Animal Care

Heartworm in Cats

Spread by infected mosquitoes, heartworm is increasingly being recognized as an underlying cause of health problems in domestic cats. Cats are an atypical host for heartworms. Despite its name, heartworm primarily causes lung disease in cats. It is an important concern for any cat owner living in areas densely populated by mosquitoes, and prevention should be discussed with a veterinarian.

You may have thought heartworm disease only affects dogs, and it’s true that the infection is less common in cats. The cat is not a natural host for the heartworm parasite, Dirofilaria immitis, and so the heartworm is not likely to complete its entire life cycle. That means that fewer and smaller worms survive, and many do not reach a cat’s heart. The worms that do survive—and the resulting immune reaction that the cat’s body sets up to kill the developing worms—can cause severe health problems.

Causes and Signs of Heartworm Disease

When a mosquito carrying the heartworm parasite, Dirofilaria immitis, bites a cat, larvae are transmitted into the bloodstream. The larvae migrate toward the heart over a period of around four to six months, maturing as they go, then settle in the heart, pulmonary arteries, and blood vessels of the lungs. Because a domestic cat is not a natural host for the heartworm parasite, many of the worms die. These—along with the living worms—cause severe inflammatory and immune responses in an infected cat.  

Cats of all ages, living in any region, can contract heartworm, but the disease is more prevalent in felines who live in areas densely populated by mosquitoes. Outdoor cats are at greater risk because of increased exposure to mosquitoes. However, indoor cats are also susceptible to mosquito bites, so it’s smart to discuss prevention with your vet. The heartworm infection can be especially life-threatening to kittens and older cats.

The following signs may indicate that your cat has been infected:

  • Persistent cough
  • Breathing difficulties (panting, wheezing, rapid or open-mouthed breathing)
  • Depression
  • Loss of appetite
  • Weight loss
  • Sporadic vomiting
  • Lethargy
  • Sudden death

Breathing difficulties that occur in the first stage of heartworm disease, caused by worms newly arriving in the heart and lungs, were likely previously diagnosed as feline asthma or bronchitis. However, these breathing problems are now thought to have actually been due to what is now called heartworm-associated respiratory disease (HARD).

Heartworm Prevention

  • There are several FDA-approved medications available that reliably prevent feline heartworm infection. Check with your vet and please remember, it’s recommended that cats are screened for heartworm infection with blood tests before being given any type of preventative medication. 
  • It’s also a good idea to limit your cat’s exposure to mosquito-infested areas and bring her in for preventative screenings during vet visits.
  • Regular checkups are key to detecting early infections and can give your cat a good chance at recovery.

 Diagnosing Heartworm Disease

Heartworm disease is not as easily diagnosed in cats as it is in dogs. 

  • Routine testing requires a combination of blood tests. 
  • When cats show signs of respiratory difficulty and heartworm is suspected, diagnosis is usually based on a cat’s history, physical examination, radiographs, echocardiogram and blood tests.

Treating Heartworm Disease

The good news is that many heartworm-infected cats are able to fight the infection themselves, and can be monitored with radiographs every few months while waiting out the worms’ lifespan. If an infected cat shows symptoms of lung disease, the cat can be given a cortisone-like medication as needed. Medication can also be given to help control coughing and vomiting.

Although some cats are able to fight the infection on their own, the following can occur if heartworms are not monitored and treated:

  • Damage to walls of heart
  • Damage to pulmonary blood vessels
  • Possible obstruction of blood flow through pulmonary arteries
  • Impaired breathing
  • Heart and lung failure
  • Kidney and liver damage
  • Sudden death


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