Heartworm prevention medication is something every cat needs—even an indoor feline.


Heartworm medicine is an important part of every cat’s preventative care—even those who live indoors. Heartworms are nasty parasites that are spread by mosquitoes and can cause serious health problems in cats. Because it’s far easier to protect cats from heartworm infection than to diagnose and treat the condition, you’ll want to partner with your veterinarian to choose the best heartworm preventative for your cat. 

What Are Heartworms?

Heartworms (also called Dirofilaria immitis) are parasites that look like cooked spaghetti and can grow to be 12 inches long. When they infect the heart or lungs of an animal, it causes a condition called heartworm disease. 

How Do Cats Get Heartworms?

Heartworms are transmitted to cats by mosquitos, who can pick up baby heartworms from infected dogs and other animals. When an infected mosquito bites a cat, it leaves these worms on the cat’s skin. The worms then crawl into the bite wound left by the mosquito, enter the cat’s bloodstream, and eventually end up in its heart or the blood vessels of its lungs, resulting in heartworm disease. 

Illustration of mosquito flying around a cat
Credit: Yeji Kim

How Do Heartworms Affect Cats?

Cats with heartworm disease typically have only one to three adult worms (as opposed to dogs, who can harbor hundreds). Some cats with heartworm disease don’t have any worms that survive until adulthood, but even baby worms can cause serious damage. 

Jackie Kleypas, DVM, a clinical instructor at the University of Missouri College of Veterinary Medicine in Columbia, Mo., says that’s because cats have special cells in their lungs that are highly effective at killing worms. And while this is in part a good thing, the death of those baby worms also results in major lung damage, which causes affected cats to display asthma-like signs (e.g. coughing, wheezing). These signs are collectively referred to as HARD (heartworm associated respiratory disease). 

Kleypas adds that another common sign of heartworm in cats is vomiting, and the third and most horrific sign is sudden death. “In some cats, one to four worms can survive to become adults,” she explains. “Typically, the death of these large adult [heartworms] is what causes cats to die suddenly. The dead adult breaks off, goes into the bloodstream, and blocks it—similar to a stroke or heart attack in a human.”

Does My Cat Need Heartworm Prevention Medicine?

Heartworm preventatives are recommended for all cats—even indoor cats!—all year long. Here’s why: 

  1. Heartworm disease can have serious and even fatal consequences in cats.
  2. Diagnosing and treating heartworm disease in cats is notoriously difficult.
  3. Because heartworms are transmitted via mosquitos, both indoor and outdoor cats are at risk of infection as mosquitoes can easily get indoors. While cats who spend time outdoors are certainly more at risk of infection, Kleypas says that roughly one in four cats diagnosed with heartworm disease is an indoor-only pet. 
  4. Though certain regions can carry an increased risk of infection, the Companion Animal Parasite Council says heartworms have been found in cats in all 50 states.

How Do I Choose a Heartworm Medicine for My Cat?

Macrocyclic lactones, the class of drugs used to prevent heartworms in cats, can be given orally or topically. Trent Eddy, DVM, of Ironhorse Veterinary Care in Leawood, Ks., says he always recommends topicals to his clients. He’s been around enough cats to know that trying to get them to swallow something they don’t want to is stressful (for both the cat and owner) and often unsuccessful. “I’m more comfortable recommending topicals because I know for sure the cat is getting the prevention it needs,” Eddy says. However, if your cat takes oral medication like a champ, you may prefer that option instead. 

Many heartworm prevention medications also protect against other parasites like fleas, ticks, ear mites, hookworms, and roundworms. For this reason, your veterinarian will likely want to know how much time your cat spends outdoors before recommending a particular product.

Regardless of whether the medication is given orally or topically, you’ll need to either purchase it from your veterinarian or get a prescription so you can get it from a pet pharmacy. There are currently no natural or over-the-counter heartworm preventatives that have been proven to be safe and effective, so you can forget about an herbal solution or essential oil that can prevent heartworms from infecting your cat. 

Does My Cat Need Heartworm Preventatives Every Month?

Even though mosquitos are considered a seasonal threat in many places, cats need heartworm prevention every month. It’s especially important to be consistent if your cat’s preventative medication includes protection against other parasites too, like fleas or mites. Set a monthly reminder on your phone or mark your calendar so you don’t miss a dose and experience a lapse in coverage. 

Are There Any Side Effects of Heartworm Preventatives I Should Watch For?

Side effects with heartworm preventatives in cats are rare, but not impossible. If your cat is sensitive to the medication, it could cause vomiting, diarrhea, lethargy or loss of coordination. If your cat has an allergic reaction, you might see itching, hives, facial swelling, seizures, or shock. 

If you witness any of these signs in your cat after giving him a heartworm preventative (even if your cat has had the medication ten times before with no problems), contact your veterinarian as soon as possible. Eddy says that another possible side effect with topical treatments could be hair loss at the point of application. If you notice this, let your veterinarian know so they can change your cat’s medication. 

Can My Dog and Cat Use the Same Heartworm Medication?

Even though dog and cat heartworm preventative drugs contain the same compounds, you shouldn’t give what’s meant for your cat to your dog (or vice versa). “The effective and safe dose for dogs is vastly different than for cats,” Kleypas explains. “For instance, there are preventatives that require four times as much drug per pound of body weight for cats versus dogs, while another preventative requires only half the amount of drug per pound of body weight for cats versus dogs.” 

Partner With Your Veterinarian to Protect Your Cat

Every cat is different, which is why it’s important to talk to your veterinarian when choosing a heartworm preventative. They will help you find the best one for your unique pet. Similarly, every preventative medication is different, so you’ll need to read and follow the instructions for whatever you choose to make sure you’re administering it correctly and your cat gets the protection he needs. Closely following the instructions is important for your safety as well—especially if your cat gets a topical product, which can rub off or be spilled without a close watch before, during, and after you administer the medicine.