Heartworm In Cats – An Owner’s Guide

Have you heard the term "heartworm" before? Sounds awful, doesn't it? If you're worried that this might be affecting your cat's health, you're not alone.

That's why we've put together a concise detailed guide for cat owners.

Heartworm in Cats: An Owner's Guide. Everything you need to know about the risk of heartworm in felines, including treatment and prevention

Heartworm (scientific name: dirofilaria immitis) is a parasitic worm that is transmitted to new hosts through the bites of infected mosquitoes.

Though its name suggests that this parasite only affects the heart of the host, the foot-long parasites can, in fact, damage the lungs and other organs in the body.

While it is commonly associated with dogs, other animals can become hosts to heartworm as well – including felines.

In this article, we will explain the lesser-discussed condition of heartworm as it occurs in cats.

Continue reading to understand how heartworm affects cats, how to notice the symptoms of a heartworm parasite, and what the proper course of treatment is.

Yes, Cats Get Heartworm Too

Veterinarians vaccinate an orange cat in the leg area to prevent rabies, leukemia, feline AIDS, and heartworm. at the clinic

Because felines are less likely to be carriers of heartworm, they are typically left out of the dog-centric discussion of this parasite.

A dog’s body is a friendlier host to heartworm than a cat’s, making the latter less vulnerable to infection.

However, this does not mean that any cat is immune to the devastating effects of this insidious worm. Indoor and outdoor cats can host heartworm. All it takes is a bite from the wrong mosquito.

A dog’s body may host anywhere between 1 and 250 worms, with the worms taking years – perhaps even a decade or more – to fully mature from their larval stage.

Cats carry a worm burden (the number of worms infecting their bodies) that is substantially lower than dogs, and in many cases the worms inside of a cat’s body will not likely live to maturity.

Don’t be fooled into thinking that heartworm is a lesser risk for cats than for dogs simply because fewer worms can exist inside of feline hosts.

Because the average cat is smaller than the average dog, even a few worms can put the cat into a “heavily infested” state. Even one heartworm carries with it significant health ramifications.

Indoor Cats are Also Vulnerable

Because it only takes one bite from an infected mosquito to introduce heartworm to a cat’s body, any cat is a potential carrier of the parasitic lifeform.

A stray mosquito getting inside your home could pose a health risk if it carries heartworm.

There is no surefire way to make sure that your cat will not encounter a mosquito carrying the worm, so get your cat on vet-prescribed preventative medication if you are seriously concerned about the matter.

In fact, when we asked cat veterinarian Dr. Letrisa Miller, DVM, about this, she said -

Surprisingly, I have seen more indoor-only cats infected with heartworm than outdoor cats.

I have talked with other practitioners that have seen the same thing, and we think it is because mosquitoes are drawn to the higher levels of carbon dioxide around homes, and cats are a magnet for mosquitoes due to their higher body temperature that attracts the insects.


Bengal cat lying on sofa and smiling.

The Symptoms of Heartworm Infections in Cats

The most problematic thing about heartworm in cats is that there can be no symptoms at all – until suddenly, there are.

A cat may exhibit no symptoms right until they die from the parasite, while other cats may be able to spontaneously rid their bodies of the worm or worms.

This problem is further compounded by the fact that many symptoms of heartworm in cats are also symptoms of other cat diseases.

Difference Between Cats and Dogs

In dogs, heart failure is a common outcome of heartworm infection. This is not the case for cats, on the other hand, who rarely experience heart failure as a result of being infected with this parasite.

Potential Symptoms in Cats

Some symptoms that you might see, if the cat displays any symptoms at all, include -

  • Vomiting
  • Decreased appetite
  • Weight loss
  • Diminished activity levels.
  • Coughing (Heartworm Associated Lower Respiratory Disease, AKA HARD)
  • Sudden death

Sudden death happens due to respiratory distress when an adult heartworm dies and causes massive inflammation in the lungs or anaphylaxis.

Respiratory problems could also be observed as the matured worms start to cause damage to the cat’s lungs.

