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) or in the eye: African eye worm (Loa loa), which causes loiasis Loiasis Loiasis is infection of tissues under the skin or under the clear outer membrane that covers the eye (conjunctiva) with the roundworm Loa loa. Itchy swellings may appear, mainly on the... read more
, or Onchocerca volvulus, which causes river blindness (onchocerciasis Onchocerciasis Onchocerciasis is infection with the roundworm Onchocerca volvulus. It causes itching, a rash, sometimes with scarring, as well as eye symptoms that may lead to blindness. The infection... read more 
)Lymph tissues: Wuchereria bancrofti, Brugia malayi, or Brugia timori, which cause lymphatic filariasis Lymphatic Filariasis Lymphatic filariasis is infection of the lymphatic system caused by one of three species of roundworms. People have a fever, swollen lymph nodes, pain in the limbs and groin, and, if the infection... read more
Most filarial infections occur in tropical and subtropical areas.
Infection with the dog heartworm Dog Heartworm Infection Dog heartworm infection is caused by the roundworm Dirofilaria immitis (or another similar roundworm), which is transmitted to people by infected mosquitoes. Dog heartworms cannot complete... read more (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 Overview of Parasitic Infections A parasite is an organism that lives on or inside another organism (the host) and benefits (for example, by getting nutrients) from the host at the host's expense. Although this definition actually... read more .)
Transmission
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 Overview of the Lymphatic System The lymphatic system is a vital part of the immune system. It includes organs such as the thymus, bone marrow, spleen, tonsils, appendix, and Peyer patches in the small intestine that produce... read more .
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.
Symptoms of Filarial Worm Infections
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.
