Heartworms in Dogs & Cats

Heartworms are parasites found in both cats and dogs. They are part of the same class of worms as roundworms, and the two are similar in appearance. Heartworms, however, are more dangerous because they grow in the right side of the heart and the blood vessels that connect the heart to the lungs. They can put a great deal of stress on a cat's respiratory system and on a dog's heart. Given the severity of heartworm disease, prevention should be a priority.


In both cats and dogs, heartworms are transmitted by mosquitoes. Mosquitoes bite an affected animal, and young heartworms are transmitted to the mosquito. The larvae can then be transmitted to another animal when the mosquito feeds again. Inside the animal, heartworms usually mature within six months, travelling through the body until they reach the heart and blood vessels in the lungs. Dogs can have up to several hundred heartworms in the heart and surrounding vessels, while cats normally have one to four. In dogs, the worms may live for five to seven years. Heartworms in cats usually only have a two-year life span.


In the early stages or in mild cases of heartworm disease, dogs may show no symptoms. Once the worms have matured and grown in the lungs, the dog may show signs of coughing, lethargy, loss of appetite and laboured breathing. Cats with heartworm disease may cough, display laboured breathing and vomit. In rare cases, a cat may die without warning from heartworm disease.


Your veterinarian can perform tests to identify a mature heartworm infection in your dog. Antibody tests can see if the dog has been exposed to heartworms, and antigen tests alert veterinarians to the presence of female heartworms within the animal. A vet may do other tests, such as a chest X-rays, ultrasounds of the heart or blood panels to confirm the diagnosis and determine how advanced the disease is. It is more difficult to identify heartworm disease in cats. The vet usually will perform both antigen and antibody tests, but the results are not always definitive.


Vets will give dogs infected with heartworms melarsomine. It contains arsenic and will kill the heartworms. The protocol depends on how severe the infection is and the overall health of the dog, but treatment usually lasts four months. Veterinarians usually keep dogs on a heartworm preventive medication during the protocol as well. Once the worms are dead, however, they may block blood vessels that lead to the lungs, causing fever, coughing and possibly heart failure. Dogs receiving treatment must be kept very quiet and calm as a result. Currently, there are no medical treatments for heartworm disease in cats, though drugs can reduce inflammation in the lungs. In both cats and dogs, surgical removal of heartworms is an option, but it is extremely risky and usually only used in severe cases.


Heartworm disease is completely preventable in both cats and dogs, so it is much better to try to prevent it than treat it. Monthly heartworm preventive medications are available for both dogs and cats. However, these preventatives do not kill existing heartworms, so it important to get your pet tested annually.

Cite this Article A tool to create a citation to reference this article Cite this Article

About the Author

Based in New York City, Jennifer Blair has been covering all things home and garden since 2001. Her writing has appeared on, World Lifestyle, and House Logic. Blair holds a Bachelor of Arts in Writing Seminars from the Johns Hopkins University in Baltimore, Maryland.