Heartworm disease is generally considered a condition that only affects dogs, but many other mammal species can suffer from it or act as carriers. Heartworms have been known to infect dogs, cats, ferrets, wolves, coyotes, sea lions, and there have even been reported cases of human infection as well. These foot-long worms colonizing the heart, lungs, and blood vessels cause severe lung disease, heart failure, and damage to other organs.

Cats, as an atypical host, can be infected, but the worms rarely survive into adulthood and so are unable to reproduce. This doesn’t mean there is no damage done—even immature worms can cause harm—but it does mean that cats often go undiagnosed because they do not have the symptoms dogs present. Cats often end up suffering from Heartworm Associated Respiratory Disease (HARD) because the medication that treats heartworm in dogs cannot be used in cats.

How are cats infected?

Heartworm disease is transmitted through mosquito bites. When a mosquito bites an infected animal, it ingests tiny worms called microfilaria. These mature over the course of 10-14 days into an “infective stage” at which time a mosquito bite transmits the larvae into a new animal. Once inside the bloodstream, the larvae mature into adult worms within 6 months, living 2 to 3 years in a cat. Adult female worms produce more microfilaria, which circulates in the bloodstream until they are picked up by a mosquito.

What are the common signs and symptoms of heartworm disease?

Heartworm disease in cats will either present very subtle or very dramatic symptoms. While some cats will have no outward signs of distress, other cats will have asthma-like attacks, difficulty walking, fainting spells, or seizures, although, in some cases, the first sign of distress is sudden collapse or death.

What tests and treatments will my veterinarian perform?

When these symptoms are seen in your cat, a veterinarian will test for heartworms with an antigen test. If it comes back positive, a second test should be performed to confirm the diagnosis as the treatment regimen for heartworms is both complex and expensive.


Diagnosis in a cat requires a physical exam, a blood count and other blood tests, an x-ray, and possibly an ultrasound as well. There is no approved drug therapy for cats as infections often resolve on their own, though damage will still remain. Treatment in mild cases consists of stabilization and management of presenting symptoms. In more severe cases hospitalization may be recommended to provide therapy including antibiotics, IV fluids, drugs to support heart and lung functions, and, in some cases, surgical removal of heartworms.

How do I protect my cat from heartworms?

To combat an initial infestation or to guard against a subsequent infection, preventative medication must be administered to the cat on a monthly basis. Preventatives are available in a topical gel or in pill form.

Some cat owners think that they can skip the preventative in the winter months as it is too cold for mosquitoes, but it is very easy to forget the first dose in the spring or to have a sudden warm spell that allows mosquito populations to build before remembering to treat your cat. It is best to have a set day every month to administer the medication to ensure the continued good health of your cat.