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.
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Filariasis (fih-luh-RYE-uh-sis) is a tropical disease caused by tiny worms.
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Filariasis is caused by different species of microscopic parasitic* roundworms that are passed to people through the bites of insects, most commonly mosquitoes. Several strains* of these worms, known as filariae (fih-LAIR-e-e), can infect humans, including Wuchereria bancrofti (voo-ker-E-re-ah ban-CROFT-e). There are also different types of filariasis itself, including cutaneous (kyoo-TAY-nee-us) or skin-related, body cavity, and lymphatic* infections. In the cutaneous disease, the worms live in the layers of the skin; in body cavity filariasis, they inhabit certain body openings and surrounding tissue; and in the lymphatic form of infection, they invade the vessels of the lymphatic system and the lymph nodes*.
- (pair-uh-SIH-tik) refers to organisms such as protozoa (one-celled animals), worms, or insects that can invade and live on or inside human beings and may cause illness. An animal or plant harboring a parasite is called its host.
- are various subtypes of organisms, such as viruses or bacteria.
- (lim-FAH-tik) means relating to the system of vessels and other structures that carry lymph, a colorless fluid, throughout the body’s tissues; the lymphatic system plays an important role in protecting the body from infections.
- (LIMF) nodes are small, bean-shaped masses of tissue that contain immune system cells that fight harmful microorganisms. Lymph nodes may swell during infections.
Lymphatic filariasis, which can progress to a condition called elephantiasis*, is the most serious form of the disease. It begins when an infected female mosquito injects worm larvae* into a person’s blood while feeding. The larvae travel to the lymphatic vessels, where they grow into adult worms. As adults, the worms can survive and reproduce for up to 7 years. The gradual buildup of worms in the vessels hinders the lymphatic system’s ability to fight infection, and causes lymph fluid to collect—typically in the arms, legs, breasts, and male genitals—leading to swelling and disfigurement.
- (eh-luh-fan-TIE-uh-sis) is the significant enlargement and thickening of body tissues caused by an infestation of parasites known as filaria.
- (LAR-vee) are the immature forms of an insect or worm that hatch from an egg.
Filariasis is most common in tropical and subtropical regions, including parts of Africa, the western Pacific, Asia (especially India), and Central and South America. In these areas, the number of cases of filariasis continues to rise. It is estimated that more than 120 million people worldwide have the lymphatic form of illness today, and approximately 40 million of them have been disabled or disfigured by the disease. Although contracting filariasis is not a risk in the United States, some recent immigrants may have it, and people who have traveled to other countries can contract the disease as well. Missionaries and Peace Corps volunteers are considered to be most at risk.
The disease does not spread from direct person-to-person contact. Instead, it is transmitted by the bite of a mosquito. When one of these insects bites someone who is infected, it takes in the parasites along with its meal of blood. The mosquito then can pass those parasites on to the next person it bites. Usually, someone must be bitten many times, typically over a long period, to develop symptoms of filariasis.
The lymphatic form of filariasis usually produces fever, swollen or painful lymph nodes in the neck and groin, pain in the testicles*, and swelling in the limbs or genitals. Males and the male urinary and genital systems are particularly likely to be affected. In elephantiasis, a severe form of chronic* lymphatic filariasis, the blocked flow of lymph causes one or both legs to swell significantly. Over time, the skin on the leg also can change, taking on a rough texture so that it resembles the skin of an elephant. Although elephantiasis is unusual, up to half of all men with lymphatic filariasis may show serious symptoms, such as swelling of the scrotum*. In some cases people may have no obvious symptoms, but they still may have serious damage to the kidneys and lymphatic system.
- (TES-tih-kuls) are the paired male reproductive glands that produce sperm.
- (KRAH-nik) means continuing for a long period of time.
- is the sac of skin that contains the testicles.
Lymphatic filariasis can progress to elephantiasis, a swelling and thickening of body tissues from accumulation of fluid. The skin may look thick, pebbly, and dark. Phototake
Knowing that the person lives in or has spent time in a country where filariasis poses a risk can help a doctor diagnose the disease. The doctor may also take skin and blood samples from the patient to look for signs of the parasite.
