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Filarial Worm Infections Overview


By Richard D. Pearson, MD, Emeritus Professor of Medicine, University of Virginia School of Medicine

Filarial worm infections are caused by certain roundworms and affect different parts of the body depending on the species of worm.

There are many species of filarial worms, but only a few infect people. Species that infect people may reside in

  • Tissues under the skin (subcutaneous tissues): African eye worm (Loa loa), which causes loiasis, or Onchocerca volvulus, which causes river blindness (onchocerciasis)

  • Lymph tissues: Wuchereria bancrofti, Brugia malayi, or Brugia timori, which cause lymphatic filariasis

Most of these infections occur in tropical and subtropical areas. Infection with the dog heartworm (Dirofilaria immitis) occurs worldwide, but this infection rarely causes symptoms in people.


Filarial worm infections are transmitted as follows:

  • An infected fly (such as a horsefly or deerfly) or mosquito bites a person and deposits larvae of the worm in the skin.

  • The larvae mature into adult worms under the skin or in lymph tissues.

  • The adult worms produce offspring called microfilariae, which circulate in the bloodstream or travel through tissues.

  • The infection is spread when the infected person is bitten by a fly or mosquito, which ingests the microfilariae.

  • Inside the insect, the microfilariae develop into larvae that can cause infection.

  • The insect then transmits these larvae when it bites another person.

These infections are not spread directly from person to person.


Inside the body, worm larvae can form lumps and thus cause symptoms. Lumps may form in the lungs, in the lymph vessels, or under the skin. Filarial worm infections and the inflammation that accompanies them can block lymph vessels, causing the face, legs, or genitals to swell and the person to be disfigured. Some symptoms may result from an allergic reaction to substances released by the worms.