Heartworms, scientifically known as Dirofilaria immitis, are parasitic roundworms that primarily affect animals. Dogs are definitive hosts, where the worms mature and reproduce, often causing serious health issues. Cats, wolves, coyotes, and other mammals can also become infected. Humans can contract heartworms, but infection is rare and typically differs from animal cases, as the worms usually do not fully mature or reproduce in the human body.
How Humans Contract Heartworms
Heartworms are transmitted to humans exclusively through the bite of an infected mosquito. Mosquitoes become infected by feeding on an animal, such as a dog, that has circulating heartworm larvae (microfilariae) in its bloodstream. These microfilariae develop into an infective larval stage within the mosquito over approximately two weeks. When the infected mosquito bites a human, it transmits these infective larvae.
Humans are considered “accidental” or “dead-end” hosts for Dirofilaria immitis. The larvae enter the human body but typically do not mature into adult worms or reproduce. This means direct transmission of heartworms from an infected pet to a human is not possible.
What Happens When Humans Are Infected
When heartworm larvae infect a human, they usually migrate to the lungs, a condition known as pulmonary dirofilariasis. Once in the lungs, the larvae often die, and the body’s immune response forms a small, inflamed tissue mass around the dead worm. These masses are referred to as “coin lesions” or granulomas due to their appearance on imaging scans. Most human infections are asymptomatic, meaning they produce no noticeable symptoms, and are often discovered incidentally during chest X-rays or CT scans performed for unrelated reasons.
If symptoms occur, they are generally non-specific and mild, such as a cough, chest pain, or a low-grade fever. These symptoms are not unique to heartworm infection and can indicate many other conditions. In rare instances, larvae may migrate to other body parts, like the eyes or subcutaneous tissues, forming nodules, but lung involvement is typical.
Identifying and Addressing Human Cases
Diagnosing human heartworm infection often begins with imaging studies, such as a chest X-ray or CT scan, which may reveal a characteristic coin lesion in the lung. Because these lesions can resemble tumors or other serious conditions like tuberculosis, they often raise concern. To definitively diagnose heartworm infection, surgical removal of the lesion is frequently performed. A biopsy of the removed tissue then allows for microscopic identification of the worm.
Since heartworms typically die in the human body and do not progress to a reproductive stage, specific anti-parasitic medications, like those used for animals, are generally not required. The primary “treatment” for human pulmonary heartworm is often surgical removal of the lesion. This procedure confirms the diagnosis, alleviates any symptoms, and rules out more serious conditions like lung cancer.
Protecting Yourself from Heartworms
The most effective way to protect yourself from heartworm infection is to minimize exposure to mosquito bites. Using insect repellents containing active ingredients like DEET, picaridin, or oil of lemon eucalyptus can deter mosquitoes. Wearing long sleeves and pants, particularly during dawn and dusk when mosquitoes are most active, also helps. Ensuring windows and doors have intact screens prevents mosquitoes from entering your home.
Reducing the overall population of infected mosquitoes indirectly lowers the risk of human infection. This is achieved by preventing heartworm in pets, especially dogs, through regular veterinary care and prescribed preventative medications. Controlling the parasite reservoir in animals reduces the chances of mosquitoes picking up and transmitting the larvae.