Ideally, treatment begins as soon as possible after the patient becomes infected. Prompt treatment may not be possible, however, because the disease can be difficult to detect in its early stages. When the diagnosis is made, treatment may include:
- medication to kill the young worms in the bloodstream and stop the parasite’s life cycle (although the medicine cannot kill adult worms)
- exercising and moving swollen limbs to improve lymph flow
- bed rest and compression bandages to treat swelling
- medications to lessen swelling and discomfort
- hospitalization and intravenous* (IV) antibiotics for secondary infections that might appear because the damaged lymphatic system is less able to assist in defending the body against infectious agents
- (in-tra-VEE-nus) means within or through a vein. For example, medications, fluid, or other substances can be given through a needle or soft tube inserted through the skin’s surface directly into a vein.
- surgical treatment for deformities, such as enlarged limbs and scrotum, sometimes with several procedures and skin grafts* to correct cases of disfigurement
- are tissue or organ transplants.
Filariasis can last a lifetime, and without treatment it can worsen. The disease can lead to permanent disfigurement and damage to the lymphatic system and kidneys, secondary infections, hardening and thickening of the skin, and sexual and psychological problems. In countries where the disease is common, a serious social stigma* often accompanies it.
- is a mark of shame.
There is no vaccine to prevent filariasis, but controlling the populations of blood-sucking insects, especially mosquitoes, can limit the spread of the disease. In some areas where filariasis is common, people are treated yearly with preventive medicine to kill any immature worms in their blood. To protect themselves, people can also:
- Stay inside as much as possible from dusk to dawn, when mosquitoes are most active.
- Sleep under mosquito netting.
- Place screens in all windows.
- Use insecticides around living areas.
- Apply insect repellent to exposed skin.
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Filariasis is a preventable parasitic disease caused by the threadworms Wuchereria bancrofti, Brugia malayi, and Brugia timori. It is endemic in over 80 countries and is considered to be the main cause of permanent disability worldwide.
When symptomatic, patients may present with severe lymphatic and limb swelling, commonly referred to as elephantiasis. The disfigurations resulting from elephantiasis and lymphodema raise issues among some communities and add to the socioeconomic impacts of this debilitating disease.
Infection is transmitted through mosquito bites, where both the vector (disease carrier) and the human host are necessary in the successful completion of the parasitic life cycle. Anti-parasitic treatment is available, but is expensive and can take around a year to eliminate the parasites. Filariasis infection has been successfully curtailed in countries such as China, which has given hope for the campaign of global eradication.
Filariasis was first recognized in its infectious form in 1866 when filarial larvae were detected in urine and identified as the causative agent.
The infection is usually acquired in childhood, but takes years to manifest and usually remains asymptomatic until adulthood. Once developed, symptoms may include chronic swelling of the lymph nodes and swelling of the arms, legs, and genitals. Elephantiasis refers to the thickening of skin and underlying tissue and often accompanies symptoms. Without presentation of symptoms, internal damage to the kidneys and the lymphatic system may also develop.
Transmission of filariasis is through mosquito bites, whereby the larval form is drawn out of blood by mosquitos, develops to the infective stage, and is injected into a new host. Here the larvae develop into the adult form, migrate to the lymph nodes, mate, and release millions of larvae into the host's bloodstream. Adult worms may live up to six years, during which time the host will remain a source of infection for others.
Filariasis is a disease largely associated with poverty with more than a billion people in over 80 countries at risk. Infection caused by W. bancrofti is endemic in tropical regions of Southeast Asia, Africa, India, and Central and South America, while B. malayi and B. timori are generally limited to areas of Southeast Asia. This disease state is rare in western countries with no cases having been reported within the United States.
Filariasis has been recognized as the leading cause of permanent disability worldwide. Of the 120 million people affected, 40 million have been left disfigured and completely incapacitated. Chronic disease symptoms present more in men than women as seen in endemic areas where from 10 to 50% of men are infected versus up to 10% of women.
Disease distribution is also affected by habitat suitability for the various forms of mosquito responsible for transmission of the parasite.
