Many dogs with heartworm disease show no symptoms at all in the early months of infection. It takes about six months for heartworm larvae to mature into adults after a dog is bitten by an infected mosquito, and visible signs often don’t appear until the worms have established themselves in the lungs and heart. By the time symptoms show up, the disease may already be moderate or advanced, which is why annual testing matters even for dogs that seem perfectly healthy.
Why Symptoms Take So Long to Appear
After an infected mosquito bites your dog, microscopic larvae enter the bloodstream and slowly migrate toward the heart and the blood vessels of the lungs. Over roughly six months, they grow into adult worms that can reach up to a foot in length. During this entire maturation period, your dog will likely look and act completely normal. A standard blood test can’t even detect the infection until the worms are mature enough to produce a specific protein, which is why the American Heartworm Society recommends testing annually with both an antigen test (which detects adult female worms) and a separate test for microfilariae, the microscopic offspring that circulate in the blood.
Early Signs: Stages 1 and 2
The first symptom most owners notice is a mild, persistent cough. It’s soft and dry, not the hacking cough you’d associate with kennel cough, and it tends to show up after physical activity. At this point, a relatively low number of worms are present, and some dogs won’t show any clinical signs at all.
As the disease moves into moderate territory, the cough becomes more frequent, and your dog may start showing reluctance to exercise. A dog that used to bound up stairs or play fetch enthusiastically might lag behind on walks, tire quickly after moderate activity, or simply lose interest in being active. You may also notice a decreased appetite and gradual weight loss. These changes can be subtle enough to chalk up to aging or a lazy day, which is part of what makes heartworm disease so easy to miss in its earlier stages.
What the Worms Are Doing Inside
Understanding the internal damage helps explain why symptoms escalate. Adult heartworms live primarily in the pulmonary arteries, the large blood vessels that carry blood from the heart to the lungs. Their physical presence obstructs blood flow, and the body’s immune response to the worms causes the artery walls to thicken and become inflamed. This combination of obstruction and inflammation forces the right side of the heart to pump harder to push blood through increasingly narrowed vessels. Over time, the increased pressure (pulmonary hypertension) weakens the heart. Research on infected racing greyhounds confirmed that this physical obstruction directly reduces exercise performance, even in otherwise athletic dogs.
Advanced Disease: Stage 3
When heartworm disease progresses to its severe stage, the signs become harder to ignore. Dogs at this point are dealing with heart failure, and the symptoms reflect it:
- Labored breathing that persists even at rest, not just after exertion
- A worsening cough that may become constant
- Swollen abdomen from fluid buildup as the failing right side of the heart causes blood to back up into the liver and abdominal cavity, giving dogs a pot-bellied appearance
- Muscle wasting and significant weight loss despite the bloated belly
- Fatigue and weakness severe enough that your dog may collapse after even minor activity
At this stage, large numbers of worms occupy the right side of the heart and the surrounding blood vessels. The damage is extensive, and even with treatment, some of it is permanent.
Caval Syndrome: A Life-Threatening Emergency
The most dangerous complication of advanced heartworm disease is caval syndrome, which happens when the worm burden becomes so heavy that worms physically block blood flow through the heart. The mass of worms disrupts the tricuspid valve (the valve between the heart’s right chambers), which often produces an audible heart murmur.
The hallmark sign of caval syndrome is dark, coffee-colored urine. This happens because red blood cells are being shredded as they pass through the tangled mass of worms, releasing hemoglobin that the kidneys then filter into the urine. Dogs with caval syndrome also show sudden weakness, depression, pale gums, poor pulse quality, difficulty breathing, and collapse. This is a true emergency. Without surgical removal of the worms, caval syndrome is fatal.
Signs That Are Easy to Overlook
Not every symptom of heartworm disease is dramatic. Some of the earliest and most commonly missed changes are behavioral rather than physical. Your dog might simply seem less enthusiastic about walks, sleep more than usual, or eat less at mealtimes. Weight loss can happen so gradually that you don’t notice it until you look at a photo from a few months earlier. Even the cough, which is the most recognizable heartworm symptom, can be intermittent enough that owners assume it’s nothing serious.
Dogs that spend a lot of time outdoors, especially in warm, humid climates where mosquitoes are active year-round, are at higher risk. But indoor dogs get heartworm too. Mosquitoes get inside, and it only takes one bite from one infected mosquito to start the process.
How Heartworm Is Detected
Because symptoms can be absent or vague for months, testing is the only reliable way to catch heartworm disease early. The standard approach uses two tests together. An antigen test detects a protein produced by adult female heartworms and is considered the gold standard for diagnosis. It’s highly sensitive and specific, meaning false results are rare. A microfilariae test checks for the larval offspring circulating in the bloodstream, which can sometimes catch infections that the antigen test misses.
There’s an important timing detail: because the worms take about six months to mature, a dog could be infected and test negative if tested too soon. This is why a dog that starts heartworm prevention later in life (at seven months or older) typically needs an initial test followed by a second test six to seven months later to confirm the result. Annual testing after that catches any new infections before they cause significant damage.