There are many species of filarial worms, but only a few infect people. Species that infect people may reside in
Lymph tissues: Wuchereria bancrofti, Brugia malayi, or Brugia timori, which cause lymphatic filariasis
Most filarial infections occur in tropical and subtropical areas.
Infection with the dog heartworm (Dirofilaria immitis) occurs worldwide, including in North America. This infection rarely causes symptoms, because the dog heartworm does not develop to maturity in people. But on occasion an immature heartworm reaches the lungs and may cause chest pain and cough. Very rarely, larvae form nodules in the eyes, brain, and/or testes.
(See also Overview of Parasitic Infections.)
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 reside in the skin.
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, adult filarial worms can migrate and form lumps in the lymph vessels or under the skin, depending on type of filarial worm causing infection. Adult female worms produce immature forms of the worm called microfilariae. Much of the damage and many symptoms caused by filarial infections result from the body's inflammatory responses to the adult worms or microfilariae.
When lymphatic tissues (cells and organs that make up the lymphatic system) are involved, adult Wuchereria or Brugia worms and the inflammation that accompanies them can block lymph vessels, causing areas of the legs, arms, or genitals to become inflamed and swell. After many years, the legs, arms, and genitals may become massively enlarged and disfigured.
Circulating microfilariae of Wuchereria or Brugia can induce allergic reactions in the lungs resulting in cough, shortness of breath, and asthma-like symptoms. Adult Loa loa worms migrate under the skin causing temporary nodules and occasionally cross the eye under the clear, outer membrane (conjunctiva). Adult Onchocerca live in nodules under the skin and produce microfilariae that cause itching and damage to the skin. They also enter the eye and cause inflammation and scarring that can result in blindness after many years.