How Heartworm Damages Cat Lungs: Dr. Miller's Insight

According to Dr. Miller, heartworm causes damage to the lungs in several ways:

  1. At the larval stage, larvae migrate through the lungs. The cat can develop HARD even if it never gets an adult worm. The inflammatory response to the larvae is all it takes.
  2. The mature worm usually resides in the pulmonary blood vessels in most cats, so it damages the lungs in cats, rather than the heart as is the case in dogs.
  3. The last type of damage the worm causes in the cat’s lungs is the most severe. When an adult worm dies and fragments, it can cause such massive and severe inflammation and blood vessel blockage that the cat dies suddenly.

"I have seen some cats that have had an adult heartworm which died, and the attentive owners were with the cat at the time," Dr. Miller says.

"Most of these cats I have been able to save because of the owner’s quick action in bringing the cat for oxygen and supportive care. Without oxygen and drug therapies, I have no doubt that the cats would have died."

Dr. Miller added that cats who show chronic coughing, vomiting or exercise intolerance should be tested for heartworm.

The Prevalence of Heartworm in Cats

A domestic striped kitten with a sad face lies on the trail. Sick pet.

A cat’s body is a hostile environment for heartworms looking to make a home.

Not only do they have the capacity to carry fewer worms than dogs, the likelihood of the worms reaching maturity is much less. This does not mean that cats cannot carry heartworm or that it is not potentially lethal to them.

Unfortunately, when cats get an adult heartworm it usually survives for 2 to 3 years.

Only when the worm dies and starts to fragment does it break into pieces that lodge in the blood vessels of the lungs. This is how circulation can be blocked (embolism) or severe inflammation caused. Many cats die suddenly when this happens.

While cats are much less likely to contract the parasite than dogs, they are not immune. All cats, indoor and outdoor, can become hosts.

The Dangers of Heartworm in Cats

Do you remember how we said that cats' bodies are not as friendly to heartworms as the bodies of dogs? This is because most of the worms that will enter the cat’s body die within a few months of infection.

This death, while sparing the cat from a massive heartworm infection, does cause a strong inflammatory response in the cat’s lungs. In reality, in many cases, the death of these worms is more harmful to the cat than the presence of live worms.

The reaction detailed above is known as Heartworm Associated Respiratory Disease, or HARD, and can lead to sudden death. Symptoms of HARD are much like symptoms of feline asthma:

  • Difficulty drawing breaths
  • Increased respiratory rate
  • Coughing

Even the presence of one worm or the migration of immature larvae through the lungs can trigger this response and end a cat’s life or cause HARD with chronic asthma as the result, making infection very dangerous for any feline infected.

Treatment Options for Cats with Heartworm

Portrait of a tricolor kitten lying on the floor. Feline depression. Sad cat. Bored cat.

At this time, there are no FDA-approved medications that can treat cats with heartworm.

This makes heartworm infection more dangerous in some ways for cats than it is for dogs, as treatment options are minimal at best.

What makes this even trickier is the fact that the blood tests used to diagnose cats with heartworm are not 100% reliable.

Veterinarians use a combination of two different blood tests to check cats for heartworm, but a positive result doesn’t necessarily indicate that the cat is infected.

Similarly, a negative result doesn’t guarantee that the cat is free and clear of infection. It is up to the vet to gauge these results, in combination with the cat’s symptoms, to determine whether the cat is host to this parasite.

The vet may utilize other testing methods, such as X-ray imaging and ultrasound, to aid in their diagnosis.

It is possible that the heartworm infestation will resolve itself in a cat’s body. When this doesn’t happen, surgery is the most reliable method of ridding cats of the parasite.

However, surgery is very risky. Should anything be left behind or a mishap occur during surgery, the cat could go into shock or die.

Instead of the surgical option, many cat owners prefer to work alongside their vet to come up with a treatment plan to manage the symptoms of the infection.

This will not cure the cat of the infection but can help them to live longer, fuller lives with heartworm.

