12 Common Signs of Heartworm in Dogs and Cats Every Pet Owner Must Know

Your dog seems a little tired lately. Your cat has been coughing on and off. Easy to brush off as nothing, until it isn’t.

Heartworm disease is a silent predator. It doesn’t announce itself with obvious symptoms right away. Instead, it quietly takes hold after a single bite from an infected mosquito, sending heartworm larvae through your pet’s blood vessels toward the heart and lungs — where they grow, multiply, and cause damage that can become irreversible. By the time most pet owners notice something is wrong, the disease has often been progressing for months.

The good news? The signs of heartworm in dogs and cats are recognizable — if you know what to look for. This guide walks you through all 12 warning signs, clearly broken down by species, so you can spot trouble early and take action before it becomes a crisis.

7 Signs of Heartworm in Dogs

Dogs are natural hosts for heartworms, meaning the worms Dirofilaria immitis can mature, reproduce, and increase in number within an infected dog over time. Heartworm disease in dogs tends to progress through clear, recognizable stages, giving pet owners a window to catch it early.

Sign 1: Persistent Cough

A dry, nagging cough that simply won’t go away is one of the earliest and most consistent signs of heartworm disease. As heartworm larvae mature into adult worms and take up residence in the lungs and surrounding blood vessels, they trigger ongoing irritation. Unlike a cough from a seasonal allergy, a persistent cough linked to heartworm disease in dogs doesn’t resolve on its own — it worsens as the infection progresses.

Sign 2: Reduced Exercise Tolerance

Does your dog tire out faster than usual on walks? Reluctance to exercise or sudden fatigue after moderate activity is a hallmark early warning sign. The heart and lungs are under constant strain from adult worms, which obstruct normal blood flow, making even light physical effort feel exhausting for your pet.

Sign 3: Lethargy and Weakness

When the heart is working harder to compensate for blocked blood vessels, the rest of the body suffers. Dogs showing signs of heartworm disease often become noticeably less active — sleeping more, disengaging from play, and losing their usual spark. This general weakness is a direct result of the cardiovascular stress caused by heartworm infections.

Sign 4: Rapid or Labored Breathing

As heartworm disease in dogs advances, the worms cause increasing lung disease by inflaming and damaging lung tissue. Breathing may become visibly faster or more strained, even when your dog is resting. If you notice your dog struggling to catch their breath without any physical exertion, that’s a sign that warrants immediate veterinary attention.

Sign 5: Loss of Appetite and Weight Loss

Internal strain from a heavy worm burden causes discomfort that suppresses appetite. Over time, this leads to noticeable weight loss and muscle wasting — a concerning sign that the body is being pushed to its limits trying to fight a growing parasitic load. If your dog is losing weight without any dietary changes, don’t ignore it.

Sign 6: Swollen Belly

A rounded, bloated abdomen — caused by fluid accumulating in the abdominal cavity — is a sign of heart failure and a signal that heartworm disease in dogs has progressed to a serious stage. At this point, the heart is no longer able to circulate blood efficiently, and fluid backs up into surrounding tissues.

Sign 7: Pale Gums, Fainting, or Collapse

In the most severe cases, an infected dog may develop what is known as caval syndrome — a life-threatening condition where a massive tangle of adult worms physically blocks blood flow back to the heart. Pale or bluish gums, dark urine, and sudden collapse are the hallmark signs. Without emergency surgical removal of the worm blockage, most dogs do not survive.

5 Signs of Heartworm in Cats

Heartworm disease in cats behaves very differently. Cats are atypical hosts, meaning most heartworm larvae don’t survive to the adult stage. But “fewer worms” doesn’t mean “less danger.” Even immature worms trigger a severe inflammatory response in the lungs — a condition known as heartworm-associated respiratory disease (HARD) — and even a single dying adult worm can be fatal to a cat. The signs of heartworm in cats are often vague, making them easy to dismiss.

Sign 8: Intermittent Vomiting

Periodic vomiting, often unrelated to meals, is one of the most frequently overlooked signs of heartworm disease in cats. It stems from the body’s systemic inflammatory response to the presence of heartworm larvae and is commonly mistaken for a digestive issue or hairball problem.

Sign 9: Coughing and Asthma-Like Attacks

Respiratory symptoms are the most prominent signs of heartworm in cats. Coughing, wheezing, and sudden asthma-like episodes are often caused by heartworm-associated respiratory disease — the lung disease triggered when immature heartworm larvae die in the pulmonary blood vessels. HARD is frequently misdiagnosed as feline asthma or bronchitis, delaying proper treatment.

