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Filariasis

DefinitionFiliariasis is the name for a group of tropical diseases caused by various thread-like parasitic roundworms (nematodes) and their larvae. The larvae transmit the disease to humans through a mosquito bite.

Filariasis is characterized byfever, chills, headache, andskin lesionsin the early stages and, if untreated,can progress to include gross enlargement of the limbs and genitalia in a condition called elephantiasis.

DescriptionApproximately 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 israrely fatal, it is the second leading cause of permanent and long-term disability in the world. The WorldHealth Organization (WHO) has named filariasis one of only six "potentially eradicable" infectiousdiseases 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 aparticular part of the body and mature to adult worms. Filariasis is classified into three distinct typesaccording 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 areasbeneath the skin and whites of the eye; and serous cavity filariasis infects body cavities but does notcause disease. Several different types of worms can be responsible for each type of filariasis, but themost common species include the following: Wucheria bancrofti, Brugia malayi(lymphaticfilariasis), Onchocerca volvulus, Loa loa, Mansonella streptocerca, Dracunculusmedinensis (subcutaneous filariasis), Mansonella pustans, and Mansonella ozzardi(serous cavityfilariasis).The two most common types of the disease are Bancroftian and Malayan filariasis, both forms oflymphatic 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 theUnited 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 bloodstreamat night, when they may be transmitted, via an insect bite, to another host. A single bite is usually notenough to acquire an infection, therefore, short-term travelers are usually safe. A series of multiple bitesover a period of time is required to establish an infection. As a result, those individuals who are regularlyactive outdoors at night and those who spend more time in remote jungle areas are at an increased riskof contracting the filariasis infection.

Causes and symptomsIn cases of lymphatic filariasis, the most common form of the disease, the disease is caused by the adultworms actually living in the lymphatic vessels near the lymph nodes where they distort the vessels andcause local inflammation. In advanced stages, the worms can actually obstruct the vessels, causing thesurrounding tissue to become enlarged. In Bancroftian filariasis, the legs and genitals are most ofteninvolved, while the Malayan variety affects the legs below the knees. Repeated episodes of inflammationlead to blockages of the lymphatic system, especially in the genitals and legs. This causes the affectedarea 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 movingbeneath the skin or beneath the white part of the eye (conjunctiva). If untreated, this disease can cause atype of blindness known as onchocerciasis.Symptoms vary, depending on what type of parasitic worm has caused the infection, but all infectionsusually begin with chills,headache, and fever between three months and one year after the insect bite.There may also be swelling, redness, andpainin the arms, legs, or scrotum. Areas of pus (abscesses)may appear as a result of dying worms or a secondary bacterial infection.

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DiagnosisThe disease is diagnosed by taking a patient history, performing a physical examination, and byscreening blood specimens for specific proteins produced by the immune system in response to thisinfection (antibodies). Early diagnosis may be difficult because, in the first stages, the disease mimicsother bacterial skin infections. To make an accurate diagnosis, the physician looks for a pattern ofinflammation 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, butbecause mosquitos, which spread the disease, are active at night, the larvae are usually only found in theblood between about 10 pm and 2 am.

TreatmentEither ivermectin, albendazole, or diethylcarbamazine is used to treat a filariasis infection by eliminatingthe 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 dosesto 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 eitherof the drug itself or the massive death of parasites in the blood. Diethylcarbamazine, for example, cancause severe allergic reactions and the formation of pusfilled sores (abscesses). These side effects can

be controlled usingantihistaminesand anti-inflammatory drugs (corticosteroids). Rarely, treatment withdiethylcarbamazine in someone with very high levels of parasite infection may lead to a fatal inflammationof the brain (encephalitis). In this case, the fever is followed by headache and confusion, then stuporandcomacaused when massive numbers of larvae and parasites die. Other common drug reactionsinclude dizziness, weakness, and nausea.Symptoms caused by the death of the parasites include fever, headache, muscle pain, abdominalpain,nausea and vomiting, weakness, dizziness, lethargy, andasthma. Reactions usually begin withintwo 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 away to drain the fluid around the damaged lymphatic vessels. Surgery may also be used to ease massiveenlargement of the scrotum. Elephantiasis of the legs can also be helped by elevating the legs andproviding support with elastic bandages.

PrognosisThe outlook is good in early or mild cases, especially if the patient can avoid being infected again. Thedisease is rarely fatal, and with continued WHO medical intervention, even gross elephantiasis is nowbecoming rare.

PreventionThe best method of preventing filariasis is to prevent being repeatedly bitten by the mosquitoes that carrythe 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, torepel insects

if sleeping in an open area or in a room with poor screens, use a bed net to avoid being bittenwhile asleep

use air conditioning, the cooler air makes insects less active.

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In addition, filariasis can be controlled in highly infested areas by taking ivermectin preventatively beforebeing bitten. Currently, there is no vaccine available, but scientists are working on a preventative vaccineat this time.

Key termsAbscess 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 theeyeball.Elephantiasis A condition characterized by the gross enlargement of limbs and/or the genitalia that isalso accompanied by a hardening and stretching of the overlying skin. Often a result of an obstruction inthe 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, wasteproducts, 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.

Resources

Organizations

Centers for Disease Control and Prevention. 1600 Clifton Rd., NE, Atlanta, GA 30333. (800) 311-3435,(404) 639-3311.http://www.cdc.gov.

Other

"Bacterial Diseases." "Health touch Online Page."http://www.healthtouch.com.Centers for Disease Control. http://www.cdc.gov/nccdphp/ddt/ddthome.htm .International Society of Travel Medicine.http:www.istm.org.

King, J. W. Bug Bytes. Louisiana State University Medical Center. http://www.ccm.lsumc.edu/bugbytes.

"Lymphatic Filariasis." Centers for Disease Control. http://www.cdc.gov/travel/yellowbk/page117.htm .Gale Encyclopedia of Medicine. Copyright 2008 The Gale Group, Inc. All rights reserved.

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