Can Humans Get Heartworm? Causes, Symptoms & Treatment

Humans can get heartworm, but the infection plays out very differently than it does in dogs. The parasite responsible, Dirofilaria immitis, reaches sexual maturity and thrives inside a dog’s heart and pulmonary arteries, but in humans it almost always dies before it can fully develop. The result is typically a small, benign lung nodule rather than the dramatic cardiovascular disease seen in pets. Most people who are infected never know it, and the condition is discovered only when chest imaging for something else picks up a spot that looks worryingly like cancer.

How Humans Get Infected

Heartworm spreads exclusively through mosquito bites. When a mosquito feeds on an infected dog, it picks up microscopic larval forms of the parasite circulating in the dog’s blood. Those larvae develop inside the mosquito over a couple of weeks into an infective stage, then get deposited under the skin of the next animal the mosquito bites. In dogs and other canids, the larvae migrate through tissue, enter the bloodstream, and eventually settle in the pulmonary arteries and heart, where they grow into adults that can reach 30 centimeters long. At least 26 mosquito species in the United States alone are capable of transmitting the parasite.¹

Humans enter the picture by accident. When a mosquito carrying infective larvae bites a person instead of a dog, the larvae get deposited just the same way. But humans are what parasitologists call aberrant or accidental hosts.² The parasite did not evolve to live in us, and it shows. The human immune system mounts a response that kills the worm before it can mature, reproduce, or establish the kind of chronic infection it builds in dogs. You cannot pass heartworm to another person, and the infection cannot cycle from human to mosquito and back again. It is a dead end for the parasite.

Why Your Immune System Usually Wins

Research in mice helps explain why. Immunocompetent mice are resistant to heartworm infection, while severely immunodeficient mice allow the larvae to survive and grow for months.³ The takeaway is that innate immune defenses, the body’s front-line response, are enough to shut down the infection without needing a highly specialized adaptive response. In humans, a similar process occurs. The larva makes it into the bloodstream, travels toward the lungs, and gets killed by the immune response before reaching adulthood.

What’s left behind is the worm’s remains. The dead parasite lodges in a small pulmonary artery and triggers a localized inflammatory reaction. Over time, the body walls it off with layers of granulomatous tissue and fibrous scarring, forming a small, round nodule in the lung.⁴ That nodule is typically a centimeter or two across and stays put. It does not grow, spread, or cause progressive damage the way heartworm disease does in dogs.

In dogs, the adult worms are remarkably good at avoiding the immune system. Their outer surface mimics the lining of blood vessels and gets coated with the host’s own proteins, making them nearly invisible to immune surveillance and resistant to blood clotting.⁵ Humans never have to deal with this problem, because the worm dies long before it develops those sophisticated evasion tactics.

What It Feels Like (Usually Nothing)

Most people with pulmonary dirofilariasis have zero symptoms. The infection is discovered incidentally when a chest X-ray or CT scan taken for another reason reveals a small, coin-shaped nodule in the lung.⁶ In a minority of cases, people do experience symptoms: cough, chest pain, coughing up blood, shortness of breath, or low-grade fever.⁷ These tend to be mild and self-limiting. Rarely, the infection has been associated with secondary heart inflammation in isolated case reports, but this is the exception, not the rule.

Because the worm dies before maturing, humans do not develop the chronic cardiovascular complications that make heartworm disease so dangerous in dogs. There is no progressive pulmonary hypertension, no congestive heart failure, and no mass of tangled adult worms clogging the heart chambers. The worst-case scenario in humans is almost always the diagnostic journey, not the infection itself.

The Cancer Scare Problem

This is where human heartworm gets genuinely stressful for patients. On a CT scan, the lung nodule left by a dead heartworm looks almost identical to a small primary lung cancer or a metastatic tumor deposit.⁶ It is a well-defined, round, solitary lesion, sometimes with a central area that looks like necrosis. For a radiologist reviewing the scan, it checks many of the same boxes as a malignancy.

Because imaging alone cannot reliably distinguish a heartworm nodule from cancer, many patients end up undergoing a biopsy or surgical removal of the nodule. In practice, minimally invasive resection with pathological examination of the tissue is often both the diagnostic test and the treatment.⁸ Under the microscope, pathologists find the characteristic layered structure: a dead, fragmented worm in the center, surrounded by a ring of inflammatory tissue and an outer shell of scar tissue.⁴ Only then does the diagnosis become clear.

Blood tests exist but play a limited role in most clinical scenarios. Serological testing can detect antibodies against the parasite, and one study on a small Italian island found that roughly a quarter of tested residents were seropositive for exposure to the parasite, up from about 8 percent a few years earlier.⁹ Seropositivity means the person’s immune system has encountered the parasite at some point, not that they currently have an active or dangerous infection. None of the seropositive individuals in that study had detectable parasite DNA in their blood, which underscores the point that the human body clears the worm efficiently.

Treatment Is Straightforward

Because the parasite is already dead by the time it is discovered, there is no live infection to treat with anti-parasitic drugs. The nodule itself is biologically inert. It does not grow, it does not release larvae, and it does not progress to anything more serious. In cases where a biopsy or surgical resection has been done to rule out cancer, the procedure itself is the treatment: the nodule is removed, the pathology comes back as dirofilariasis rather than malignancy, and no further intervention is needed.⁸

If the nodule is found incidentally and the clinical picture makes a cancer diagnosis unlikely (for instance, a young, healthy non-smoker in a region with high heartworm prevalence), some clinicians may opt for watchful waiting with serial imaging rather than immediate surgery. But given the difficulty of distinguishing the nodule from cancer on imaging alone, many cases still proceed to tissue sampling. The relief patients feel when the biopsy comes back “just a worm” rather than a tumor is probably the most significant clinical outcome of the whole experience.