Preventing Heartworm in Cats

While there is no approved method of curing cats of heartworm, there are several options available to help prevent the presence of heartworm in the first place.

Oral and topical treatments can be prescribed by a vet to keep heartworms away - if there's a major risk of infestation in your area.

Some of these medications even help to prevent the occurrence of other types of infestations like hookworm, roundworm, and fleas.

Because you can’t always keep mosquito netting around your cat, there’s little you can do to prevent the bite of a mosquito from infecting your feline. Prevention is essential, and these medications are the best option.

You may have heard that heartworm prevention medication can be harmful – even lethal – if the animal is already a host to the worm. Dr. Miller explains that this is true for dogs, but not necessarily for cats:

Most heartworm preventatives for cats can be given without testing and do not pose a risk because cats only get microfilaria in extremely rare cases, so they are different from dogs in this respect as well.

It is the massive death of microfilaria that causes the reaction to preventatives that is seen in infected dogs.

Is Heartworm Contagious?

The key to the spread of heartworm is the common mosquito. The emergence of heartworm in any animal relies on an infected mosquito carrying microfilariae, microscopic baby worms that move throughout the bloodstream of an infected animal that it’s bitten.

Over the span of about two weeks, the microfilaria reaches the larval stage that poses a risk to our pets.

When a mosquito that carries these young worms bites another animal, they enter the creature’s bloodstream through the wound made during the mosquito’s bite.

In this way, heartworm can be transmitted to numerous animals.

Because the mosquito is the essential link between infected animals and uninfected ones, you can rest assured that your pets cannot transmit heartworm to each other through any form of contact.

It is not contagious like a cold. Other animals cannot get it from your pet, and neither can you.

Having said that, if you have an infected dog or cat in your home, the likelihood of mosquitoes becoming infected and then transferring the parasite is higher.

Heartworm is a Real Concern for Cats

A Scottish cat is lying on the sofa.A sick cat is lying on the couch. The cat misses its owner. Emotional state of the cat.

No, your cat is not likely to contract or die from heartworm. But that does not mean that they cannot become infected, whether indoor or outdoor, or that it poses no risk to their health.

Heartworm can kill a cat, who may present no symptoms until their death.

For this reason, it is a wise move to get your cat heartworm tested by their veterinarian and then have them prescribe a heartworm preventative medication.

Prevention is easier and less expensive than treatment for heartworm infections or their symptoms. Many of these medications are also effective at combating other types of worms, fleas, and ticks.

Outdoor cat owners especially should consider this step in caring for their pets. Especially due to the increased exposure risk posed out in nature.

The fact of the matter is that heartworm spreads fast. The pattern of new infection varies wildly from year to year.

The infection of wild animals causes the parasite to spread out further. Additionally, certain climate activities could help encourage the spread.

It is very possible to unknowingly travel with your cat to an area that’s hot with heartworms. This is because it’s not something that can easily be tracked.

If your veterinarian recommends it, avoid the worry and fretting over your cat playing host to heartworm by taking precautions to ensure their health and longevity.

Discuss what risk factors of your area could increase your pet’s risk of heartworm infection and work with your vet to keep Kitty protected.

To Recap - The Heartworm in Cats FAQ

Sad sick young gray cat lies on a white fluffy blanket in a veterinary clinic for pets. Depressed illness, suppressed by the disease animal looks at the camera. Feline health background

We've put together a list of common questions and the quick answers. Most of these have been discussed above in this article, but are worth another mention.

Do cats need to take preventive heartworm medication?

That depends on the risk factors in your area. If you're worried about your cat getting heartworm from mosquitoes, discuss this with your veterinarian and follow his or her advice.

Is heartworm in cats fatal?

It can be, especially if left untreated. A qualified veterinarian may be able to save the life of a cat, but only if the vigilant owner gets Kitty to the vet in time.

If your cat has difficulties with breathing and begins to cough - call your veterinarian right away. Whether or not the cause is heartworm - acute respiratory distress is always an emergency.

How to get rid of heartworm in cats?

