Dirofilariasis

Dirofilariasis is an infection by parasites of the genus Dirofilaria.

Dirofilariasis

Causes

It can be caused by:

• Dirofilaria immitis
• Dirofilaria repens
• Dirofilaria tenuis

• 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.
• 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.

Diro Pulmonary Life Cycle

Human infection

• 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.

Dirofilaria immitis

Pulmonary granulomas

• 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.
• 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.

Geographic distribution

• 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.

Clinical Presentation

Dirofilaria immitis most often causes pulmonary disease in the human host, but can also uncommonly cause nodules in other tissues. Humans are suboptimal hosts and larvae that migrate to the heart usually die. Dead worms produce infarcts when they lodge in pulmonary vessels; these infarcts are usually referred to as “coin lesions” on chest radiography, which may be mistaken for malignancy. Following embolization, patients may present with vague systemic symptoms (e.g. malaise, fever, chills) and respiratory distress. There is often a mild eosinophilia.

Diro SQ LifeCycle lg

Subcutaneous nodules

Species of Dirofilaria in the subgenus Nochtiella usually (but not always) manifest as subcutaneous nodules. These nodules are often tender, and may be fixed or migratory. D. repens-associated lesions can occur in a variety of locations, the most typical being in exposed sites (e.g. scalp, arms, legs, eyelids, chest), but occasionally have been found in deeper tissue such as the breast, epididymis, spermatic cord, and subconjunctiva. Many reports of D. tenuis involve the facial region (e.g. ocular and periocular sites, oral mucosa, cheek) and breast nodules. Unlike D. immitis, some of these species may achieve a degree of development beyond the larval stage in the human host, as adult-stage D. repens and D. tenuis are often discovered in human infections. However, most do not develop to sexual maturity. There are very rare reports of patent D. repens infections in humans with detectable microfilariae in the blood.

Treatment

The definitive treatment of Dirofilaria infection in humans is surgical removal of lung granulomas and subcutaneous nodules; this treatment is also curative. In many cases, no drug treatment is necessary.

Dirofilariasis Resources
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