What are Dirofilaria?

Dirofilaria are long, thin parasitic roundworms that infect a variety of mammals. Infection is transmitted by mosquito bites. There are many species of Dirofilaria, but human infection is caused most commonly by three species, D. immitis, D. repens, and D. tenuis. The main natural hosts for these three species are dogs and wild canids, such as foxes and wolves (D. immitis and D. repens) and raccoons (D. tenuis). D. immitis is also known as “heartworm.” D. repens is not found in the United States, and D. tenuis appears to be restricted to raccoons in North America.

What is dirofilariasis?

Dirofilariasis is the disease caused by Dirofilaria worm infections. In dogs, one form is called “heartworm disease” and is caused by D. immitis. D. immitis adult worms can cause pulmonary artery blockage in dogs, leading to an illness that can include cough, exhaustion upon exercise, fainting, coughing up blood, and severe weight loss.

Like dogs, humans become infected with Dirofilaria through mosquito bites. In persons infected with D. immitis, dying worms in pulmonary artery branches can produce granulomas (small nodules formed by an inflammatory reaction), a condition called “pulmonary dirofilariasis.” The granulomas appear as coin lesions (small, round abnormalities) on chest x-rays. Most persons with pulmonary dirofilariasis have no symptoms. People with symptoms may experience cough (including coughing up blood), chest pain, fever, and pleural effusion (excess fluid between the tissues that line the lungs and the chest cavity). Coin lesions on chest x-rays are not diagnostically specific for pulmonary dirofilariasis. Therefore, discovery of these lesions have led to invasive diagnostic procedures to exclude other, more serious causes, including cancer. Rarely, D. immitis worms have been found in humans outside the lungs, including in the brain, eye, and testicle. A review of cases of human dirofilariasis in the United States published in 2005 lists a total of 81 cases reported in the literature since 1941, the year of the first reported case.

D. repens, which is not found in the United States, is the leading cause of human dirofilariasis in Europe. Both D. immitis and D. repens have been found to cause human dirofilariasis in other parts of the world. Among persons infected with D. repens and D. tenuis, the most common findings have been nodules under the skin and under the conjunctiva of the eye. The prevalence of Dirofilaria infection in humans who have no symptoms from their infection is unknown.

The larvae that enter the body through a mosquito bite often do not survive the passage through tissue under the skin and when they do survive, the adult worms remain sexually immature. Therefore, Dirofilaria infection in humans does not result in the production of microfilariae and humans are not able to transmit the infection to other hosts. This is unlike the infection in other mammalian hosts, such as dogs. In these hosts, the infection can be transmitted to other hosts because microfilariae are produced and then ingested by mosquitoes in blood meals.

D. immitis is commonly reported as the cause of human dirofilariasis in the United States. In the United States, infection in dogs and humans is most common in the east and southeast. D. repens, not found in the United States, is the Dirofilaria species most commonly reported to cause human dirofilariasis in Europe. Both species have been found to cause human dirofilariasis in other parts of the world.

Where is dirofilariasis found?

Dirofilariasis is found throughout the world where Dirofilaria species are common. In the United States, canine dirofilariasis has been reported from all states, and D. tenuis in raccoons is common in many areas where raccoons are found. Canine and human dirofilariasis are most prevalent in eastern and southeastern states, although rates are increasing rapidly in a number of western states. D. immitis is the Dirofilaria species most commonly reported to cause dirofilariasis in humans in the United States. D. repens is the Dirofilaria species most commonly reported to cause dirofilariasis among humans in Europe. One or both of these species have been found to cause human dirofilariasis in other parts of the world.

How is dirofilariasis transmitted?

The definitive mammalian hosts for Dirofilaria are primarily domestic dogs, wild canids (such as wolves and foxes), and raccoons. In these hosts, sexually mature worms produce microfilariae that circulate in the blood and are ingested by mosquitoes during a blood meal. In mosquitoes, the microfilariae develop into larvae that migrate to the proboscis (the long, tubular part of the mouth of the mosquito that punctures the skin during a blood meal), where they are ready to infect another host during a blood meal. Several types of mosquitoes are capable of transmitting Dirofilaria infection, including Aedes, Anopheles, and Mansonia.

Humans and a wide range of other mammals are accidental hosts that play no role in the transmission of Dirofilaria. In these hosts, Dirofilaria larvae can develop into adult worms but the worms remain sexually immature and no microfilariae are produced.

Can dirofilariasis be transmitted person-to-person?

No. Dirofilariasis is transmitted neither person-to-person nor person-to-mosquito-to-person. The transmission of dirofilariasis requires mosquitoes as the intermediate host as well as the production of microfilariae, which does not take place in humans.

Human infections with D. immitis can result in areas of inflammation induced by dying adult worms in pulmonary arteries that appear as coin lesions on chest x-rays. Coin lesions can also be caused by cancer and other serious diseases, and a coin lesion discovered accidentally on chest x-ray usually leads to an invasive procedure to learn the cause. Most human cases of pulmonary dirofilariasis are diagnosed from samples taken during these procedures. Most reported cases of D. immitis infection in humans have been in persons with no symptoms. People with symptoms can have cough (including coughing up blood), chest pain, fever, and pleural effusion (excess fluid between the tissues that line the lungs and the chest cavity). Rarely, D. immitis worms have been found in humans at outside the lungs, including the brain, eye, and testicle. When D. repens and D. tenuis infection is reported in humans, it is generally as the cause of nodules under the skin, but on occasion, worms are found in the conjunctiva.

In humans, dirofilariasis is diagnosed most frequently by the examination of tissue from areas of inflammation in the lung obtained as part of the diagnostic investigation of coin lesions (small, round abnormalities) on chest x-rays or from the examination of tissue in nodules under the skin. Blood tests are not yet helpful in the diagnosis of dirofilariasis in humans.

The definitive treatment of Dirofilaria infection in humans is surgical removal of lung granulomas and nodules under the skin; this treatment is also curative. In many cases, no treatment with medicines is necessary.

Dirofilariasis can be prevented by avoiding mosquito bites in areas where mosquitoes may be infected with Dirofilaria larvae. The risk of such mosquito bites can be reduced by leaving as little skin exposed as possible, by the use of insect repellent when exposed to mosquitoes, and by sleeping under an insecticide-treated bednet in areas where Dirofilaria-infected mosquitoes bite at night and have access to sleeping areas.