Heartworm disease, caused by the parasitic worm Dirofilaria immitis, is a concern for pet owners, especially dog owners. While often associated with canine health, humans can also contract this parasite. Though rare, human infections typically manifest differently and with less severity than in animals.
Human Susceptibility
Humans are considered accidental or “dead-end” hosts for Dirofilaria immitis, meaning the parasite cannot complete its full life cycle or reproduce within the human body. Unlike in dogs, where heartworm larvae mature into adult worms causing significant damage to the heart and lungs, the parasite’s development in humans is limited. The human immune system effectively recognizes and responds to immature worms, preventing them from fully maturing. This physiological difference explains why human cases are uncommon and less severe compared to the disease in dogs.
How Humans Contract Heartworm
Humans contract heartworm through the bite of an infected mosquito. Mosquitoes become infected by feeding on an animal, such as a dog, that carries immature heartworm larvae, known as microfilariae, in its bloodstream. Inside the mosquito, these microfilariae develop into an infective larval stage. When the infected mosquito then bites a human, these infective larvae are transmitted into the skin.
Once inside the human body, the larvae do not mature into adult worms or reproduce. Instead, they die, and the body’s immune response to these dying parasites leads to localized inflammation. This limited development means infected humans do not have circulating microfilariae in their bloodstream, preventing them from transmitting the parasite to mosquitoes, unlike primary animal hosts.
Recognizing and Diagnosing Human Heartworm
Human heartworm infections are often asymptomatic. When symptoms do occur, they are non-specific and mild, such as a cough, chest pain, or wheezing. The most common manifestation in humans is pulmonary dirofilariasis, where dying heartworm larvae lodge in the pulmonary arteries of the lungs, causing inflammation. This leads to the formation of small, round lesions or nodules, often called “coin lesions,” which may be incidentally discovered during a chest X-ray or CT scan.
Diagnosing human heartworm involves imaging techniques like chest X-rays or CT scans to identify these characteristic pulmonary nodules. Because these lesions can resemble more serious conditions, such as lung cancer, a biopsy is often performed to determine their exact nature. Microscopic examination of the tissue sample then confirms the presence of the parasitic worm.
Treatment and Prevention for Humans
In many human heartworm cases, specific medical intervention is not required, particularly if the infection is asymptomatic. If symptoms are present or a lesion is concerning, surgical removal of the worm or affected nodule is the most common approach. There is no specific medication for treating heartworm in humans comparable to treatments used for animals.
Prevention for humans focuses on avoiding mosquito bites, as mosquitoes are the sole vectors for heartworm transmission. Strategies include using insect repellents with active ingredients like DEET or picaridin, wearing protective clothing, and minimizing outdoor activities during peak mosquito hours, such as dawn and dusk. Reducing mosquito breeding sites around homes by eliminating standing water in containers like old tires, birdbaths, or clogged gutters also lowers the risk of exposure.