What is filariasis?

Definition

Filariasis is an infection with microscopic, threadlike
worms transmitted from person to person by repeated mosquito
bites. The adult worm lives and reproduces in the lymph system and produces
microscopic worms known as microfilariae. These microfilariae circulate throughout the blood
vessels of the body. When a mosquito bites an infected person and moves on to the
next person, the microfilariae are deposited into the skin and move to the
lymphatic system. The disease is often referred to as lymphatic filariasis,
especially when symptoms occur.

Causes

The mosquito is the vector or disease-carrying insect that transmits filariasis from person to person. In Africa, the
Anopheles mosquito is the most common vector. The
Culex quinquefasciatus mosquito in the Americas and the
Mansonia mosquito in Asia and the Pacific Rim transmit the infection.

Three species of worms cause lymphatic filariasis. Most infections are caused by
Wuchereria bancrofti, but in Asia the infection filariasis is also caused by
Brugia malayi and B. timori. The adult worms live up to seven years. Multiple mosquito bites over time are needed before symptoms of the disease occur.

Risk Factors

Living in a subtropical or tropical area where the infection is common is the greatest risk factor. Exposure to repeated mosquito bites increases the risk. Persons with occasional, short visits to tropical or subtropical countries with a history of the disease are at very low risk. There have been no reported cases originating in the United States in recent years.

Symptoms

The worm that causes filariasis damages the lymph system. Persons who are
infected may have no symptoms or may never develop clinical symptoms. Lymphedema, or lymph fluid collection in the tissues, is the
primary symptom. Lymphedema is swelling that maybe seen in the legs, arms,
breasts, and genitalia. Swelling of the scrotum in men may occur due to infection
with one of the causative worms. There is a decreased function of the lymph
system, making it difficult for the body to fight infection. Hardening of the skin
from bacterial
infection may also be a symptom. In Asia, pulmonary filarial
infection may cause a cough, wheezing, and difficulty breathing.

Screening and Diagnosis

There is no screening test for filariasis. The diagnosis is made with a blood smear that is stained and viewed under a microscope. The microfilariae circulate in the blood at night, so blood collection is more accurate when done at night. A serology test for immunoglobulin G4 may show elevated levels. Because lymphedema symptoms may occur many years after infection, laboratory tests may be negative.

Treatment and Therapy

Certain drugs may be used to kill the microfilariae and the adult worms. However, if clinical symptoms such as lymphedema are present, drugs are not used because an active infection is rarely present. A lymphedema therapist will provide help with skin care, exercise, and treatments to manage symptoms. Surgery may be indicated for infection of the scrotum.

Prevention and Outcomes

To prevent infection, one should avoid mosquito bites in tropical and subtropical areas of the world by sleeping in an air-conditioned room or under a mosquito (sleeping) net, wearing long sleeves and pants, and using mosquito repellent, especially between dusk and dawn.

Bibliography

Jong, Elaine C., and Russell McMullen, eds. Travel and Tropical Medicine Manual. 4th ed. Philadelphia:Saunders/Elsevier, 2008. A useful reference manual with advice on preventing, evaluating, and managing diseases that can be acquired in tropical environments and countries outside the United States.

Lu, S., et al. “Localized Lymphedema (Elephantiasis): A Case Series and Review of the Literature.” Journal of Cutaneous Pathology 36 (2009): 1-20.

McDowell, Julie, and Michael Windelsprecht. The Lymphatic System. Santa Barbara, Calif.: Greenwood Press, 2004.

Marquardt, William C., ed. Biology of Disease Vectors. 2d ed. New York: Academic Press/Elsevier, 2005. This textbook is geared to graduate students and researchers, but most of the information is accessible to general readers.

Muller, Ralph. Worms and Human Disease. 2d ed. New York: CABI, 2002. An advanced-student textbook that covers all human worm infections with emphasis on diagnosis, treatment, clinical manifestations, pathogenesis (disease development), epidemiology, and control.