2005). "Macrofilaricidal activity after doxycycline treatment of Wuchereria bancrofti: a double-blind, randomised placebo-controlled trial". Lancet 365 (9477): 2116–21. doi:10.1016/S0140-6736(05)66591-9. PMID 15964448.
- ^ Outland, Katrina (2005 Volume 13). "New Treatment for Elephantitis: Antibiotics". The Journal of Young Investigators. http://www.jyi.org/news/nb.php?id=361.
- ^ a b c d e The Carter Center, "How is Lymphatic Filariasis Treated?", http://www.cartercenter.org/health/lf/treatment.html, retrieved on 2008-07-17
- ^ U.S. Centers for Disease Control and Prevention, "Preventing Two Diseases with One Net", http://www.cdc.gov/malaria/features/nigeria_bednets_program.htm, retrieved on 2008-07-17
- ^ BBC World Service, "'End in sight' for elephantiasis", http://news.bbc.co.uk/2/hi/health/7659222.stm, retrieved on 2008-10-08
External links
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