How Do Dogs Get Heartworm: From Mosquito to Heart

Dogs get heartworm through the bite of an infected mosquito, and there is no other natural route of transmission. The parasite, a threadlike roundworm called Dirofilaria immitis, depends on mosquitoes to shuttle its larvae from one animal to another, and the journey from that initial bite to a full-blown infection lodged in the heart and pulmonary arteries takes roughly six to seven months. What happens in between involves a surprisingly complex series of developmental stages, tissue migrations, and immune interactions that determine whether a dog ends up with a handful of worms or a life-threatening blockage.

Why Mosquitoes Are the Only Way

Heartworm cannot spread from dog to dog by casual contact, shared water bowls, or even direct blood transfer. The parasite requires a mosquito as an intermediate host because a critical phase of its development happens inside the insect. When a mosquito feeds on an infected dog, it ingests microscopic larvae called microfilariae circulating in the dog’s blood. Those microfilariae are immature and incapable of infecting another animal on their own. They need roughly two weeks inside the mosquito’s body to molt through two larval stages before reaching the infective third stage, known as L3. Only then can they be passed on.

Dozens of mosquito species can serve as heartworm vectors. Research comparing mosquito populations in Oklahoma found that urban areas had more of the Asian tiger mosquito (Aedes albopictus) and a higher overall likelihood of heartworm infection, but rural areas had their own competent species. Because so many different mosquitoes can carry the parasite, avoiding any single species does not eliminate the risk.¹

Temperature Decides Whether Larvae Mature

The parasite’s development inside the mosquito is temperature-dependent. Larvae need sustained warmth to reach the infective L3 stage. Below about 14°C (57°F), development stalls entirely. This is why heartworm transmission has historically been seasonal in temperate climates: mosquitoes may be biting in early spring or late fall, but the larvae inside them cannot mature if nights are too cold.

The relationship between temperature and development speed is not as simple as a fixed threshold, though. Research on Aedes aegypti mosquitoes showed that larvae experiencing daily temperature swings (cycling around 19°C with 9-degree fluctuations) actually developed faster than larvae held at a constant 19°C. The warm peaks during the day pushed development along even when overnight lows dipped near the threshold.² This means that in regions where average temperatures hover near the cutoff, daily spikes can still allow transmission during months that would seem too cool on paper.

What Happens at the Moment of the Bite

When an infected mosquito lands on a dog and begins feeding, it does not inject larvae directly into the bloodstream the way it delivers saliva. Instead, the L3 larvae are deposited on the dog’s skin in a tiny droplet of the mosquito’s own body fluid (hemolymph). The larvae then actively crawl into the bite wound.³ This detail matters because it means anything that disrupts the mosquito’s feeding or causes it to leave before finishing can still result in larvae being left behind on the skin, though the likelihood of successful entry drops.

Several mosquito genera are confirmed vectors, including Aedes, Culex, Anopheles, and Mansonia. Wild canids like coyotes and wolves are also definitive hosts, meaning mosquitoes can shuttle larvae between wildlife and domestic dogs.⁴

Migration Through the Body

Once L3 larvae enter through the bite wound, they do not head straight for the heart. They first take up residence in the subcutaneous tissue, the layer just beneath the skin. Over the next one to two weeks, they molt into fourth-stage larvae (L4). Over the following weeks, L4 larvae continue to grow and eventually molt again into immature adults, called juvenile or young adult worms. It is only at this point, roughly two to three months after infection, that the worms begin migrating into the bloodstream and making their way toward the pulmonary arteries.

Occasionally, worms end up somewhere they should not. One documented case involved adult heartworms found in a dog’s abdominal aorta rather than the expected pulmonary arteries, causing widespread damage to small arteries throughout the abdomen and a dead segment of small intestine.⁵ Aberrant migration like this is uncommon in dogs but shows that the worms’ route is not perfectly programmed.

Settling Into the Pulmonary Arteries

The worms’ final destination is the pulmonary arteries, the large blood vessels that carry blood from the right side of the heart to the lungs. Adult heartworms can grow to 12 inches or longer, and a heavily infected dog may harbor dozens of them. They physically crowd the vessels and provoke a severe inflammatory reaction in the arterial lining called endarteritis. This thickening and scarring of the vessel walls reduces the arteries’ elasticity, increases resistance to blood flow, and over time drives up pressure in the pulmonary circulation.⁶

The damage is not just mechanical. The same study found that pulmonary endarteritis persisted for at least ten months after the adult worms had been killed and eliminated. This means a dog treated successfully for heartworm still faces months of ongoing vascular damage and elevated blood pressure in the lungs. Chronic pulmonary hypertension eventually strains the right side of the heart, which has to pump harder to force blood through narrowed, stiffened arteries. That strain can lead to right-sided heart failure.

