Matters of the Heart

May 27, 2025

Yes, Cats Can Get Heartworms Too

Nancy Kerr

Most people automatically associate heartworms with dogs. However, cats and ferrets can also get them. “Feline heartworm disease is a potentially life-threatening condition that occurs when a parasitic worm called Dirofilaria immitis gets into an animal’s system,”¹ resulting in severe lung disease, heart failure, other organ damage, and death in pets. Both indoor and outdoor cats are at risk for heartworm disease. “Up to 1/3 of infected cats live indoors only.”² “In the United States, heartworm disease is most common along the Atlantic and Gulf coasts from the Gulf of Mexico to New Jersey and along the Mississippi River and its major tributaries, but it has been reported in dogs in all 50 states.”³ “Female worms are 6-14″ long (15-36 cm) and 1/8″ wide (3 mm). Males are about half the size of the females.”²

Heartworm disease is much more common in dogs than in cats. “Cats are relatively resistant to adult heartworm infection when compared to dogs, with the infection rate in cats reported to be 5-20% of the rate in dogs in the same geographic location; however, infection can still occur.”² This means that cats will typically have fewer adult worms than dogs.

How Do Cats Get Heartworms?

The heartworm life cycle is complex and requires two host animals to complete it. “The dog is the definitive host, meaning that the worms mature into adults, mate, and produce offspring while living inside a dog. The mosquito is the intermediate host, meaning that the worms live inside a mosquito for a short transition period in order to become infective (able to cause heartworm disease). The worms are called ‘heartworms’ because the adults live in the heart, lungs, and associated blood vessels of an infected animal.”³

“As many as 30 species of mosquitoes can act as host and transmit heartworms.”² Since the worms do not thrive as well inside a cat’s body, it is not considered a natural host.

“Mosquitoes ingest immature heartworm larvae, called microfilariae, by feeding on an infected cat or, more commonly, an infected dog. The microfilariae develop further for 10–14 days in the mosquito’s gut and then enter its mouthparts. When an infected mosquito bites a cat, infective larvae move into the cat. The larvae migrate through the cat’s tissues as they mature and enter the bloodstream. The larvae end up in the blood vessels that carry blood from the heart to the lungs and right side of the heart. Many immature worms die at this point, as cats are not natural hosts for heartworm. Those that survive mature further into adult heartworms capable of reproduction within about 7–8 months.”²

“If there is a male and female worm, they will produce a new crop of microfilaria that will live in the cat’s blood for about one month. Cats are resistant hosts, and few circulating microfilariae are generally found. Unlike in dogs, adult heartworms only live for 2-4 years in cats.”² “Usually, a cat has only one or two worms. However, due to its relatively small body size, a cat with only a few worms is still considered to be heavily infected.”³ A cat can only become infected with heartworms if it is bitten by an infected mosquito. Heartworms have to be transmitted by the mosquito and cannot be transmitted directly from one cat to another or from a dog directly to a cat. When temperatures are above 50°F (10°C), mosquitoes are actively feeding and there is typically a greater risk of infection.

Symptoms

There are no specific signs of an infection, and common signs such as coughing and rapid breathing may also be signs of other diseases. Other signs may include weight loss, vomiting, and decreased activity and appetite. “Cats with heartworm disease rarely show signs of heart failure.”³

Cats exhibiting obvious symptoms can be attributed to the lung damage caused by the heartworms. Cats typically show symptoms of heartworm disease at “two time points – when the immature heartworms arrive in the arteries of the heart and lungs and when the adult heartworms die.”² “Sudden death is thought to be due to a reaction within the lungs to the young heartworms, or to a reaction to heartworms entering the bloodstream and obstructing the flow of blood to the lungs.”³ In some cases, heartworm disease is diagnosed post-mortem for an otherwise healthy-looking cat. “When the adult heartworms die, they release toxins into the cat’s bloodstream which cause lung damage, leading to respiratory problems or sudden death. Even the death of one worm can be fatal for a cat.”³