Not Just the Lungs, and Not Just One Species

While pulmonary nodules are the most common presentation of human heartworm in the Americas, the parasite occasionally ends up elsewhere. Case reports have documented nodules in the abdominal wall, and one case involved an asymptomatic abdominal lesion discovered during surgery for an unrelated cancer.¹⁰ These extrapulmonary cases are rare and, like the pulmonary version, involve a dead worm walled off by the body.

It is worth knowing that Dirofilaria immitis is not the only species in this genus that infects humans. In Europe and parts of Asia, Dirofilaria repens is a more common cause of human dirofilariasis. While D. immitis tends to produce lung nodules, D. repens typically causes subcutaneous lumps or, memorably, ocular infections where the worm migrates into the tissue around the eye. Subcutaneous cases present as swelling, sometimes with a sensation of something moving under the skin. Ocular cases cause visual disturbances and the deeply unsettling discovery of a worm in or near the eye.¹¹ In rare cases, D. repens can also affect the lungs and pleura, making the two species hard to distinguish without molecular testing or detailed examination of the worm’s anatomy.

Where the Risk Is Highest

Human heartworm cases follow canine heartworm geography, since infected dogs are the reservoir that keeps the parasite cycling through mosquito populations. In the United States, the highest risk is concentrated in the southeastern states and along the Gulf Coast, where warm, humid conditions support both mosquito activity and parasite development.¹² Risk drops toward the western mountains and deserts, and remains low across most of the country during winter months, increasing through spring and peaking in summer.

In Europe, the historically endemic zones for canine heartworm disease sit in the southern countries around the Mediterranean: Italy, Spain, Portugal, Greece, and parts of France. Over the past couple of decades, the disease has been expanding northward into central and northern European countries that were previously considered heartworm-free.¹³ South Texas is a notable hotspot in the United States, where even urban mosquito species typically associated with biting humans, like Aedes aegypti, have been found contributing to transmission among dogs.¹⁴

More dogs infected means more larvae circulating in the mosquito population, which means more opportunities for an accidental human bite to deliver the parasite. Living in an area with poor canine heartworm prevention or large populations of stray, untreated dogs increases background risk for everyone, even though human cases remain uncommon.

Climate Change and Expanding Risk

The parasite’s range is not static. Heartworm larvae need a certain amount of warmth to develop inside the mosquito, and climate models project that warming temperatures will push suitable conditions into areas that are currently too cool for reliable transmission. In the United States, projections for 2100 suggest rising infection risk in the Midwest, the Northeast, and higher-altitude regions that have historically had minimal heartworm activity.¹² These shifts are driven by temperature alone, independent of whether mosquito ranges change, because existing mosquito species in these regions may become capable of supporting parasite development during longer warm seasons.

In Europe, similar modeling shows expanding zones where the parasite can complete its development cycle in mosquitoes, potentially bringing heartworm into densely populated areas that have not dealt with it before.¹⁵ Mapping monthly risk across Europe has been proposed as a tool for guiding prevention efforts and alerting veterinarians and physicians in newly affected regions.¹⁶ For humans, the practical upshot is that as canine heartworm expands geographically, the small but nonzero chance of an accidental human infection expands with it.

The Role of Wolbachia Bacteria

One piece of the heartworm puzzle that has gotten increasing attention involves bacteria of the genus Wolbachia that live inside the worms themselves. These bacteria are not just hitchhikers. They play essential roles in the parasite’s biology, including its ability to molt, reproduce, and trigger inflammatory responses in the host. When adult worms die in dogs, the release of Wolbachia into the bloodstream worsens the damage to blood vessel linings and lung tissue, encouraging clot formation and pulmonary hypertension.¹⁷

In veterinary medicine, this has led to the strategy of treating infected dogs with antibiotics that target Wolbachia before killing the worms, which can reduce the inflammatory fallout of worm die-off. For humans, the clinical relevance is less clear, since the worm typically dies on its own when it is still small, and the resulting inflammatory reaction is localized and self-contained rather than systemic. Still, the Wolbachia connection is a reminder that parasitic infections are often more biologically complex than they appear, involving not just the parasite but its own internal ecosystem.

Protecting Yourself and Your Pets

There is no human heartworm preventive medication, because the condition is rare enough and benign enough that prophylaxis would be overkill. The best way to reduce your already-low risk is indirect: keep your dogs on year-round heartworm prevention. Monthly preventives for dogs work by killing larval heartworms before they mature, which breaks the transmission cycle. When fewer dogs carry adult heartworms, fewer mosquitoes pick up larvae, and fewer mosquitoes deliver them to anyone, canine or human.

Standard mosquito avoidance measures also apply, especially if you live in a high-prevalence area during peak season. Screens on windows, long sleeves at dusk, and reducing standing water around your home all limit mosquito bites. None of this is specific to heartworm, but it is the same advice that applies to every mosquito-borne disease, and it works by reducing the number of bites you receive from any mosquito that might be carrying any pathogen.

If you are ever told that a chest scan has found a small lung nodule and you live in an area where heartworm is common, it is worth mentioning that to your doctor. Pulmonary dirofilariasis is not the first thing most physicians think of when they see a lung nodule, and the diagnosis sometimes only becomes apparent after tissue has been removed and examined. Awareness of the possibility will not change the diagnostic workup in most cases, since ruling out cancer is still the priority, but it can provide useful context for the clinician making decisions about how urgently to biopsy.