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Filiariasis is the name for a group of tropical diseases caused by various thread-like parasitic round worms (nematodes) and their larvae. The larvae transmit the disease to humans through a mosquito bite. Filariasis is characterized by fever, chills, headache, and skin lesions in the early stages and, if untreated, can progress to include gross enlargement of the limbs and genitalia in a condition called elephantiasis.
Approximately 170 million people in the tropical and subtropical areas of southeast Asia, South America, Africa, and the islands of the Pacific are affected by this debilitating parasitic disease. While filariasis is rarely fatal, it is the second leading cause of permanent and long-term disability in the world. The World Health Organization (WHO) has named filariasis one of only six "potentially eradicable" infectious diseases and has embarked upon a 20-year campaign to eradicate the disease.
In all cases, a mosquito first bites an infected individual then bites another uninfected individual, transferring some of the worm larvae to the new host. Once within the body, the larvae migrate to a particular part of the body and mature to adult worms. Filariasis is classified into three distinct types according to the part of the body that becomes infected: lymphatic filariasis affects the circulatory system that moves tissue fluid and immune cells (lymphatic system); subcutaneous filariasis infects the areas beneath the skin and whites of the eye; and serous cavity filariasis infects body cavities but does not cause disease. Several different types of worms can be responsible for each type of filariasis, but the most common species include the following: Wucheria bancrofti, Brugia malayi (lymphatic filariasis), Onchocerca volvulus, Loa loa, Mansonella streptocerca, Dracunculus medinensis (subcutaneous filariasis), Mansonella pustans, and Mansonella ozzardi (serous cavity filariasis).
The two most common types of the disease are Bancroftian and Malayan filariasis, both forms of lymphatic filariasis. The Bancroftian variety is found throughout Africa, southern and southeastern Asia, the Pacific islands, and the tropical and subtropical regions of South America and the Caribbean. Malayan filariasis occurs only in southern and southeastern Asia. Filariasis is occasionally found in the United States, especially among immigrants from the Caribbean and Pacific islands.
A larvae matures into an adult worm within six months to one year and can live between four and six years. Each female worm can produce millions of larvae, and these larvae only appear in the bloodstream at night, when they may be transmitted, via an insect bite, to another host. A single bite is usually not enough to acquire an infection, therefore, short-term travelers are usually safe. A series of multiple bites over a period of time is required to establish an infection. As a result, those individuals who are regularly active outdoors at night and those who spend more time in remote jungle areas are at an increased risk of contracting the filariasis infection.
Causes and symptoms
In cases of lymphatic filariasis, the most common form of the disease, the disease is caused by the adult worms actually living in the lymphatic vessels near the lymph nodes where they distort the vessels and cause local inflammation. In advanced stages, the worms can actually obstruct the vessels, causing the surrounding tissue to become enlarged. In Bancroftian filariasis, the legs and genitals are most often involved, while the Malayan variety affects the legs below the knees. Repeated episodes of inflammation lead to blockages of the lymphatic system, especially in the genitals and legs. This causes the affected area to become grossly enlarged, with thickened, coarse skin, leading to a condition called elephantiasis.
In conjunctiva filariasis, the worms' larvae migrate to the eye and can sometimes be seen moving beneath the skin or beneath the white part of the eye (conjunctiva). If untreated, this disease can cause a type of blindness known as onchocerciasis.
Symptoms vary, depending on what type of parasitic worm has caused the infection, but all infections usually begin with chills, headache, and fever between three months and one year after the insect bite. There may also be swelling, redness, and pain in the arms, legs, or scrotum. Areas of pus (abscesses) may appear as a result of dying worms or a secondary bacterial infection.
The disease is diagnosed by taking a patient history, performing a physical examination, and by screening blood specimens for specific proteins produced by the immune system in response to this infection (antibodies). Early diagnosis may be difficult because, in the first stages, the disease mimics other bacterial skin infections. To make an accurate diagnosis, the physician looks for a pattern of inflammation and signs of lymphatic obstruction, together with the patient's possible exposure to filariasis in an area where filariasis is common. The larvae (microfilariae) can also be found in the blood, but because mosquitos, which spread the disease, are active at night, the larvae are usually only found in the blood between about 10 pm and 2 am.
Either ivermectin, albendazole, or diethylcarbamazine is used to treat a filariasis infection by eliminating the larvae, impairing the adult worms' ability to reproduce, and by actually killing adult worms. Unfortunately, much of the tissue damage may not be reversible. The medication is started at low doses to prevent reactions caused by large numbers of dying parasites.
While effective, the medications can cause severe side effects in up to 70% of patients as a result either of the drug itself or the massive death of parasites in the blood. Diethylcarbamazine, for example, can cause severe allergic reactions and the formation of pusfilled sores (abscesses). These side effects can be controlled using antihistamines and anti-inflammatory drugs (corticosteroids). Rarely, treatment with diethylcarbamazine in someone with very high levels of parasite infection may lead to a fatal inflammation of the brain (encephalitis). In this case, the fever is followed by headache and confusion, then stupor and coma caused when massive numbers of larvae and parasites die. Other common drug reactions include dizziness, weakness, and nausea.
