Heartworm Disease in Cats: Everything You Need to Know

By: BeChewy EditorsUpdated:

heartworm in cats: surprised kitty
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Heartworm Disease in Cats: Everything You Need to Know

Heartworm in cats is a scary disease that easily can hit close to home. Signs of heartworms in cats range from no symptoms at all to sudden collapse or death.

If your cat has been diagnosed with heartworm disease, there are things you can to do help them through it and ways to prevent it from ever happening again. Prevention is key!

FAQs about Heartworms in Cats

Here are the most common questions about heartworm in cats, answered by Bianca A. Zaffarano, DVM, Director at Veterinary Emergency Group in New York.
Heartworm disease in cats
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How common is heartworm in cats?

A: Heartworm disease occurs in every state in the U.S. Outdoor cats are more susceptible, but indoor cats can acquire heartworm if a mosquito makes its way indoors. The disease is less common in cats as in dogs.


Is heartworm contagious in cats?

A: No, heartworm in cats is not contagious. Because cats are not natural hosts for heartworms, the parasites do not reproduce inside cats. Heartworms’ natural hosts are dogs. Mature heartworms living in a dog’s tissue and heart can produce baby worms (microfilariae). These microfilariae are then spread by mosquitoes.


How do you know if your cat has heartworms?

A: The best way to know if your cat has heartworms is to take him to your veterinarian for testing. Though they can show some signs, which we discussed earlier, they also can demonstrate no symptoms at all.


Are there any natural home remedies for heartworm in cats?

A: There are no home remedies for heartworm in cats. The only “natural” or “organic” home remedy for heartworm in cats is to consider treatments that center on preventing or warding off mosquito bites.


When should I start heartworm prevention for my cat?

A: To prevent heartworm disease in your cat, begin heartworm prevention as soon as possible after your veterinarian tests for the disease. Heartworm prevention products are safe for cats 6 weeks of age and older, and they should be continued year-round for the entire life of your pet.

Preventing Heartworm in Cats

The best way to treat feline heartworm disease is to avoid it in the first place. To accomplish this, use heartworm preventives for cats year-round, no matter where you live, says Bruce Gordon Kornreich, DVM, a board-certified veterinary cardiologist and associate director of the Cornell Feline Health Center at Cornell University College of Veterinary Medicine in Ithaca, New York.

Keep your cat on a safe, proven-effective heartworm preventive medicine that contains ivermectin or selamectin. These products, available only through your veterinarian, are formulated to kill heartworms in their active, pre-adult stage.

Heartworm preventatives come in two forms:

  1. Topical: Topicals are poured on top of the shoulder blades. Revolution Plus for cats, for example, is a topical product that contains selamectin. It’s also used for flea control.

  2. Pill: The chewy pill form looks like a treat. Heartgard for cats is a chewable option that contains ivermectin. It’s also used to control hookworm infection.

“They have the same class of heartworm preventive drug, and some brands have other things in it, like drugs that will prevent intestinal parasites,” Dr. Zaffarano says.

Heartgard Chew for Cats, 5-15 lbs, (Purple Box), 6 Chews (6-mos. supply)

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Revolution Plus Topical Solution for Cats, 5.6-11 lbs, (Orange Box), 6 Doses (6-mos. supply)
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What Are Heartworms?

Heartworms in cats

Heartworms, Dirofilaria immitis, are foot-long worms that live in the heart as well as lungs and blood vessels of affected animals.

Spread by over 70 species of mosquitoes, heartworms can cause lung disease, heart failure and damage to other organs in the body, says Dr. Zaffarano.

In addition to cats, they can infect a range of other mammal species, she adds, including dogs, ferrets, wolves, coyotes, foxes, sea lions, otters and, in rare cases, humans.

How Do Cats Get Heartworms?

