Evaluating Heartworm Medications for Cats

Preventive heartworm medications can help protect your cat, but are they necessary? Read this quick primer first.

Heartworm medication side effects in cats are rare.

Heartworms are worms that migrate and grow in the hearts and lungs of animals. Most commonly associated with cats and dogs, heartworms begin their cycle through infected mosquito bites as larvae and grow inside the animal. Some can grow to as large as 12 inches long and restrict the blood flow from the heart.

Cats generally have fewer heartworms than dogs, and theirs do not live as long. Heartworms live an average of two years in cats as opposed to 5-7 years in dogs. Cats are mostly affected by the damage done to the respiratory system, and it may take months or longer for symptoms to appear. The time elapsed from bite to internal larvae reproduction (adult larvae mating and producing additional larvae) averages around eight months in cats.

Below are the four preventive medicines most commonly used:


Also on Petful

See Dave’s favorite dog food brand…

Show Me


Interceptor (Novartis)

  • Generic Name: Milbemycin Oxime
  • Administration: Cats 6 weeks of age and up, weighing over 1.5 pounds (also dogs)

Revolution (Pfizer Animal Health)

  • Generic Name: Selamectin
  • Administration: Cats 8 weeks of age and up (also dogs)

Advantage Multi (Bayer)

  • Generic Name: Imidacloprid, Moxidectin
  • Administration:Cats 9 weeks of age and up, weighing more than 2 pounds (also dogs)

Heartgard Chewable (Merial)

  • Generic Name: Ivermectin
  • Administration: Cats 6 weeks old and up

Diagnosing Heartworm Disease

Heartworms can be present without any symptoms, but the most common one is mild to moderate asthma symptoms in cats, meaning difficulty breathing, coughing or wheezing. Other possible — but rare — symptoms may include:

  • Lethargy
  • Change in appetite or weight loss
  • Unable to digest/keep food down
  • Passing out/fainting
  • Convulsions or seizure-type movements
  • Difficulty seeing or loss of sight

Any out-of-the-ordinary symptoms should send you to your vet, but the above afflictions should not be ignored. Filter tests are not recommended since they rely on the adult female heartworm being present and can give a false negative. The feline’s body makes an antibody to defend against heartworms and a test is available that detects its presence. This test may also provide a false negative if the heartworms have not matured.

More reliable testing options include blood tests that can be done in the office and other batch tests that are sent (typically) to an outside laboratory. Heartworm testing in cats is also more involved than in dogs. Blood is sent out for antigen and antibody testing, chest films are made, and sometimes an echocardiogram is required.


There is no safe treatment to eradicate heartworms in cats. Once heartworms have been identified, cats are usually administered monthly heartworm preventive medication and anti-inflammatory medication (these corticosteroids can help a great deal with the asthma-like symptoms). Follow-up visits will be necessary to monitor the success of the medications. In extreme cases where adult worms are occupying a large portion of the heart, surgery may be necessary.

Be sure to treat areas outside your home for mosquitoes and have a regular pest control solution to minimize future infections.

Side Effects

According to our own Dr. Debora Lichtenberg, VMD, side effects of feline heartworm preventive medicines should be evaluated by “risk assessment vs. prevention and, honestly, if it’s worth the cost to the owner. Most [veterinarians] don’t think too much about the side effects of the medications because of the high safety rate of them. But we do worry about making our clients pay for another product…. There is no product available yet that is effective against fleas, ticks and heartworms. So cat owners are often looking at purchasing two products a month if they want to protect against all three.”

Additional Resources

Photo: MowT/Flickr

Please share this with your friends below:


Also Popular

Do NOT follow this link or you will be banned from the site!