12 Heartworm Symptoms in Cats You Shouldn’t Ignore
Heartworm in cats is not talked about as much as heartworm in dogs, but cats can become infected with heartworms, too. In some cases, the prognosis is deadly. Heartworm symptoms in cats can be subtle or even nonexistent, so it’s important to keep cats protected from becoming infected in the first place.
According to Mark Cousins, DVM, DABVP (Feline Practice), of The Cat Practice Veterinary Hospital in New Orleans, Louisiana, it’s hard to know the true prevalence of heartworm disease in cats because most veterinarians do not routinely screen cats for heartworm. However, a 2020 study comparing heartworm prevalence for a random sample of 100 shelter dogs and 100 shelter cats from a heartworm-endemic region found that the rate of infection may be more similar between dogs and cats than previously thought (1).
“As a feline practitioner from New Orleans—where heartworm rates are known to be quite high in dogs—I can tell you that heartworm is also very common in cats,” says Dr. Cousins, who is on the board of directors for the American Heartworm Society. “Anyone with a cat who lives in an area known to have heartworm in dogs should also be concerned about protecting their cats.”
How Do Cats Get Heartworm?
Cats become infected with heartworms when they are bitten by a mosquito that carries heartworm larvae. The larvae develop into immature adults within the cat’s tissues and then enter the bloodstream. The immature adults become mature adult heartworms after reaching their ultimate destination—the heart and its associated vessels.
Unlike dogs, cats are not ideal hosts for heartworms, so although cats can become infected, heartworm disease is quite different in cats than dogs. “[Cats] can and do develop adult worms, but the majority of infections in cats are from immature worms that are eliminated before they reach the adult stage, thanks to the cat’s immune system,” Dr. Cousins says. “These immature worms are not harmless; they cause disease in the cat.”
According to Dr. Cousins, cats usually harbor less than six adult heartworms, and single-worm infections are common. Heartworm-infected dogs generally have a dozen or more adult heartworms, sometimes even carrying over 100 worms.
12 Heartworm Symptoms in Cats You Shouldn’t Ignore
Heartworm symptoms in cats range from very mild to severe. Because cat heartworm symptoms resemble those of other illnesses and diseases, they can sometimes be overlooked. According to the American Heartworm Society, some common signs of heartworm in cats include:
- Chronic coughing
- Asthma-like attacks
- Lack of appetite
- Weight loss
- Difficulty walking
- Fluid accumulation in the abdomen (ascites)
- Sudden death
“Death from heartworm disease in cats typically occurs when an adult worm living inside the cat dies,” Dr. Cousins says. “This causes an acute respiratory disease syndrome. The death of just one adult worm inside a cat can cause a fatal reaction.”
Cat Heartworm Symptoms: Next Steps
If you’re worried that your cat might have heartworms, do not delay scheduling an appointment with your veterinarian. There are two simple blood tests veterinarians can use to screen cats for heartworm infection:
Antigen tests, which are used to screen dogs for heartworms, only detect the presence of adult female heartworms. Antigen tests are a useful tool to detect adult heartworms in cats, but only if a cat is harboring adult female worms.
“A better screening test for cats is an antibody test, which tells the veterinarian that the cat has been infected with heartworms at some point in its past,” Dr. Cousins says. “This tells the veterinarian and owner that the cat has been exposed to and infected with heartworms and therefore is at risk.”
If a cat has a positive antibody test, the vet can perform an antigen test and/or heat-treated antigen test (which helps rule out potential false-negative test results) to see if adult worms are present. Chest X-rays and an ultrasound of the heart (echocardiogram) may also be recommended.
Dr. Cousins points out that most cats with adult heartworms can survive, especially if they receive good supportive care. “Anecdotal evidence suggests that anti-inflammatory medications like steroids, as well as a class of drugs called leukotrienes, can be helpful,” he says.
Owners of cats that test positive for adult worms should talk to their veterinarian about spotting signs of acute respiratory disease, which can indicate that worm death is occurring. Supportive care at the veterinary clinic can help cats survive this critical time.
Do Cats Need Heartworm Prevention?
Pet parents might think that indoor cats don’t need to worry about heartworms, or that prevention is only needed in the summer months, but both of these assumptions are false. The American Heartworm Society recommends year-round heartworm prevention in all cats, including those that live primarily indoors.
“Studies have shown that heartworm in cats is not limited to outdoor pets,” Dr. Cousins says. “Mosquitoes love to come indoors! And it only takes one feeding session by a mosquito to infect a cat. No one knows when the first and last mosquitoes of a so-called season will be present.”
Mosquitoes can survive in colder climates due to elevated temperatures within urban “heat islands” (areas of warmth created by buildings and concrete). Mosquitoes can even live indoors, so it’s impossible to identify an entirely safe season when cats are not at risk of becoming infected with heartworms.
“I also find that cat owners are better at adhering to heartworm prevention if they give a pill or apply a spot-on medication on a routine, year-round basis,” Dr. Cousins says.
Although effective treatment options are available for dogs with adult heartworms, there are no FDA-approved treatments for heartworms in cats. “This makes prevention especially important for cats, and there are safe and highly effective preventives out there for them,” Dr. Cousins says.
Every heartworm preventive for cats protects against multiple parasites. Depending on the product you choose, you can keep your cat safe from heartworms—as well as from harmful intestinal worms like roundworms and hookworms, and/or external parasites like fleas, ticks and mites—year-round.