Diagnosing Heartworms

“The immature heartworms arrive in the heart and lung arteries about 3 to 4 months after a cat is bitten by an infected mosquito. Many of these immature heartworms die, causing a strong inflammatory response in the cat’s lungs in the pulmonary vessels, small airways, and lung tissue. This response is called heartworm associated respiratory disease (HARD) because respiratory signs, such as trouble breathing, increased respiratory rate, and cough, are the most obvious. It may be difficult to distinguish HARD from feline asthma or feline bronchitis. In cats, it takes 7 to 8 months for infective larvae to mature into adult heartworms and produce microfilariae. This is about one month longer than in dogs. The presence of microfilariae in a cat’s bloodstream is uncommon. Only 20 percent of cats with heartworm disease have microfilariae in the bloodstream, compared to 80 to 90 percent of dogs with heartworm disease. Also, the presence of microfilariae in the bloodstream is inconsistent and short-lived in cats.”³

It is harder to detect heartworm infections in cats than in dogs. Since no single method is 100% accurate, multiple methods have to be used to determine if a cat has been infected. Veterinarians generally use two types of blood tests (the heartworm antibody test and heartworm antigen test) in combination to test a cat for heartworms. However, negative test results do not rule out infection, and positive test results may or may not mean that there is an active infection. A veterinarian will then use the results of both blood tests (along with the cat’s symptoms and the results of other tests such as x-rays and an ultrasound of the heart) to determine if a cat has the disease. In many cats, adult heartworms can be seen in ultrasound images. However, because most infected cats have a low number of worms, this is not always the case.

“Adult heartworms have a more severe effect in cats than dogs, because cats’ blood vessels are so much smaller. Even one adult heartworm can cause fatal lung disease. Adult heartworms cause heartworm disease through their effect on the heart and blood vessels that lead from the heart to the lungs. They cause chronic inflammation that leads to scarring and narrowing of the pulmonary arteries and thickening (fibrosis) of the surrounding lung tissues.”²

Treatment

There is no “silver bullet” drug to treat heartworm disease, but symptoms can be managed with medications. In most cases, cats diagnosed but not showing severe clinical signs will be monitored to see whether the condition will resolve itself on its own.

Periodic chest X-rays can be performed to monitor cases where there is evidence of the disease in the lungs and associated blood vessels. Small, gradually-decreasing doses of prednisone are recommended for cats with clinically confirmed evidence of lung disease. Cats with definite signs of heartworm disease may require additional supportive treatment in the form of intravenous fluids, oxygen therapy, cardiovascular drugs, and antibiotics. Surgical removal of adult heartworms is risky and the heartworms may not be removed intact, leading to potentially serious complications, such as shock and death.

Prevention

The best way to deal with heartworms is with prevention. There are several products to prevent heartworms in cats. There are several monthly topical and oral options that can be prescribed by your veterinarian. Some heartworm preventives contain other ingredients that are effective against certain intestinal worms (such as roundworms and hookworms) and other parasites (such as fleas, ticks, and ear mites). These include NexGard Combo^®, Revolution^®, Revolution^® Plus, Advantage Multi™, Interceptorä, Heartgard^®, Bravecto^® Plus, and Centragard™. Talk to your veterinarian about which preventive treatment is right for your cat,  and look into ways to control the mosquito population around your home and yard.

 

Sources

1. Heartworm in Cats, Accessed 03/15/2025, https://www.vet.cornell.edu/, https://www.vet.cornell.edu/departments-centers-and-institutes/cornell-feline-health-center/health-information/feline-health-topics/heartworm-cats
2. Malcolm Weir, DVM, MSc, MPH; Krista Williams, BSc, DVM, CCRP; Malcolm Weir, DVM, MSc, MPH; Catherine Barnette, DVM; Ernest Ward, DVM, Heartworm Disease in Cats, Accessed 03/15/2025, https://vcahospitals.com/, https://vcahospitals.com/know-your-pet/heartworm-disease-in-cats
3. Keep the Worms Out of Your Pet’s Heart! The Facts about Heartworm Disease, 12/22/2022, Accessed 03/15/2025.https://www.fda.gov/, https://www.fda.gov/animal-veterinary/animal-health-literacy/keep-worms-out-your-pets-heart-facts-about-heartworm-disease