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

By Sarah Mouton Dowdy
Updated June 16, 2020
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Heartworm in cats is a serious and sometimes fatal condition caused by parasites known as heartworms 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. Prevention is far easier than treating the condition, and the best way to protect your cat is with a monthly heartworm preventative. 

What Is Heartworm Disease?

When heartworms (also 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  in cats can actually be traced back to infected dogs, foxes, wolves, and coyotes, but mosquitoes play a key role in transmission. Kleypas says the process looks like this:

  1. Dogs, foxes, wolves, and coyotes infected with heartworms have baby worms in their bloodstream.
  2. When a mosquito bites and feeds on these infected animals, it picks up the baby worms in the blood. 
  3. Once inside the mosquito, these baby worms develop into infectious worms.
  4. When the infected mosquito bites a cat, it leaves infectious worms on the cat’s skin.
  5. 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. 

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. 

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.”

How is Heartworm in Cats Diagnosed and Treated?

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 even 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 Help Prevent Heartworm in Cats

Because cats 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.” Your cat’s veterinarian will work with you to find the best option for your pet.

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.