Lymphatic filariasis is an infection caused by the roundworms (nematodes) Wuchereria bancrofti, Brugia malayi, and Brugia timori.
Lymphatic filariasis is transmitted to people by infected mosquitoes.
People have a fever, swollen lymph nodes, pain in the limbs and groin, and, if the infection becomes chronic, swelling that can become constant and disfiguring.
The infection is diagnosed when doctors identify the worm larvae (microfilariae) in a sample of blood or tissue.
People are usually treated with the medication diethylcarbamazine, which kills the immature larvae in the blood and some of the adult worms.
Helminths are parasitic worms that can infect humans and animals. There are 3 types of helminths: flukes (trematodes), tapeworms (cestodes), and roundworms (nematodes). Wuchereria bancrofti, Brugia malayi, and Brugia timori are a type of roundworm called a filarial worm.
Lymphatic filariasis is caused by Wuchereria bancrofti, Brugia malayi, or Brugia timori. Wuchereria bancrofti is present in tropical and subtropical areas of Africa, Asia, the Pacific, and the Americas, including Haiti. Brugia malayi and Brugia timori are common in South and Southeast Asia.
In 2000, the World Health Organization (WHO) launched its Global Programme to Eliminate Lymphatic Filariasis. As a result, substantial progress has been made in stopping the spread of infection through large-scale, annual treatment of eligible people in areas where infection is present. As of 2018, approximately 51 million people were infected, which is a decline of 74% since the programme started. In 2023, more than 657 million people in 39 countries required this preventive treatment to stop the spread of infection.
(See also Overview of Parasitic Infections.)
Transmission of Lymphatic Filariasis
Lymphatic filariasis is transmitted when an infected mosquito bites a person and deposits larvae of the worm in the skin. The larvae travel to the person's lymphatic system, which includes lymph nodes, and mature into adult worms. Adult female Wuchereria bancrofti worms may be 3 to 4 inches (7 1/2 to 10 centimeters) long.
The adult females produce millions of worm larvae (called microfilariae) that circulate in the bloodstream and lymphatic system. The infection is spread when a mosquito bites an infected person and ingests the microfilariae. Inside the mosquito, the microfilariae develop into larvae that can cause infection. The mosquito then transmits these larvae when it bites another person.
Symptoms of Lymphatic Filariasis
Lymphatic filariasis symptoms are caused by adult worms. Microfilariae do not cause symptoms and gradually disappear from the bloodstream after people leave the area where the parasites are common.
Early (acute) infection
Early in the infection, people may have episodes of fever, swollen lymph nodes in the armpits and groin, and pain in the limbs and groin that last for 4 to 7 days. Pus may collect in a leg and drain through the skin's surface, resulting in a scar.
Bacterial infections of the skin and tissues under the skin are more likely because the worms block the lymphatic vessels (tubes that carry a fluid called lymph through the body). This blockage makes the immune system less able to defend the skin and nearby tissues from bacteria.
Often, symptoms resolve, then return. They are more severe when people who do not live in the area are exposed to the infection for the first time.
Chronic infection
After many years of infection, the blocked lymphatic vessels widen. Most people have no symptoms. But, in a few people, the widened lymphatic vessels cause swelling that gradually becomes permanent (chronic). The legs are affected most often, but the arms, breasts, and genitals may be also. This swelling (called lymphedema) develops because
The adult worms live in the lymphatic system and reduce the drainage of lymph fluid from tissues, causing the fluid to accumulate and the tissues to swell.
The worms trigger a response from the immune system that produces inflammation and swelling.
Lymphedema makes the skin spongy. Pressing on the skin leaves an indentation that does not disappear right away (called pitting). Chronic pitting lymphedema may make the skin hard and thick (called elephantiasis). Elephantiasis can occur in the legs and occasionally in the arms and sometimes in the scrotum.
Bacterial and fungal infections of the skin are common in people with lymphatic filariasis. These infections, along with the inflammation caused by the worms, can cause pain and discomfort. These infections also contribute to the development of lymphedema.
Some people have mild joint pain and blood in the urine.
Less commonly, the lungs are affected by microfilariae in the bloodstream, resulting in a disorder called tropical pulmonary eosinophilia. People may have a low-grade fever, feel short of breath, cough, or wheeze. If the infection persists, scar tissue (fibrosis) may form in the lungs.
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Diagnosis of Lymphatic Filariasis
Examination of a blood sample or biopsy sample
Blood tests
Doctors diagnose lymphatic filariasis when they identify microfilariae in a sample of blood or in a biopsy sample of lymphatic tissue examined under a microscope.
When an ultrasound is done, doctors can see adult worms moving in the widened lymph vessels.
Doctors may do blood tests to identify antibodies to the worms or antigens of the worms. Antibodies are proteins produced by the immune system to help defend against infection, including by parasites. Antigens are substances produced by the worms that can trigger an immune response in the body. (See also Tests That Detect Antibodies to or Antigens of Microorganisms.) However, the value of blood tests is limited because they cannot distinguish between worms that cause lymphatic filariasis and some other worms nor between past and current infection.
Treatment of Lymphatic Filariasis
Diethylcarbamazine
Treatment of long-term problems
Typically, doctors prescribe diethylcarbamazine for people with lymphatic filariasis. This medication is taken by mouth for 1 or 12 days. It kills microfilariae and some adult worms. In addition to diethylcarbamazine, doctors may give people other medications such as doxycycline or albendazole.
Before treating people with diethylcarbamazine, doctors check them for other filarial worm infections called loiasis and onchocerciasis because diethylcarbamazine can cause serious side effects in people who have these infections at the same time.
Treatment of long-term problems
Chronic lymphedema requires meticulous skin care. People must be careful not to damage the skin and to thoroughly clean any minor cuts and scrapes. Such care helps prevent bacterial infections.
Swelling may be reduced by wrapping elastic bandages around the affected limb or by elevating the limb (see also Treatment of Lymphedema).
If elephantiasis, including swelling in the scrotum, is severe, surgery may be done to improve drainage in the lymphatic system.
Bacterial skin infections are treated with antibiotics given by mouth. The antibiotics may slow or prevent progression to elephantiasis.
For lung-related problems, diethylcarbamazine is taken for 14 to 21 days. However, the infection recurs in approximately 25% of people. For them, treatment must be repeated.
Prevention of Lymphatic Filariasis
To reduce the number of mosquito bites, people can do the following:
Use insect repellents on exposed skin
Wearing clothing that has been treated with the insecticide permethrin
Wearing loose-fitting, long-sleeved shirts and long pants
Using netting over beds
In areas where lymphatic filariasis is common, the World Health Organization's Global Programme to Eliminate Lymphatic Filariasis uses various medication regimens to help prevent the spread of infection. These medications reduce the number of microfilariae in the blood of infected people and thus reduce transmission by mosquitoes.
