Heartworm is a parasitic infection that damages the blood vessels in a dog’s lungs, forces the heart to work harder, and can eventually cause heart failure. The worms themselves are long, spaghetti-like parasites that live inside the arteries connecting the heart to the lungs, where they can survive for five to seven years. What makes heartworm especially dangerous is that dogs often show no symptoms until significant damage has already occurred.
How Dogs Get Infected
Heartworm spreads exclusively through mosquito bites. When an infected mosquito feeds on a dog, it deposits microscopic larvae into the skin. These larvae burrow into the bite wound and begin a slow migration through the dog’s body, molting twice as they mature. It takes roughly six months from the initial mosquito bite for the larvae to reach the pulmonary arteries and grow into adults. By that point, the worms can be up to 12 inches long.
Adult worms settle primarily in the pulmonary arteries, the large blood vessels that carry blood from the right side of the heart to the lungs. In heavy infections, they spill back into the right ventricle of the heart itself. A single dog can harbor dozens of worms at once, and each female produces tiny offspring called microfilariae that circulate in the bloodstream, ready to be picked up by the next mosquito.
How Heartworms Damage the Body
The real harm isn’t the worms eating tissue. It’s the body’s inflammatory response to their presence. The lining of the pulmonary arteries reacts to the worms by thickening and scarring, a process where cells multiply and migrate abnormally along the vessel walls. This thickening narrows the interior of the arteries, reduces their elasticity, and obstructs smaller branches entirely. The result is a chronic increase in blood pressure within the lungs, called pulmonary hypertension.
As pulmonary blood pressure rises, the right side of the heart has to pump harder to push blood through increasingly stiff, narrowed vessels. Over months or years, this extra workload causes the right ventricle to enlarge and weaken. Eventually the heart can no longer keep up, leading to right-sided congestive heart failure. Fluid backs up into the abdomen, the liver swells, and the dog’s ability to circulate oxygen drops significantly.
The kidneys and liver also take collateral damage. Microfilariae and immune complexes (clumps of antibodies stuck to worm proteins) can lodge in small blood vessels throughout the body, triggering chronic inflammation in these organs. Dogs with long-standing infections sometimes develop kidney disease or liver dysfunction that persists even after the worms are eliminated.
Symptoms at Each Stage
Veterinarians classify heartworm disease into four stages based on severity, and the progression from one to the next can take months or years depending on the number of worms and the dog’s activity level.
- Class I (mild): No visible symptoms, or an occasional cough. Most dogs appear completely normal. This is the stage where infections are typically caught by routine blood tests rather than clinical signs.
- Class II (moderate): A persistent cough develops, and dogs tire more easily during walks or play. A veterinarian may hear abnormal lung sounds with a stethoscope.
- Class III (severe): Coughing becomes frequent, and exercise intolerance is obvious. Dogs may faint during exertion, develop a swollen belly from fluid accumulation, and show signs of labored breathing. Heart and lung sounds are clearly abnormal. Death is possible at this stage.
- Class IV (caval syndrome): This is a medical emergency. A large mass of worms migrates backward from the pulmonary arteries into the heart chambers and the large veins feeding into them. The worms physically interfere with the heart’s valves, preventing them from closing properly. Red blood cells are shredded as they pass through the tangle of worms, causing severe anemia and producing dark, tea-colored urine. Dogs collapse rapidly with signs of acute heart failure, kidney failure, and liver dysfunction. Caval syndrome carries a high mortality rate even with emergency intervention.
Why It’s Hard to Catch Early
The six-month gap between infection and detectable disease creates a significant blind spot. Standard blood tests look for proteins released by adult female worms, but these proteins don’t appear in the bloodstream until the worms are mature. If a dog was bitten by an infected mosquito three months ago, the test will come back negative even though larvae are actively developing inside the body.
Even after worms mature, false negatives happen. Tests can miss infections when only male worms are present (since the proteins detected come from females), when the worm count is very low, or when the dog’s immune system has bound up the worm proteins in antibody complexes that the test can’t detect. The American Heartworm Society notes that even the most sensitive tests can fail under these circumstances, which is why annual testing is recommended even for dogs on preventive medication.
What Treatment Looks Like
Treating heartworm is far more difficult, risky, and expensive than preventing it. The process involves killing the adult worms with a series of deep muscle injections over the course of several months. As the worms die, their bodies break apart and are carried by blood flow into smaller branches of the pulmonary arteries, where they form clots. This is expected and unavoidable, but it’s also dangerous. The dying worms can trigger blood clots in the lungs, acute inflammation, fever, coughing, loss of appetite, and in some cases, life-threatening clotting disorders.
The single most important part of treatment is strict exercise restriction for at least one month after each round of injections. Physical activity increases heart rate and blood flow, which pushes more worm fragments into the lungs faster than the body can safely absorb them. Even moderate exertion, like running to greet someone at the door, can cause a fatal embolism in a dog whose pulmonary arteries are already compromised. Most treatment protocols involve weeks of crate rest, leash-only bathroom breaks, and no play.
Injection site reactions are the most common side effect: pain, swelling, and firm lumps that can persist for weeks. In rare cases, dogs develop neurologic complications including weakness or paralysis in the hind legs. The overall risk of complications correlates directly with how many worms are present, which is one reason early detection matters so much. A dog with a class I infection faces a much safer treatment course than one already in heart failure.
Prevention Is Nearly 100% Effective
Monthly preventive medications work by killing the immature larvae before they can reach the pulmonary arteries and mature. In controlled studies, monthly dosing achieved 100% efficacy at preventing established infection. These preventives are available as oral chewables, topical liquids, or injectable formulations that last six to twelve months.
The key is consistency. A single missed dose during mosquito season can leave a window long enough for larvae to develop past the stage where preventives can kill them. Because mosquito exposure is difficult to predict (mosquitoes can survive indoors and in surprisingly cool conditions), year-round prevention is standard guidance regardless of climate. The cost of monthly prevention is a fraction of what treatment runs, and the health risks are essentially zero by comparison.