When Worms Back Up Into the Heart

In severe infections, adult worms can migrate backward from the pulmonary arteries into the right chambers of the heart and even into the large veins feeding the heart (the venae cavae). This retrograde movement creates a condition called caval syndrome, one of the most dangerous complications of heartworm disease. The worms physically obstruct blood flow and mechanically damage red blood cells, causing them to rupture. Signs of caval syndrome include anemia and dark-colored urine from the released hemoglobin.⁷

Not every dog with worms in the heart chambers develops caval syndrome, but the condition is a veterinary emergency when it does appear. Treatment usually involves physically extracting worms through a catheter inserted into the jugular vein before proceeding with standard heartworm treatment.

The Bacteria Living Inside the Worms

One of the more surprising aspects of heartworm biology is that the worms themselves carry passengers: bacteria called Wolbachia. These organisms live inside heartworm cells and are essential to the worm’s survival and reproduction. When worms die, whether naturally or from treatment, Wolbachia are released into the dog’s bloodstream and trigger an inflammatory immune response that adds to the damage already caused by the worms themselves.⁸

This discovery opened up an unexpected treatment angle. Giving dogs doxycycline, a common antibiotic, before standard heartworm treatment targets the Wolbachia bacteria. Killing the bacteria weakens the worms, reduces microfilarial loads, impairs female worm fertility, and, critically, reduces the inflammatory surge that accompanies worm death. In cases where the standard arsenic-based adulticide treatment is too risky or unavailable, a combination of monthly preventive medication and a four-week course of doxycycline targeting Wolbachia has been considered as an alternative approach.⁹

Research on naturally infected dogs found that all heartworm-positive animals had significant antibody responses to a Wolbachia surface protein, and that the protein could be detected in the lungs, liver, and kidneys, particularly in the kidney’s filtering structures. This suggests that Wolbachia-driven inflammation contributes to the organ damage seen in advanced heartworm disease beyond just the lungs and heart.¹⁰

Why Testing Is Not Always Straightforward

Most veterinary clinics test for heartworm using a quick blood test that detects proteins (antigens) shed by adult female worms. The test is highly specific, meaning a positive result is almost certainly real. But sensitivity can be an issue. A study that compared multiple testing methods found that standard point-of-care antigen tests caught only about half to 57% of infections when measured against a more sensitive heat-treated antigen test. The heat-treatment step breaks apart antigen-antibody complexes that can mask the parasite’s presence, revealing infections the standard test misses.¹¹

Several factors can produce a false negative. Low worm burdens, all-male infections (since the antigen is produced primarily by females), and very early infections before worms have matured can all cause a negative result in an infected dog. Antibody-based tests exist as well but trade improved sensitivity for lower specificity, occasionally flagging dogs that are not truly infected. In practice, veterinarians sometimes combine test types or use heat-treated assays for dogs with clinical signs suggestive of heartworm but a negative standard test.

How Preventive Medications Work and Why Resistance Matters

Monthly heartworm preventives belong to a drug class called macrocyclic lactones, which includes ivermectin, milbemycin oxime, moxidectin, and selamectin. These drugs kill heartworm larvae during the tissue-migration phase, before the worms reach the pulmonary arteries. They are effective against L3 and L4 larvae, so a single monthly dose eliminates any larvae deposited by mosquitoes during the preceding weeks. The drugs are not instantaneous paralytics at the concentrations that reach the worms. Recent evidence suggests they work partly by disrupting the larvae’s ability to evade the dog’s immune system, essentially making the larvae visible to immune cells that then clear them.¹²

The emergence of drug resistance has complicated this picture. Reports of preventive failure first surfaced in 2005, concentrated in the lower Mississippi River valley. A decade later, controlled studies provided direct proof: heartworm strains from regions with frequent preventive failure established infections in dogs despite correct dosing with ivermectin or injectable moxidectin. Researchers identified genetic markers in the resistant worms that could predict which populations are likely to survive treatment.¹³ Understanding what drives resistance is now considered important for maintaining control of the parasite broadly.¹⁴

For dog owners, this does not mean preventives are unreliable. Resistance is still concentrated in certain geographic hot spots, and compliance failures (missed or late doses) remain a far more common reason for breakthrough infections than genuine drug resistance. But it does mean that annual testing remains important even for dogs on year-round prevention.

Treating an Active Infection

When a dog tests positive for adult heartworms, treatment typically involves an arsenic-based drug called melarsomine, delivered by deep intramuscular injection. The goal is to kill the adult worms, but the treatment itself carries risk. As worms die, their fragments are carried downstream into smaller pulmonary vessels, causing inflammation and potentially blocking blood flow to parts of the lung. Common signs during treatment include cough, exercise intolerance, rapid breathing, and occasionally coughing up blood. Severe reactions like acute hypersensitivity after melarsomine injection are uncommon but possible.¹⁵

Strict exercise restriction during and after treatment is essential. Physical activity increases blood flow through the lungs and can dislodge worm fragments, worsening embolism. Many veterinarians now pre-treat with doxycycline to target Wolbachia and reduce the inflammatory load before administering melarsomine, which may improve outcomes.

Coyotes and Other Wildlife Reservoirs

Heartworm is not just a disease of domestic dogs. Wild canids, especially coyotes, are natural hosts and serve as reservoirs that keep the parasite circulating in an area regardless of how well domestic dogs are protected. A study across Texas found heartworm infections in coyotes sampled from many counties, with higher rates in southern regions. The researchers described coyotes as indicators of relative heartworm risk for dogs and cats in the same area.¹⁶

Earlier work in the Gulf coastal prairies of Texas and Louisiana found heartworms in 71% of coyotes, 83% of coyote-red wolf hybrids, and all red wolves sampled. The infection appeared to be a meaningful factor in the morbidity and mortality of these wild populations.¹⁷ The practical takeaway is that even if every pet dog in a neighborhood were on prevention, the local mosquito-coyote cycle would keep heartworm larvae circulating. Stopping preventive medication because an area “seems safe” ignores this wildlife reservoir.

Heartworm in Cats Is a Different Disease

Cats can be infected by the same parasite, but the disease looks and behaves very differently. Cats are resistant hosts, meaning fewer larvae survive to adulthood, and a typical feline infection involves only one to three worms. Even so, the arrival and death of immature worms in the pulmonary arteries can trigger coughing and breathing difficulty as early as three months after infection, far earlier than in dogs. Adult worms suppress certain immune cells in the lungs, which paradoxically reduces clinical signs during chronic infection.¹⁸

Roughly 80% of cats with no symptoms end up clearing the infection on their own. But for symptomatic cats, the median survival time is about a year and a half. Aberrant migration, where worms end up in the brain, eyes, or other unusual locations, is more common in cats than dogs. Perhaps most unsettling, sudden death with no prior warning signs can occur when even a single worm dies and causes an acute embolism. There is no approved adulticide treatment for cats, so prevention is the only reliable strategy.

Can Humans Get Heartworm?

Technically, yes, though the result is nothing like canine heartworm disease. Humans are accidental dead-end hosts. When an infected mosquito bites a person, the larvae may enter the skin, but the human body is an unsuitable environment for the parasite. The worm cannot complete its lifecycle, so it dies before reaching adulthood. As it dies, it gets carried to the lungs, where it lodges in a small pulmonary vessel and triggers localized tissue death. The body walls off the dead worm, forming a small nodule.¹⁹

The clinical significance is not the infection itself but the diagnostic confusion it creates. These pulmonary nodules show up on chest imaging and can look identical to lung tumors, sometimes prompting invasive biopsies or even surgery before the true cause is identified.²⁰ Most human cases are discovered incidentally during imaging for other reasons and require no treatment once correctly identified.

Climate Change and Expanding Geographic Risk

Because heartworm transmission depends on warm temperatures to develop inside mosquitoes, climate trends directly affect where and when dogs are at risk. Modeling of global temperature data found that the zone where temperatures never get warm enough for heartworm larval development has been shrinking by about 0.45% per decade, while the zone with seasonal transmission potential has been expanding by roughly 0.76% per decade.²¹ In practical terms, regions that historically had a short or nonexistent heartworm season are seeing that window widen.

Projections for the United States specifically suggest strong geographic expansion of transmission risk, with new areas becoming suitable for heartworm as temperatures rise.²² For dog owners in northern states or higher elevations who have historically viewed heartworm as a Southern problem, these trends are a reason to reconsider year-round prevention.

Income, Geography, and Uneven Risk

Heartworm prevalence is not evenly distributed, and the reasons go beyond climate. A large analysis of factors influencing canine heartworm rates across the United States found that median household income was a significant predictor: lower-income areas had higher prevalence. Temperature, precipitation, humidity, elevation, surface water coverage, and the presence of specific mosquito species all mattered, but socioeconomic conditions showed independent predictive power.²³

A suburban-focused study reinforced this pattern, finding that the number of heartworm-positive dogs in an area was best predicted by a combination of higher mosquito diversity and lower household income.²⁴ The income connection likely reflects the cost of year-round preventive medication and regular veterinary testing, both of which are straightforward in principle but represent a recurring expense that not all owners can consistently maintain. Combined with the wildlife reservoir that keeps the parasite cycling regardless of how many pet dogs are treated, these disparities help explain why heartworm remains stubbornly common in certain regions despite effective prevention being available for decades.