There are many diseases and conditions common to both cats and dogs — including heartworm. Dogs are considered the natural host for heartworms, meaning that after being infected, the parasite can mature, mate, and multiply inside a dog fairly efficiently. Cats, on the other hand, are considered an atypical host. After a cat is infected, most larvae never make it to the adult stage. The cats who do develop adult worms typically have only one to three of them — compared to the 30 (upwards to 100) adult heartworms that a dog might harbor.

One might think that fewer worms means less risk of damage, however that is not necessarily the case. In cats, even a single adult heartworm, or a small number of worms that never fully mature, can trigger a serious inflammatory reaction in the lungs and airways. This condition is called Heartworm Associated Respiratory Disease, or HARD, and it can cause lasting respiratory damage in cats.

Heartworms can cause damage to the heart, lungs, and blood vessels in any infected animal, but they don’t always have obvious signs or symptoms of infection. In cats, symptoms can vary widely — from nothing at all, to subtle or dramatic — and can include:

• Coughing or asthma-like breathing attacks
• Vomiting (not necessarily related to eating)
• Loss of appetite or weight loss
• Difficulty walking, fainting, or seizures
• Fluid buildup in the abdomen

Because symptoms are often vague or easy to miss, many cases of feline heartworm disease go undiagnosed entirely. In the most severe of cases, the first visible sign could be sudden collapse or sudden death, particularly if an adult worm dies inside the cat and triggers a shock-like reaction. 

The American Heartworm Society recommends annual heartworm testing for dogs and cats but in cats testing looks a little different.^1,2 Because cats are less likely than dogs to have adult worms, a single antigen test (the standard test for dogs) can miss feline infections. Veterinarians typically recommend combining an antigen test with an antibody test, which detects past exposure to heartworm larvae even if adult worms never developed. X-rays or ultrasound may also be used to look for signs of disease in the heart and lungs.

Dogs that test positive for heartworm can undergo treatment. However, because there is no approved drug or treatment to kill adult heartworms in cats, if a cat tests positive, the focus is on management rather than cure. Monitoring and management of cats that have heartworm may include:

• Periodic chest X-rays (every 6 to 12 months)
• Treatment with anti-inflammatories (e.g. prednisolone to manage inflammation)
• Cats with severe symptoms may require hospitalization for IV fluids, oxygen support, or medications to manage heart and lung symptoms
• Ongoing prevention because a cat that's had heartworm disease has proven it's susceptible

Some infections do resolve on their own. Worms do die naturally, but the respiratory damage caused along the way can be permanent.

As in dogs, year-round preventives are recommended for cats. The American Heartworm Society recommends starting kittens on a heartworm preventive as early as the product label allows, and no later than 8 weeks of age.² Because dosage is based on body weight rather than age, and kittens can grow unevenly (fast) in their first few months of life, owners should consult their veterinarian about anticipated dosage changes rather than assuming one prescription will be sufficient. Monthly preventives (available as topical spot-on treatments or chewable tablets) are the standard for cats and, as with dogs, they need to be given strictly on schedule to be effective.

Some mistakenly assume that an indoor-only cat doesn't need heartworm prevention. Unfortunately, mosquitoes can make their way indoors (e.g. through doors, windows, and gaps), and studies have found that a significant number of cats — reported to be indoor-only — have been diagnosed with heartworm disease.

The bottom line is that the exposure risk is just as real for cats as it is for dogs. This point was further reinforced by findings reported by Banfield Pet Hospital. A 2023 review of patient medical records (over a period of five years) showed a 47% increase in cats diagnosed with heartworm. And yet, according to a survey they conducted of pet owners, only 22% of owners were aware that there is no safe treatment for feline heartworm disease.³

Heartworm disease in cats is harder to detect and, in some ways, is more unpredictable than in dogs. We, also, currently have no approved treatment for infection in cats. Cat owners should be proactive. If your cat isn't currently on a year-round heartworm preventive – whether or not it ventures outdoors – talk to your veterinarian about heartworm testing and preventive options. 

Resources:

1. The American Heartworm Society - Heartworm Prevention for Dogs

2. The American Heartworm Society - Heartworm in Cats

3. New pet owner survey suggests common myths fueling significant gaps in prevention of deadly parasite for U.S. dogs and cats. PR Newswire. April 2024