Your veterinarian can provide you with pills that will either get rid of the worm or help keep your cat healthy as a carrier.

What to do if you suspect Kitty has heartworm?

Cat on womans hands at reception in vet clinic. Animal health care concept

Talk to your veterinarian. If the cat is experiencing any of the symptoms described above, call your vet immediately - this could be an emergency.

Can humans be infected with heartworm?

According to the CDC, human beings are not the preferred host of heartworm but they can become "accidental hosts". That means that if a human becomes infected, the parasite won't be able to multiply or complete its life cycle.

You can't catch heartworm from your cat or dog. It takes the bite of an infected mosquito to do that. The CDC recommends avoiding bites by covering up and using insect repellent in areas where Dirofilaria is common.


Please share this article to help raise awareness of heartworm in cats. And don't forget to share your own experience with this condition by leaving a comment below. If you have any questions - please take those to the cat health forum.

We'd like to thank Dr. Miller again for her help with preparing this guide! Dr. Letrisa Miller is a feline-only veterinarian who owns and operates the Connecticut Feline Medicine and Surgery, LLC at Manchester, CT.

Dr. Miller is a founding fellow of the International Association of Cat Doctors and served as president in 2012 and 2013. She is also a member of the American Veterinary Medical Association, the Connecticut Veterinary Medical Association, the International Society of Feline Medicine, and the Veterinary Information Network. From 2006 through 2011 Dr. Miller was a board member of the American Association of Feline Practitioners AAFP) and for several years she was the chair of the AAFP's research committee. In 2010 she represented the AAFP at the founding of the Cat Health Network, a collaboration of the American Veterinary Medical Foundation, Morris Animal Foundation, Winn Feline Foundation, and AAFP to fund and promote research in feline medicine.You can read more about Dr. Letrisa Miller on her website.

Note: We may get commissions for purchases made through links on this page.

8 comments on “Heartworm In Cats – An Owner’s Guide

Jcatbird April 6, 2020
I came back to let everyone know that my rescued heart worm positive cat, BJ, has recovered. He now tests negative for heart worms. I kept him on preventatives and over time he managed to eliminate the problem. He will remain on preventatives and never be returned to being an outside kitty.
Jcatbird July 12, 2019
Thank you for posting this information to TCS! I adopted a cat with heart worms and the information here was very helpful.
JMJimmy April 10, 2016
It should be mentioned that diagnosis in cats is *rare*.    What I would do is ask the nearest vet if there have been any cases in the area.  If not the chances are pretty slim that you'll have a problem with it.  If there are cases then you can buy Invermectin (Heartguard) or Selamectin (Revolution) as a preventative step.  If you've got indoor/outdoor cats you'll probably want to treat them with Selamectin anyway to keep fleas/mites out of your home.
raysmyheart June 5, 2015
I started to worry about heartworm lately, because Speedy has been out on her harness in this nice weather, but the mosquitos are so thick!  I give her topical treatment monthly of Revolution and was relieved to see that it helps fight off heartworm.  I thought it was only fleas, ticks and mites, but as I said, I am relieved to read in the pharmacy info that it is for heartworm too!  Anyway, we will be going out earlier in the day because the mosquitos are driving us crazy.
helen drye May 16, 2015
How cat just got sick yesterday.She just turn 6 and she feel trying to get on her cat seat and became dizzy.I took her to the vet right away.They say she had brain damage from a worm.She has not been outside since we got her when she was 3mths.She has had all the Meds she was to get every month as a cat should have.We don't have special vets here and I scared.We did every thing right.They said she could have gotten this before we got her.How does something like this show up after 6yrs.
Anne May 25, 2014
A diagnosis can always help. Simply knowing what the cat has means it won't have to go through treatments, for example antibiotics for suspected bacterial infections, which may do nothing more than weaken it. It also means the cat can receive the preventive medicine to hold infection at bay and good supportive care in case of severe heartworm infection. 
splasha1 May 24, 2014
If there is no treatment for heartworm in cats then how does diagnosis save life ?

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