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Heartworm symptoms in cats and how to prevent them

Heartworms in cats – it’s not a pleasant topic, but it’s something all cat owners need to be aware of when looking after the health and wellbeing of their furry family member. Heartworm disease is a serious condition which can eventually result in devastating organ damage, lung disease and heart failure. It’s a scary list of entirely preventable outcomes.

With the help of PETstock VET, Dr Natalia Li, we’ll tackle the following questions about heartworm in cats:

What are heartworms?

When we talk about heartworms, including heartworm in cats, the reference is to a parasitic worm, scientifically named Dirofilaria immitis. Visually, heartworms look a bit like angel hair or white spaghetti strands, whereas larvae (pre-adult form, or baby heartworm) are not visible to the naked eye. In fact, a vet will usually identify one under a microscope from a blood smear of an infected animal.

Heartworm in cats is different than in dogs

It’s also worth knowing that while cats can contract heartworm, the experience is different to that of a dog who has heartworm. The way a dog or cat contracts heartworm is the same; however, a cat is considered an atypical host for heartworm. This is because many of the worms inside a cat fail to evolve into the full adult stage, meaning a lot less potential for harm – but still considered a dangerous health issue. Typically, a cat may be infected by a couple of adult heartworms, while dogs can be the host of hundreds.

Luckily, a cat’s immune system is very reactive to heartworm larvae, preventing heartworm from growing into its adult stage. However, any number of heartworms in your cat’s organs can be fatal and cause damage to surrounding tissues, so there is still cause to be vigilant with preventative treatment.

“In endemic regions, nearly 100% of unprotected dogs will be infected by heartworm, though cat incidence is much lower than that, and it’s a relatively easy-to-prevent infection. Unfortunately, heartworm infection can easily cause heart failure, pulmonary hypertension, thromboembolism, and subsequent sudden death,” - Dr.Natalia Li, PETstock VET.

The lifecycle of a heartworm

  • It all starts with a mosquito. A mosquito bites an infected host and then transmits the worm/larva into its next host. For the larva to transmit into the next host, it develops first inside the mosquito for some time before further transmission.
  • *Note: A heartworm cannot be transmitted from one pet to another, only through mosquito bites.
  • The larva then develops further in the host/cat before reaching adult stage, finding a way to the heart and blood vessels of the lungs.
  • Once a thriving adult, the heartworm breeds to produce microfilaria in the bloodstream, where the cycle begins again with a mosquito bite.

Common heartworm symptoms in cats

Cat stares out glass door, with seemingly no interest in the food in its bowl.

Symptoms of heartworm in cats are challenging to differentiate from other health issues, so this can be very dangerous. However, the symptoms can also appear very mild and can be perceived as non-life-threatening to the average person.

So, if you do notice the below symptoms, please speak to your vet just to be sure, especially for the more severe symptoms.

Chronic infections

  • Heavy/fast breathing
  • Random episodes of vomiting
  • Lethargy or exercise intolerance

Severe acute infection:

  • Coughing or coughing up blood
  • Wheezing
  • Loss of appetite
  • Difficulty breathing/gasping for air


  • Weight loss

Just remember, any symptoms like the above may be a sign of heartworm in your cat, so it’s best to check in with your local vet whenever you feel your cat is sick or in distress.

PETstock VET

Heartworm treatment for cats

As is often said, prevention is always better than cure for both human and animal-related illness. This is especially the case for heartworm in cats. Unfortunately, due to the nature of heartworm symptoms, you may not detect it, which is more than half the battle before treatment.

Once your cat has contracted and is diagnosed with heartworm disease, treatment at your local vet may look like this:

  • X-rays to determine level or evidence of disease in the lungs and associated blood vessels.
  • Treatment to eradicate the heartworms in the body.
  • Supportive treatment of intravenous fluids, oxygen therapy, cardiovascular medicine and antibiotics.
  • Your cat will need monitoring over a couple of days during the treatment period.

The intention of treating a heartworm infection is to essentially eliminate worms from the infected animal’s body as soon as possible. To do this, your vet will use an anti-parasitic to treat adult worms, with strict rest a must during the treatment period. Once the adult worms are successfully removed, a vet will administer another anti-parasitic to treat the microscopic larvae.

Further tests will then be completed to ensure the treatment was a success, along with adjunctive treatments of anti-inflammatories and antibiotics. In some cases, heartworm treatment in cats can pose certain risks, where a specialist may need to look at an alternative treatment plan.

Treatment of heartworm in cats can vary depending on your cat’s current condition or level of infection, so it’s best to contact your local vet for more information.

How to prevent heartworm in cats?

Tired, lethargic looking cat naps on bed.

Preventing heartworm in cats is very straightforward - if you remember to do it. Just like for dogs, heartworm prevention is available through monthly chews or liquid spot treatments. We recommend you add a reminder in your calendar so you don’t forgot!

A whole range of reputable brands like NexGuard, Revolution, Advocate and Selapro have heartworm prevention options; make sure the one you choose specifies heartworm on the packaging. In addition, some ‘all wormers’ do not cover heartworm, so make sure you don’t make this mistake when making your purchase.

Now you’ve got the tips and insights on heartworm, keep reading more health advice for your cat.


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