Cats get infected with heartworms via heartworm-positive mosquitos. Here’s how it works:

  1. First, a mosquito bites an animal that already is infected with heartworms. At this time, the mosquito picks up baby worms in the blood called microfilariae (the youngest larvae), and these microfilariae live inside the insect for 10 days to two weeks as they mature into their infective larvae stage.
  2. When that mosquito bites a cat, the infective larvae are deposited on the surface of the animal’s skin and enter the new host through the mosquito’s bite wound, Dr. Zaffarano explains.
  3. Once they’re inside the cat’s tissue, they wriggle through the body and are whisked into the bloodstream. “It takes about six months for them to move around the body and through the tissues and into the bloodstream,” she adds.
  4. From there, the infective larvae mature into adult worms in the heart, lungs and blood vessels.

The ideal host for heartworms is dogs, so many of the worms in cats die before even reaching their adult stage, Dr. Zaffarano says. As a result, a cat with heartworms typically has very few of them—one to three at the most—and the worms usually live two to three years. However, during that short period of time, they can cause some serious damage.

Signs of Heartworms in Cats

heartworm in cats: tired kitty

Photo: iStock.com/Svetlana Sultanaeva

About four months after a mosquito with microfilariae infects a cat, the first signs of heartworms can start to appear with symptoms ranging from difficulty breathing to sudden death, Dr. Zaffarano says. Or, your cat can show no symptoms at all.

“The symptoms can be very subtle, and people can mistake them for other things, or they can be very dramatic,” she says.

Cats with immature worms may show the following signs, which often resemble those of feline asthma:

  • Vague malaise
  • Intermittent coughing
  • Difficulty breathing
  • Heartworm-associated respiratory disease (HARD)
  • Vomiting
  • Lethargy
  • Anorexia
  • Weight loss

Cats with mature worms may show:

  • Intermittent vomiting
  • Fluid in the belly
  • Lethargy
  • Coughing
  • Difficulty breathing
  • Acute respiratory distress
  • Shock
  • Death

While they mainly live in the heart, lungs and blood vessels, heartworms can travel to other parts of the cat’s body. This process is known as aberrant migration.

“They can migrate to the central nervous system—the brain, the spinal cord—as well as the eyes and other organs,” Dr. Zaffarano says. “The damage they cause in other parts of the body results in all kinds of other complications and inflammation when the worms die.”

Diagnosing Heartworm in Cats

Vets use two tests to diagnose heartworm in cats: an antigen blood test and an antibody blood test.

  • An antigen test checks for the presence of heartworm proteins made by the female.
  • The antibody test checks for exposure to heartworm larvae and for the cat’s immune response against the parasite.

Why do cats need both tests? Because cats are much less likely to have adult heartworms, they often have a single-sex infection, which means there may not be any female worms to detect, Dr. Zaffarano says.

If the veterinarian thinks the cat is positive, they may choose to do an X-ray, a radiograph or an ultrasound to confirm a diagnosis. Sometimes a good cardiologist can actually see the worms on the ultrasound, Dr. Zaffarano adds.

Treatment of Heartworm in Cats

Unfortunately, the go-to drug that destroys both mature and immature heartworm in dogs—melarsomine dihydrochloride—is not safe for cats. So, treating feline heartworm disease in cats can be difficult.

There’s no way to kill the adult heartworms, so cats have to live with any mature heartworms until the heartworms die, Dr. Zaffarano says.

Instead, you’ll work with your vet to alleviate symptoms of heartworm disease.

“You can try to diminish their inflammation, reduce their cough and, if they have liver problems, you can try and support liver function,” Dr. Zaffarano says. “Sometimes, you may have to go down the antibiotics or anti-inflammatory path to help with heart function.”

Dr. Kornreich agrees.

“With cats, it’s supportive maintenance and dealing with inflammation that occurs from these worms, and hopefully you don’t have any aberrant migration,” he says. “If the cat is having trouble with heartworm-associated respiratory disease, your veterinarian can give [him] oxygen and corticosteroids to help with inflammation.”

The only thing you can really do is nurse them through it, hospitalize them for supportive care and hope for a full recovery.

“But prevention is key,” Dr. Zaffarano emphasizes.

Not sure whether to see a vet? Use PetMD's Symptom Checker. Answer a few questions about your pet's symptoms, and the vet-created online tool will give you the most likely causes and next steps.
Chat with your vet on the best heartworm prevention treatment for your cat.


By: BeChewy EditorsUpdated: