Heartworm is a parasitic worm that lives inside a dog’s heart and lung blood vessels, causing progressive damage that can be fatal without treatment. The parasite, spread exclusively through mosquito bites, takes about six to seven months to mature inside a dog’s body. By the time symptoms appear, significant internal damage has often already occurred.
How Dogs Get Heartworm
Dogs cannot catch heartworm from other dogs directly. The parasite requires a mosquito to complete its life cycle. When a mosquito bites a dog already carrying heartworm, it picks up microscopic baby worms (called microfilariae) circulating in the dog’s blood. Over the next 10 to 14 days, those baby worms develop inside the mosquito into a larval stage capable of causing infection. When that mosquito bites another dog, it deposits the infective larvae into the skin through the bite wound.
At least nine mosquito species in the United States can transmit heartworm, and the larvae need warm temperatures to develop inside the mosquito. Development stalls below roughly 57°F (14°C) and requires a cumulative amount of warmth before the larvae become infective. This is why heartworm transmission is seasonal in cooler climates but can occur year-round in warmer regions. The American Heartworm Society tracks prevalence nationwide every three years, and cases have been reported in all 50 states.
What Happens Inside the Dog’s Body
Once larvae enter through a mosquito bite, they migrate through the dog’s tissues and into the bloodstream, eventually settling in the pulmonary arteries (the blood vessels connecting the heart to the lungs). Over six to seven months, the larvae molt twice more and grow into adult worms. Adults can reach 10 to 12 inches in length. Once mature, they mate and release new microfilariae into the dog’s bloodstream, completing the cycle if another mosquito feeds on that dog.
The real danger is what the worms do to blood vessels. Heartworm disease is primarily a disease of the lung vasculature, not the heart itself, at least initially. The presence of worms triggers inflammation inside the artery walls. Smooth muscle cells multiply and migrate inward, creating a rough, thickened lining that narrows the vessels. Over time, the entire network of pulmonary blood vessels becomes stiff and loses its ability to expand and contract normally. This drives up blood pressure in the lungs, a condition called pulmonary hypertension.
When worms die, whether naturally or from treatment, the situation can worsen acutely. Dead worm fragments lodge in smaller vessels and trigger blood clots, causing sudden spikes in lung blood pressure and inflammation. In advanced cases, the sustained high pressure in the lungs forces the right side of the heart to work harder and harder until it begins to fail. This is right-sided congestive heart failure, and it represents the final stage of untreated heartworm disease.
Symptoms by Stage
Heartworm disease is often invisible in its early months. Dogs can carry a developing infection for the entire six-to-seven-month maturation period without showing any outward signs. Even after worms mature, mildly infected dogs may appear completely normal.
As the disease progresses, the first sign is usually a mild, persistent cough. This comes from the irritation and inflammation in the lung vessels. With more worms and more vascular damage, dogs become exercise intolerant: they tire more quickly on walks, may lag behind, or seem reluctant to play. Some dogs lose weight or develop a poor appetite. In moderate to severe infections, you might notice labored breathing even at rest.
The most dangerous presentation is caval syndrome, which occurs when the worm burden is so heavy, or pulmonary hypertension so severe, that worms are pushed backward from the lung arteries into the right side of the heart and the large veins feeding into it. This physically obstructs blood flow. Dogs with caval syndrome collapse suddenly, often with pale gums, dark or bloody urine, and severe difficulty breathing. Without emergency surgical removal of the worms from the heart, caval syndrome is almost always fatal.
How Heartworm Is Diagnosed
Veterinarians primarily use a blood test that detects proteins (antigens) produced by adult female heartworms. The test is quick and can be done in-clinic, but it has important limitations. It cannot detect infection until worms have matured, which means a dog infected within the past six to seven months may test negative despite carrying developing larvae. Antigen tests are highly specific (about 97%), so a positive result is reliable. Sensitivity is lower (around 78%), meaning some infected dogs, particularly those with low worm burdens or only male worms, can test falsely negative.
Because no single test is perfect, veterinarians consider test results alongside a dog’s symptoms, travel history, and geographic risk. A second type of blood test looks for microfilariae directly under a microscope, which can help confirm infection but misses cases where adult worms are present without producing offspring.
Treatment for Heartworm Disease
Treating an active heartworm infection is a months-long process that carries real risks, which is why prevention is so strongly emphasized. The current standard protocol recommended by the American Heartworm Society involves three phases.
First, at diagnosis, a dog is started on a monthly preventive medication to kill any new larvae and stop the infection from getting worse. The dog also receives a four-week course of an antibiotic that weakens the adult worms by killing bacteria they depend on for survival. This pre-treatment phase makes the eventual worm-killing step safer.
About two months after diagnosis, the dog receives an injection of an adulticide drug designed to kill the mature worms. One month later, two more injections are given 24 hours apart. A steroid is prescribed around the time of these injections to reduce the lung inflammation that occurs as worms die and break apart inside the blood vessels.
Exercise restriction is critical throughout the entire treatment period. Physical activity increases blood flow through the lungs, which raises the risk that dead worm fragments will cause dangerous blockages. From the time of diagnosis, dogs should avoid running, jumping, and off-leash activity. After the final injections, restrictions become even stricter: only short, slow, leashed walks. Activity is gradually reintroduced six to eight weeks after the last injection. For many dog owners, this months-long confinement is one of the hardest parts of treatment.
Treatment success rates are around 89% with the full three-injection protocol, but roughly 10% of treated dogs may still have surviving adult worms when retested a year later.
Preventing Heartworm
Prevention is far simpler, safer, and cheaper than treatment. All currently licensed heartworm preventives belong to a single drug class that kills larval-stage worms before they can mature and cause damage. These medications come in several forms:
- Monthly oral tablets or chewables are the most common option and typically contain one of three active ingredients. Many are combined with drugs that also prevent intestinal parasites.
- Monthly topical treatments are applied to the skin between the shoulder blades and work systemically after absorption.
- Long-acting injections are administered by a veterinarian and provide six or twelve months of protection per dose, removing the need to remember monthly dosing.
These preventives work retroactively, killing any larvae a dog picked up from mosquito bites over the preceding weeks. This is why consistent, on-time dosing matters. A gap of even a few months can allow larvae to mature past the point where preventives are effective. Because heartworm testing cannot detect infections younger than six to seven months, annual testing is recommended even for dogs on year-round prevention, as a safety net against missed or late doses.