Heartworm disease in cats is notoriously difficult to detect because the symptoms mimic other common conditions, especially asthma and bronchitis. Many cats show only vague, intermittent signs for months before anyone suspects heartworms. Understanding what to watch for matters more in cats than in dogs, because there is no FDA-approved treatment to kill adult heartworms in cats once they’re established.
The Most Common Warning Signs
The symptoms of feline heartworm disease center on two body systems: the lungs and the digestive tract. Respiratory signs are the hallmark. Cats with heartworms often develop rapid or labored breathing, coughing, and gagging that looks almost identical to a feline asthma attack. These episodes may come and go over weeks or months, which makes them easy to dismiss as a minor respiratory issue.
Intermittent vomiting is the other telltale sign, and it’s one that surprises most cat owners. A cat with heartworms may vomit food, bile, or occasionally blood at irregular intervals. Because cats vomit for so many reasons, this symptom alone rarely triggers suspicion. Diarrhea, loss of appetite, weight loss, and general lethargy round out the picture. No single symptom confirms heartworms on its own. The pattern to watch for is a combination of recurring respiratory trouble and unexplained vomiting or weight loss that doesn’t resolve with standard treatment.
Why It Looks Like Asthma
When a mosquito transmits heartworm larvae to a cat, those immature worms travel to the lungs and begin growing inside the pulmonary arteries. Even before they reach adulthood, these larvae trigger intense inflammation in the lung tissue and surrounding blood vessels. This early-stage damage has its own name: heartworm-associated respiratory disease, or HARD.
HARD produces coughing, wheezing, and difficulty breathing that is, for all practical purposes, indistinguishable from feline asthma during a routine exam. A cat can develop significant lung inflammation from immature worms that never survive to adulthood. This means your cat can suffer real respiratory damage from heartworms even if the parasites eventually die off on their own. It also means a cat diagnosed with “asthma” that doesn’t improve with typical asthma treatments may actually have heartworm disease.
Sudden, Severe Complications
One of the more alarming aspects of feline heartworm disease is that some cats show no obvious symptoms at all until a crisis hits. Cats typically harbor only one to three adult worms (compared to dozens or even hundreds in dogs), but even a single worm dying inside a cat’s body can cause a massive inflammatory reaction. This can lead to acute respiratory distress, collapse, or sudden death. In some cases, the first sign that anything was wrong is the worst possible outcome. This is a key reason veterinarians emphasize prevention over detection in cats.
Indoor Cats Are Not Safe
If your cat never goes outside, you might assume heartworms aren’t a concern. That assumption is wrong. Mosquitoes get indoors through open doors, windows, and gaps in screens. A study at North Carolina State University found that over one-fourth of cats diagnosed with heartworm disease were described by their owners as strictly indoor pets. Any cat living in an area where mosquitoes exist is at some level of risk.
How Vets Diagnose Heartworms in Cats
Diagnosing heartworm disease in cats is considerably harder than in dogs. There’s no single test that gives a definitive yes-or-no answer, so vets typically use a combination of approaches.
Two blood tests are commonly used. An antibody test checks whether the cat’s immune system has responded to heartworm larvae at any point, meaning it can detect exposure even from immature worms. A positive result means the cat has been infected but doesn’t confirm that adult worms are currently present. An antigen test detects proteins released by adult female heartworms. The catch: because cats carry so few worms, and because the test only detects females, a cat with one or two male worms can test negative on the antigen test despite being infected. Vets often run both tests together to get a clearer picture.
Chest X-rays provide additional clues. Characteristic signs include enlarged pulmonary arteries (especially on the right side of the chest), an enlarged heart silhouette, and patchy changes in the lung tissue. Enlargement of the arteries in the lower lung lobes is one of the most frequently observed abnormalities. Vets may measure the width of the right pulmonary artery relative to a nearby rib. A ratio above 1.6 has been associated with heartworm infection in cats.
Echocardiography, or ultrasound of the heart, can sometimes allow a vet to directly visualize worms inside the heart or pulmonary arteries. When worms are visible on ultrasound alongside positive blood tests, the diagnosis is considered highly reliable. But small worm burdens can be missed, so a normal ultrasound doesn’t completely rule out infection.
Treatment Options Are Limited
Here is the difficult reality: there is no FDA-approved medication for killing adult heartworms in cats. The drug used to treat heartworm disease in dogs is too dangerous for cats because even a single worm dying and breaking apart inside a cat’s small pulmonary vessels can trigger a fatal reaction.
Instead, management focuses on controlling symptoms while the cat’s body attempts to outlive the worms. Adult heartworms in cats survive roughly two to three years, compared to five to seven years in dogs. During that time, vets may use medications to reduce lung inflammation and manage respiratory distress. Cats with confirmed infections need regular monitoring with X-rays and blood tests to track the disease’s progression. In rare, severe cases, surgical removal of worms is attempted, but this carries significant risk.
Because treatment is so limited and risky, monthly heartworm prevention is the single most effective thing you can do. Several preventive products are available for cats, and they work by killing heartworm larvae before they mature. This applies to indoor and outdoor cats alike, year-round, in any region where mosquitoes are present.