Cardiology – General Practice & Preventative Medicine


Feline heartworm disease is a serious parasitic disease caused by a long, thin worm, Dirofilaria immitis, that lives in the blood vessels and heart of infected cats.

The disease is spread from dog to cat by mosquitoes. The mosquito bites a dog with heartworm infection, collects some of the microscopic heartworm offspring, and then after a couple of weeks, passes these on to another dog or to a cat. Inside the cat, the microscopic heartworm can grow into a parasite exceeding a foot in length. The life cycle is somewhat complicated. The important thing is to prevent worm development using safe and effective preventative drugs.

Heartworms are present (endemic) in most parts of the United States and in many parts of North America. Mosquitoes are the key – without them the disease cannot spread. The highest rates of infection are found in subtropical climates like those of the southeastern United States, the Gulf States and Hawaii. However, heartworms are also found throughout the central and eastern United States, particularly near oceans, lakes and rivers. When compared to dogs, cats are naturally resistant to heartworm disease (estimated at about one-fifth as likely to become seriously infected as dogs in the same region); however, heartworm disease in cats is often more severe than in dogs. The presence of even a single adult heartworm in a cat can result in very serious consequences.

Heartworm disease injures the lungs, the arteries within the lungs and the heart. Symptoms include tiring, coughing, vomiting, weight loss, difficult breathing and even sudden death. Heartworm infection in cats can be difficult to diagnose. Blood tests are available, but the results may sometimes be misleading.

Prevention of heartworm disease is simple. “Preventatives” kill microscopic larvae that are left behind by mosquitoes when they bite a cat. Please consult with your veterinarian for the preventative that is best for your cat.


Owners of all cats living in areas endemic for heartworms should discuss the pros and cons of preventative care with their veterinarian.

If dogs in the area receive heartworm prevention, it is likely that cats also may benefit from this protection. Do NOT use your canine heartworm medicine in your cat! The drug dosing is very different between species.

Speak to your veterinarian about the need for preventative therapy, administration guidelines and when to start and stop prevention treatments. Some recommend that before beginning heartworm prevention, any cat over 7 months of age should first have a heartworm blood test.