Heartworm (Dirofilaria immitis) is a parasitic roundworm that primarily threatens dogs and, less often, cats. In its natural host, the parasite resides in the heart and pulmonary arteries, where it grows and reproduces, causing severe and potentially fatal disease. Humans can become infected, but these cases are rare occurrences. The infection is typically benign because humans are considered accidental, or “dead-end,” hosts, meaning the parasite cannot mature into the reproductive adult stage or complete its life cycle.
Transmission: The Role of the Mosquito
The mosquito acts as the intermediate host and vector for heartworm transmission. When a mosquito takes a blood meal from an infected dog, it ingests microscopic larval forms of the parasite, called microfilariae, circulating in the bloodstream. Inside the mosquito, the microfilariae develop over approximately ten to fourteen days, undergoing two molts to reach the infective third-stage larvae (L3). This development requires warmth and is highly dependent on environmental temperatures.
When the infected mosquito bites a human or another animal, it deposits the infective L3 larvae onto the skin surface near the bite wound. The larvae then actively migrate into the new host through the small opening created by the mosquito bite. Once inside the human body, the immune response and temperature prevent the parasite from completing its life cycle. The larvae cannot fully mature into the reproductive adult worms that cause heart disease in dogs, effectively ending the infection.
How the Human Body Responds to the Parasite
After the infective L3 larvae enter the human body, they are met with a strong immunological reaction. The majority of the larvae die quickly, often before traveling far from the bite site. A small number of these immature worms may survive long enough to enter the bloodstream and be carried to the right side of the heart. From there, they are swept into the pulmonary arteries, which carry blood to the lungs.
The dying parasite becomes lodged in a small peripheral artery within the lung tissue, obstructing blood flow and causing a localized pulmonary infarction. The immune system responds by surrounding the dead worm and damaged tissue with inflammatory cells, forming a dense mass known as a granuloma. This response, called human pulmonary dirofilariasis, results in a solitary, peripheral nodule in the lung. These nodules are often described as “coin lesions” due to their distinct, round appearance on a chest X-ray.
Symptoms, Diagnosis, and Management
In the vast majority of human heartworm infections, the formation of the pulmonary nodule is asymptomatic. Since the parasite dies, the infection is self-limiting, and no specific medical treatment is necessary. In rare instances, if the nodule is located near the outer lining of the lung (the pleura), a person might experience mild, non-specific symptoms such as chest pain, a cough, or a low-grade fever.
Diagnosis is usually incidental, occurring when a chest X-ray or CT scan is performed for an unrelated reason and the solitary pulmonary nodule is discovered. Because these coin lesions resemble a cancerous tumor, they present a diagnostic challenge. Management involves observation if the lesion is small and not suspicious, but surgical removal is often pursued to rule out lung cancer. Excision of the nodule provides tissue for a histopathological examination, which confirms the benign diagnosis by identifying the remnants of the dead parasite within the granuloma.