Heartworm in Cats and Dogs

Heartworm in Cats and Dogs

Animals Affected

Heartworm disease affects dogs most frequently. Cats suffer from heartworm disease less often but are still at risk. Heartworms have been found in people in rare cases.


Heartworm is a blood parasite of dogs and cats that is spread by mosquitoes. Heartworm cannot be spread by direct contact between infected individuals. Heartworms start as microscopic larvae that are injected into animals when they are bitten by mosquitoes. Over several months, they develop into worms that are up to eight inches long and live in the heart and the arteries leading out of the heart. They can cause severe damage to the heart, and can cause heart failure.


Animals with light infestations may not show symptoms. When symptoms are present, they typically are symptoms of heart disease, including:

  • Coughing in both dogs and cats.
  • Shortness of breath
  • Weakness or reduced ability or desire to exercise

In cats, vomiting is a frequent symptom.


Risk Factors and Prevention

Geographical location is the most important risk factor for heartworm. Although the disease occurs worldwide, it is most common in warm and humid environments.


Because mosquitoes are required for transmission of heartworm, animals in areas with large numbers of mosquitoes are at greater risk of the disease.

Animals that spend significant time outdoors are more likely to be bitten by mosquitoes and therefore are at greater risk.

Animals with short hair are more vulnerable because mosquitoes can more readily bite them. However, long-haired animals can be bitten on the face or muzzle and are still at risk.

Heartworm can be prevented with a number of safe and effective medicines, including Interceptor®, Sentinel®, Advantage Multi®, and Heartgard®. When used properly, these products virtually eliminate the possibility of heartworm infection. Some veterinarians advocate the use of Revolution® for heartworm prevention in cats, but it is not considered reliable in dogs.

In some circumstances, animals should undergo blood testing for heartworm disease before starting a heartworm preventative.



Heartworm disease can lead to heart failure and premature death. It is a very serious disease.


Diagnosis of heartworm is made through blood tests, X-rays, and occasionally ultrasound evaluation of the heart.  False negative results are possible with all types of heartworm tests.


Treatment of heartworm disease is expensive, complex, and carries the risk of complications. Considering the safety and efficacy of heartworm preventatives, prevention of heartworm is a much better option than treatment.



Routine blood testing for heartworm disease may be recommended for some individuals, especially if heartworm prevention has lapsed.



Although heartworms have been found in people in rare instances, they generally are not believed to pose a risk to human health.

Many heartworm preventatives have the added benefit of preventing eliminating some intestinal worms.

Heartworm disease cannot spread directly from one animal to another.

The lifecycle of the heartworm parasite includes a developmental stage inside of a mosquito. Therefore, if a mosquito bites an infected pet and then immediately bites an non-infected animal, the disease will not be transmitted.

Copyright © Eric Barchas, DVM All rights reserved.
The contents of this page are provided for general informational purposes only. Under no circumstances should this page be substituted for professional consultation with a veterinarian.