Symptoms caused by the death of the parasites include fever, headache, muscle pain, abdominal pain, nausea and vomiting, weakness, dizziness, lethargy, and asthma. Reactions usually begin within two days of starting treatment and may last between two and four days.
No treatment can reverse elephantiasis. Surgery may be used to remove surplus tissue and provide a way to drain the fluid around the damaged lymphatic vessels. Surgery may also be used to ease massive enlargement of the scrotum. Elephantiasis of the legs can also be helped by elevating the legs and providing support with elastic bandages.
The outlook is good in early or mild cases, especially if the patient can avoid being infected again. The disease is rarely fatal, and with continued WHO medical intervention, even gross elephantiasis is now becoming rare.
The best method of preventing filariasis is to prevent being repeatedly bitten by the mosquitoes that carry the disease. Some methods of preventing insect bites include the following:
- limit outdoor activities at night, particularly in rural or jungle areas
- wear long sleeves and pants and avoid dark-colored clothing that attracts mosquitoes
- avoid perfumes and colognes
- treat one or two sets of clothing ahead of time with permethrin (Duramon, Permanone).
- wear DEET insect repellent or, especially for children, try citronella or lemon eucalyptus, to repel insects
- if sleeping in an open area or in a room with poor screens, use a bed net to avoid being bitten while asleep
- use air conditioning, the cooler air makes insects less active.
In addition, filariasis can be controlled in highly infested areas by taking ivermectin preventatively before being bitten. Currently, there is no vaccine available, but scientists are working on a preventative vaccine at this time.
Abscess — An area of inflamed and injured body tissue that fills with pus.
Antibody — A specific protein produced by the immune system in response to a specific foreign protein or particle called an antigen.
Conjunctiva — The mucous membrane that lines the inside of the eyelid and the exposed surface of the eyeball.
Elephantiasis — A condition characterized by the gross enlargement of limbs and/or the genitalia that is also accompanied by a hardening and stretching of the overlying skin. Often a result of an obstruction in the lymphatic system caused by infection with a filarial worm.
Encephalitis — Inflammation of the brain.
Lymphatic system — The circulatory system that drains and circulates fluid containing nutrients, waste products, and immune cells, from between cells, organs, and other tissue spaces.
Microfilariae — The larvae and infective form of filarial worms.
Nematode — Round worms.
Subcutaneous — The area directly beneath the skin.
Centers for Disease Control and Prevention. 1600 Clifton Rd., NE, Atlanta, GA 30333. (800) 311-3435, (404) 639-3311. http://www.cdc.gov.
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any infection with filariae; the organism causing the most common form is Wuchereria bancrofti. Most often encountered in central Africa, the southwest Pacific, and eastern Asia, the disease also occurs in the West Indies and in tropical South and Central America. It is transmitted by the Culex mosquito or by mites or flies. The larvae invade lymphoid tissues and then grow to adult worms 2 to 5 cm long. The resulting obstruction of the lymphatic circulation causes swelling, inflammation, and pain. Repeated infections over many years, with impaired circulation and formation of excess connective tissue, may cause enlargement of the affected part, usually a limb or the scrotum. In cases of extreme enlargement, known as elephantiasis, the part may swell to many times its normal size. The larvae can be killed by treatment with diethylcarbamazine. Edema of the legs can be reduced by rest and by the use of pressure bandages. The prognosis is favorable for all but the most severe cases.
Presence of filariae in the tissues of the body or in blood (microfilaremia) or tissue fluids (microfilariasis), occurring in tropic and subtropic regions; living worms cause minimal tissue reaction, which may be asymptomatic, but death of the adult worms leads to granulomatous inflammation and permanent fibrosis causing obstruction of the lymphatic channels from dense hyalinized scars in the subcutaneous tissues; the most serious consequence is elephantiasis or pachyderma.
n. pl. filaria·ses (-sēz′)
Disease caused by infestation, especially of the lymphatic system, with filarial worms.
Presence of filariae in the tissues of the body or in blood (microfilaremia) or tissue fluids (microfilariasis), occurring in tropical and subtropical regions; living adult worms cause minimal tissue reaction, which may be asymptomatic, but death of the adult worms may cause granulomatous inflammation and permanent fibrosis. Some species of filarial worms can damage lymphatic channels, thus permitting onset of obstruction of the lymphatic channels from dense hyalinized scars in the subcutaneous tissues; the most serious consequence is elephantiasis or pachyderma.
filariasisA group of parasitic worm diseases transmitted by mosquitos and other biting flies in tropical Africa, South-east Asia, the South Pacific and parts of South America. The insect vector injects large numbers of microscopic worms (microfilariae) into the blood and these settle in the tissues and grow into adult worms of from 2 to 50 cm in length. These breed thousands of new microfilariae which enter the blood and are taken up by insects and carried to other people. The filarial diseases include river blindness (ONCHOCERCIASIS), LOA-LOA and CALABAR SWELLINGS. Repeated infection with worms that inhabit the lymphatics causes blockage and ELEPHANTIASIS.
Presence of filariae in the tissues of the body or in blood or tissue fluids; occurring in tropic and subtropic regions.