Do Cats With Heartworm Disease Show Symptoms?

It can be hard to spot 😬

cat heartworm symptoms

You’ve probably been warned about the dangers of heartworm disease in cats. But if your cat does get infected, you’ll want to know how to spot the signs yourself so you can get him treated ASAP.

But unfortunately, it’s incredibly difficult to identify whether or not your cat has heartworm because symptoms don’t really show up. And if they do, they’re often misdiagnosed, especially since heartworm testing can be unreliable.

So how can you know if your cat has heartworms?

The Dodo spoke with Dr. Zach Marteney, a veterinarian and medical director at Meadowlands Veterinary Hospital in New Jersey, to find out everything you need to know about heartworm symptoms in cats — or a lack thereof.

What is heartworm in cats?

Heartworm disease is when worms infect your cat’s heart after being transmitted through the bite of an infected mosquito. The worms will live inside your cat’s pulmonary artery, which carries blood from his heart to his lungs, and can cause lots of damage that can be fatal for your cat.

Cats are actually more resistant to a heartworm infection compared to dogs, and an infection is more difficult to spot when testing cats.

Cats will also often have fewer worms in their systems than dogs — but even a small amount of worms can be life-threatening.

Signs of heartworm in cats

The scary part about heartworm disease is that most cats don’t show any symptoms at all — at least not until the worms die.

“Between the worms' arrival and their death, many cats are asymptomatic,” Dr. Marteney told The Dodo.

Once the worms die, that’s when complications can get really serious and pretty dangerous.

“When the worms die, they can travel deep into the vessels of the lungs, blocking blood flow to those areas and preventing oxygen from being able to exchange across the lung-blood barrier,” Dr. Marteney said. “These pulmonary thromboembolisms are very acute and are often fatal.”

According to Dr. Marteney, that means sudden death could tragically end up being the first and only sign that your cat has heartworm disease.

If your cat somehow manages to show symptoms of heartworm disease, they’re going to look a lot like asthma, which can often lead to a misdiagnosis.

“The cat's immune response results in inflammation of the pulmonary arteries, which can clinically look much like asthma and is often misdiagnosed as such,” Dr. Marteney said.

Signs that your cat has heartworm disease include:

  • Coughing
  • Wheezing
  • Gagging
  • Difficulty breathing
  • Rapid breathing
  • Vomiting
  • Weight loss

But these asthma-like symptoms aren’t necessarily constant throughout the course of your cat’s infection. If they do show up, they’d be the most noticeable at the beginning of the infection and at the end of the worms’ life cycles.

“These clinical signs are most significant when the worms first arrive in the pulmonary arteries and again when the adult worms die at the end of their natural lives — usually about two to three years after infection,” Dr. Marteney said.

Heartworm test for cats

Regular annual testing for heartworm in cats is important but not always reliable.

“It's very likely that there are many more cats with heartworm disease than are actually diagnosed,” Dr. Marteney said.

That’s because most heartworm tests for cats can only spot female worms and not the males. So the test can completely miss your cat’s infection if there are only male worms present.

“Oftentimes the worms in an infected cat are both the same sex, which prevents continued reproduction of the worms and breaks the life cycle,” Dr. Marteney said. “However, the standard heartworm test done at veterinary hospitals looks for an antigen produced by the female worm, which makes testing very difficult in cats — an all-male worm load will not test positive.”

In addition to standard testing, you could try an antibody test, which will check whether or not your cat’s immune system contains antibodies for heartworm disease.

However, he might not test positive for antibodies a while after being infected, and even then there’s no way of knowing if his antibodies are due to a current or past infection.

“An antibody test is also available that looks for the cat's immune system response, but it takes several weeks after infection to show up positive,” Dr. Marteney said. “And a positive test doesn't mean there is a live-worm infection — just that the cat has been exposed to heartworms at some point in its life.”

And even then, it can be pretty difficult to track down just how recently your cat was infected.

“The cat can remain antibody-positive for several years after exposure,” Dr. Marteney said.

Heartworm prevention for cats

Preventing your cat from getting heartworm in the first place is crucial, since it can be almost impossible to spot an infection before it’s too late.

There are two main types of heartworm prevention: topical ointments and oral medications. These are prescription products, so you’ll need to talk to your vet about which option is right for your cat.

Check out Heartgard chewable tablets from Chewy for $39.99

Or Revolution Plus topical solution from Chewy for $124

“These preventatives also treat and prevent intestinal parasites, so I recommend using them year-round, not just during ‘mosquito season,’” Dr. Marteney said.

Heartworm prevention is super important to maintain regularly, since most cats with heartworm will show no symptoms at all — and if they do, chances are it’ll be misdiagnosed as asthma.

And even regular screenings can be unhelpful, since they only detect female worms and can’t determine whether or not your cat is currently infected.

So make sure you stay on top of those preventatives to protect your cat and his heart.

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