Heartworms affect cats differently than dogs. Learn the signs and symptoms of heartworm disease in cats to help prevent heartworm-associated respiratory disease (HARD).

Heartworm disease in cats is a serious and sometimes fatal condition caused by parasites that are transmitted by mosquitoes. Heartworm disease is less common in cats than in dogs, but cats can experience more serious effects and have fewer treatment options. 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. Heartworm prevention medicine is an important part of every cat's preventative care—even those who live indoors.

What Is Heartworm Disease?

When heartworms (called Dirofilaria immitis) infect the heart or lungs of an animal, it causes a condition called heartworm disease. These worms look like cooked spaghetti and can grow to be 8 to 12 inches long. According to Jackie Kleypas, DVM, a clinical instructor at the University of Missouri College of Veterinary Medicine in Columbia, Mo., the disease is seen mostly in dogs. "Typically a veterinarian will diagnose heartworm disease much more often in dogs than cats—roughly ten dogs to one cat—but it's still a very serious disease in felines," she says. The American Heartworm Society notes that heartworms can also live in other animals, including ferrets, foxes, wolves, and coyotes. 

How Do Cats Get Heartworms?

Heartworm disease in cats can actually be traced back to infected dogs, foxes, wolves, and coyotes, but mosquitoes play a key role in transmission. Heartworms are actually transmitted to cats by mosquitos, who can pick up baby heartworms (heartworm larvae) 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. 

The Companion Animal Parasite Council notes that humans can become infected with heartworms, but it's very rare and the parasites can't be directly transmitted from cats to humans.

Illustration of a golden cat sitting outside by grasses with mosquito buzzing around his head
Credit: Corinne Mucha

Know the Signs of Heartworm in Cats

Heartworm disease affects cats differently than dogs. According to the American Heartworm Society, dogs are natural hosts of heartworms, meaning the parasites are able to mate and produce offspring inside them. Dogs left untreated can have hundreds of worms in their body, and adult worms can live for several years in dogs.

The American Heartworm Society says that in cats, however, most worms don't survive to become adults. Cats affected by heartworm disease typically have only one to three adult worms, and these adult worms survive about half as long as they do in dogs. Surprisingly, some cats with heartworm disease don't have any adult worms, but even baby worms can cause serious damage.

Kleypas says that the signs and symptoms of heartworm disease in cats are more varied than they are in dogs. "One common sign is coughing or wheezing, which can look similar to asthma," she explains. "Another common sign is vomiting, and the third and most horrific is death. Some cats with heartworms may appear completely normal and then suddenly have what looks like a stroke and die."

According to Kleypas, the signs of heartworm disease in cats that mimic asthma are collectively referred to as HARD (heartworm associated respiratory disease). Once the baby infectious worms reach the lungs, a huge battle takes place between the worms and a special type of cell found in cats' lungs that are excellent worm killers, she explains. "That's why many of the baby worms don't survive to become adult heartworms like they do in dogs," says Kleypas. "This is good, except for the fact that the death of these worms causes major damage to the lungs, and this damage is what causes the asthma-like signs."

The presence of the worm-killing cells in cats explains why they can have HARD without having a single adult heartworm present. "But in some cats, one to four worms can survive to become adults," Kleypas says. "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."

gray cat looking up at camera
With their beautiful silvery-grey coat and big green eyes, it's easy to identify a Russian blue.
| Credit: Westend61 / Getty

Diagnosis and Treatment of Heartworm Disease in Cats

"Unfortunately," Kleypas says, "it's much more difficult to diagnose heartworms in cats than in dogs, and many go undiagnosed." She explains that the standard test done in dogs (called an antigen test) can be done in cats, but it doesn't work as well since cats tend to have far fewer worms than dogs. Another type of test (known as an antibody test) is often used and can detect the presence of both baby and adult worms. Chest X-rays and ultrasound imaging can be helpful as well.

Treating heartworm disease is also more difficult in cats than in dogs. According to Kleypas, the drug used to kill adult heartworms in dogs is toxic to cats, and there is no safe, effective alternative. Instead, she explains that cats with heartworm disease receive supportive care in the form of steroids to minimize inflammation, and there are drugs available to treat the signs of HARD. Infected cats are also placed on heartworm preventative medications in hopes that the cat will outlive the worm, Kleypas says. Recovery is possible, but it is likely the cat will have permanent damage from the infection.

How to Prevent Heartworm Disease in Cats

Because they are infected via mosquito bite, cats who spend a lot of time outdoors are at a higher risk, but indoor cats aren't completely safe either. Kleypas says that roughly one in four cats diagnosed with heartworm disease is an indoor-only pet. The Companion Animal Parasite Council also notes that heartworms have been found in cats in all 50 states.

That means the best way to prevent your cat from being infected with heartworms is with preventative drugs. "The same class of drugs (called macrocyclic lactones) used to prevent heartworms in dogs is available as a monthly treatment for cats," Kleypas says. "And as an added bonus, many of these products will also prevent fleas, intestinal parasites, and ear mites." Feline heartworm prevention comes in oral or topical forms, and your cat's veterinarian will work with you to find the best option for your pet.

Trent Eddy, DVM, of Ironhorse Veterinary Care in Leawood, Ks., says he 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. 

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. 

Even though mosquitos are considered a seasonal threat in many places, heartworm preventatives are recommended for all cats—even indoor cats—all year long. 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 a 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 Prevention?

Kleypas cautions that while 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," she 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." It's important to talk with your veterinarian to be sure the right preventative heartworm treatment is used for your cat to help keep them safe from infection.

Every cat is different, which is why it's important to work with 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.