Treatment of this disease consists of a multifaceted approach. Treatment of the infection itself may be achieved with a concurrent dosage of strong anti-parasitic drugs such as diethylcarbamazine citrate (DEC), DEC fortified salt, and albendazole. This proves to be 99% effective in removing microfilaria from the blood after one year of treatment.
Lymphodema and elephantiasis are often exacerbated due to bacterial and fungal infections taking advantage of the patient's compromised lymphatic condition. Following rigorous hygiene routines as prevention against infection from opportunistic pathogens and completing exercises to improve lymph flow often helps to reduce these causes of swelling. Surgery may be necessary for treatment of severe genital swelling in men.
Prevention of filariasis is achievable by reducing exposure to mosquito bites and treating endemic communities, in total, to remove the pool of infection. Some means of prevention are relatively simple and inexpensive. Using mosquito netting in sleeping areas, wearing clothing that covers the arms and legs, applying mosquito repellant on exposed skin, and remaining indoors during peak times of mosquito activity all reduce the risk of exposure to filariasis. These measures are the basis of a global campaign to eradicate filariasis.
Filariasis causes permanent—and often painful—disability in over 40 million people worldwide. Most cases occur in developing or underdeveloped regions. Many disabled by filariasis cannot regularly farm, work, or attend school. Malnourishment rates are often higher among those affected by filariasis than in the surrounding non-affected population.
The social impacts of filariasis present not only in the loss of work labor due to incapacitation, but also in community relations. People disfigured by the disease are frequently shunned by society and chronic complications are often deemed shameful. Marriage is considered a near impossibility among those suffering genital manifestations, which then puts growth and development of the community at risk.
Filariasis is a disease with potential for eradication, but one that remains endemic in poor communities. The continued increase in infection may be attributed to the rapid growth of cities and the resultant spread of poverty-stricken housing areas. These regions provide breeding sites for the mosquitoes and aid in disease transmission. It is for these reasons that endemic areas are trapped in a vicious cycle of infection that has proven difficult to break.
WORDS TO KNOW
LYMPHATIC SYSTEM: The lymphatic system is the body's network of organs, ducts, and tissues that filter harmful substances out of the fluid that surrounds body tissues. Lymphatic organs include the bone marrow, thymus, spleen, appendix, tonsils, adenoids, lymph nodes, and Peyer's patches (in the small intestine). The thymus and bone marrow are called primary lymphatic organs, because lymphocytes are produced in them. The other lymphatic organs are called secondary lymphatic organs. The lymphatic system is a complex network of thin vessels, capillaries, valves, ducts, nodes, and organs that runs throughout the body, helping protect and maintain the internal fluids system of the entire body by both producing and filtering lymph, and by producing various blood cells. The three main purposes of the lymphatic system are to drain fluid back into the bloodstream from the tissues, to filter lymph, and to fight infections.
SOCIOECONOMIC: Concerning both social and economic factors.
VECTOR: Any agent, living or otherwise, that carries and transmits parasites and diseases. Also, an organism or chemical used to transport a gene into a new host cell.
IN CONTEXT: RELATED DISEASE GROUPS
Filiariasis is a group of tropical diseases caused by thread like parasitic round worms (nematodes of the order Filaraiidae, commonly called filariae) and their larvae. The group affects humans and animals. 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. There are hundreds of described filarial parasites, but only eight that cause infections in humans: Brugia malayi, Wucheria bancrofti, Brugia timori, Onchocerca volvulus, Loa loa, Mansonella streptocerca, Mansonella ozzardi, and Mansonella perstans.
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Directors of Health Promotion and Education (DHPE). “Lymphatic Filariasis.” 2005 <http://www.dhpe.org/infect/Lymphfil.html> (accessed March 5, 2007).
World Health Organization (WHO). “Lymphatic Filariasis.” September 2000 <http://www.who.int/mediacentre/factsheets/fs102/en/> (accessed March 5, 2007).
fil·a·ri·a·sis / ˌfiləˈrīəsəs/ • n. Med. a tropical disease caused by the presence of filarial worms, esp. in the lymph vessels where heavy infestation can result in elephantiasis.