Sign 10: Rapid or Difficult Breathing

Cats typically show respiratory distress at two critical points during the infection: when heartworm larvae first enter the heart and the lung arteries, and again when adult worms die and release toxins into the bloodstream. Both events cause lung disease and breathing difficulties that can come on suddenly and severely.

Sign 11: Sudden Lethargy and Decreased Appetite

Because cats have small body sizes, even one or two worms can exert a disproportionate effect. Cats that show signs of heartworm disease often become withdrawn, stop eating, and lose interest in activities. Because this mimics so many other feline conditions, it’s easy to miss — but combined with any respiratory sign, it should prompt an urgent vet visit.

Sign 12: Sudden Collapse or Seizures

Perhaps the most heartbreaking reality of heartworm in cats is that for some, the first visible symptom is collapse — or sudden death. Without any prior warning signs, the death of even a single adult worm can cause an acute, often fatal crisis. This makes routine prevention and screening absolutely critical for cats of all ages.

Why Heartworm Signs Differ Between Dogs and Cats

The core difference comes down to host biology. Dogs are natural hosts, so heartworm larvae consistently develop into adult worms, reproduce, and accumulate over time — sometimes numbering in the hundreds in a heavily infected dog. The damage builds gradually, which is why signs of heartworm disease in dogs tend to follow recognizable stages.

Cats, on the other hand, are accidental hosts. Most worms called heartworms don’t reach adulthood in cats. But the immune system’s battle against even immature worms triggers heartworm-associated respiratory disease, causing acute lung disease that can be severe and unpredictable. The American Heartworm Society emphasizes that because there is no approved drug to treat heartworm infections in cats, prevention is the only reliable protection.

Heartworm Preventives that Actually Work

• Senergy Topical Solution: A monthly spot-on treatment using selamectin that prevents heartworm disease and also controls fleas, ear mites, roundworms, and hookworms. Available in multiple weight variants for small to extra-large dogs.
• Selarid Topical Solution for Dogs: Contains the same active ingredient as Revolution at a more budget-friendly price point. Broad-spectrum protection against heartworm larvae, fleas, ticks, and mites for dogs across all size ranges.
• Interceptor Chewables: A palatable monthly chewable tablet that prevents heartworm disease and treats adult hookworm, roundworm, whipworm, and tapeworm infections in dogs.
• MilbeGuard Tablets: A cost-effective generic to Interceptor, offering the same broad-spectrum milbemycin oxime coverage. Suitable for dogs and cats, preventing heartworm and controlling intestinal parasites with a single monthly dose.
• Selarid Topical Solution for Cats: A monthly spot-on heartworm preventive for cats formulated with selamectin. Controls flea infestations, heartworm disease, and ear mites. Safe for cats from 8 weeks of age.

Always consult your veterinarian before starting any heartworm preventive, and ensure your pet is tested for existing heartworm infections before beginning a new prevention program.

FAQs about Signs of Heartworm in Dogs and Cats

Q. What are the first signs of heartworm in dogs?

A: The earliest signs of heartworm in dogs are often subtle — a mild persistent cough, reduced stamina during exercise, and slight weight loss. These early symptoms are easy to miss, which is why annual testing is essential even when a dog appears healthy.

Q. What are the first signs of heartworm in cats?

A: The most common early signs of heartworm in cats include intermittent vomiting, lethargy, and coughing or wheezing that resembles asthma. Some cats are entirely asymptomatic, making routine preventive care the only reliable safeguard.

Q. Can indoor pets get heartworm disease?

A: Yes. Both indoor dogs and cats are at risk. Infected mosquitoes can enter homes easily, and even one bite from a mosquito carrying heartworm larvae is enough to start an infection. No pet is truly safe without a consistent heartworm preventive.

Q. How long does it take for heartworm signs to appear after infection?

A: In dogs, heartworm larvae take approximately 6 months to mature into adult worms before signs of heartworm disease appear. In cats, the process takes 7–8 months, though HARD symptoms from dying larvae can emerge as early as 3–4 months after the initial mosquito bite.

Q. Is there a heartworm preventive for cats available at HardyPaw?

A: Yes. HardyPaw carries several vet-approved cat-specific options, including Selarid Topical Solution for Cats and Centragard — both designed to prevent heartworm infections in cats with monthly application. Since there is no approved drug to treat heartworm in cats, prevention is critical.

Prevention is always easier than treatment. Explore HardyPaw’s full range of heartworm preventives for dogs and cats, and speak with your veterinarian about the best option for your pet.