Five Important Things to Know About Feline Heartworm Disease

Photo Credit: "Monster Cat" by murchik_dasananda on Flicker CC.

If you think dogs are the only ones who get heartworm disease, I invite you to reconsider. Although dogs are the more natural host for this disease, cats are also susceptible to heartworm infection. It is estimated that, in any given community, the incidence of heartworm infection in cats is approximately 5% to 15% percent that of dogs who are not on preventive medication.

While the canine and feline versions of heartworm disease share some similarities, there are some striking differences.

I discuss these differences below, within the five feline facts pertaining to heartworm disease.

  1. The disease targets the lungs

Most dogs with heartworm disease involve many worms and the heart is the prime target for damage. In contrast, only one or two worms are typically present in an affected cat, and the disease takes its primary toll on the lungs. The cat’s immune system when it goes into overdrive in response to immature heartworms located within the lungs and/or fragments of dying adult heartworms that enter blood vessels feeding into the lungs. The result of this immune system activity is a whole lot of inflammation that can wreak havoc within the lungs. The acronym HARD (heartworm associated respiratory distress) is used to describe feline lung disease caused by heartworms.

  1. Common symptoms

Common symptoms of heartworm disease include coughing, rapid breathing, labored breathing, decreased appetite, lethargy, and weight loss. In rare cases, more severe symptoms and even sudden death can occur. One of the most surprising symptoms that occurs in cats with heartworm disease, but not in dogs, is intermittent vomiting that is unrelated to eating.

Not all cats with heartworm disease show symptoms. For those cats who test positive for the disease on routine screening but are free of symptoms, careful monitoring over the course of two to three years (the lifespan of the adult worms) is recommended.

  1. Diagnosing heartworm disease

The most reliable screening test for heartworm infection in dogs is called an antigen test. Performed on a blood sample, it detects microscopic particles (antigen) produced by adult female heartworms. In cats, it’s not unusual to have a male only population, given that often only one to two worms are present. Additionally, many cats develop symptoms and are therefore tested when the worms are immature. For these reasons, cats with active heartworm disease often have negative antigen test results. However, if the antigen test is positive, this is proof of heartworm disease

The more useful diagnostic tool for cats is blood antibody testing. The presence of antibodies means that the cat’s immune system has been exposed to heartworm disease. A negative antibody test is good evidence that a cat has not been infected. On the other hand, a positive antibody test can mean that either there is an active infection, or the cat experienced heartworm infection in the past. Antibody levels can remain elevated long after the heartworms have died.

The American Heartworm Society recommends that initial screening for feline heartworm disease includes both antigen and antibody testing. If results support the possibility or probability of heartworm disease, ultrasound of the heart and X-rays of the chest to evaluate the lungs are recommended to confirm or deny the diagnosis.

  1. No treatment for feline heartworms

Unlike the canine version of this disease, feline heartworm infection is not specifically treatable. Melarsomine, the drug of choice to kill adult heartworms in dogs, is toxic for cats. For this reason, feline heartworm disease is considered to be manageable rather than treatable. Corticosteroids such as prednisone or prednisolone are commonly used for their potent anti-inflammatory effects. Treatment often continues until the adult worms have died and are cleared from the lungs (a two to three year process).

  1. Prevention

Disease prevention is the best strategy, particularly in areas where mosquitoes proliferate. The American Heartworm Society recommends orally administered, once a month preventive medication, beginning at eight weeks of age for all cats in heartworm-endemic areas. Depending on the weather in a particular region, preventive medication may be recommended seasonally or year-round.

An indoor feline lifestyle is not a guarantee against heartworm infection. In fact, one in four cases of heartworm disease occur in cats that live exclusively indoors.

Do you have a heartworm prevention strategy for your cat?

Best wishes,

Nancy Kay, DVM

Diplomate, American College of Veterinary Internal Medicine
Author of Speaking for Spot: Be the Advocate Your Dog Needs to Live a Happy, Healthy, Longer Life
Author of Your Dog’s Best Health: A Dozen Reasonable Things to Expect From Your Vet
Recipient, Leo K. Bustad Companion Animal Veterinarian of the Year Award
Recipient, American Animal Hospital Association Animal Welfare and Humane Ethics Award
Recipient, Dog Writers Association of America Award for Best Blog
Recipient, Eukanuba Canine Health Award
Recipient, AKC Club Publication Excellence Award
Become a Fan of Speaking for Spot on Facebook

Please visit to read excerpts from Speaking for Spot and Your Dog’s Best Health.   There you will also find “Advocacy Aids”- helpful health forms you can download and use for your own dog, and a collection of published articles on advocating for your pet’s health. Speaking for Spot and Your Dog’s Best Health are available at,, local bookstores, and your favorite online book seller.







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3 Comments on “Five Important Things to Know About Feline Heartworm Disease

  1. Thank you Dr. Kay for providing this very valuable information. I had no idea heartworm was so different in cats than dogs. I own both. My dogs receive HW prevention although my cats do not and my vet never explained why he didn’t recommend it nor did I ask. I’ve shared this on my facebook for other cat owners.

  2. Dr. Nancy,

    The only cats we have are feral. But, you got me thinking about their healthcare and I found the following information to help our feline neighbors, the cats/kittens.

    The San Antonio Humane Society (no kill) offers healthcare and makes the following statement:

    “There is some confusion in cats on the difference between “feral” and “stray”. The San Antonio Humane Society values the sterilization of both populations of cats, however, the care needed and grants available for the different types of cats is varied.

    “Stray cats and feral cats share the same basic characteristics in that both are cats that do not have an obvious owner, who wander around the neighborhood without a human family they call home. They could or would have spent most of their life on the street. The major and Very important difference is that a stray cat enjoys being touched or is at very least handleable. They will approach humans to ask for food or affection. They can be pet or picked up and are easily transported in a common cat carrier without undue stress.

    “A feral cat is much like a raccoon. Simply crossing within 20ft of the cat will cause them to run away and seek shelter behind a structure, under a house, or up a tree/fence. They cannot be touched, pet, or picked up. The only way to catch them is to trap them in a humane trap. When the trap is approached with the cat inside, the cat will try to escape by hitting itself against the bars frantically or sink to the bottom of the cage in a small ball, growling, hissing, and being aggressive or scared. A towel or blanket over the cage will reduce the stress and damage the cat may due to itself during transport.

    “Feral cats IN TRAPS can be brought in on a first come first serve basis. There is a 10 feral cat limit Monday – Thursday. Calling ahead to check for surgery is recommended as there are days when the surgery suite is closed for maintenance or staff training. All cats presented as feral in a trap will receive an ear tip procedure. Community cats or strays that are tame and enjoy human companionship must have a scheduled appointment. Any cat not in a humane trap MUST be handable by surgery staff to be accepted for surgery.

    “Although it is easy to see that there could be some confusion when trapping the cats in the neighborhood, with a little diligence it is easy to see the difference between a stray and a feral.

    Feral cats are un-socialized cats. They were either born outside and have never known human companionship, or they were pets that got lost or abandoned, and after an extended time without human contact, have learned to fear humans. Feral cats form colonies around a food source such as a dumpster. Reproducing to the limits of the food supply, they lead meager lives often shortened by malnutrition, disease, and trauma. Most kittens born never make it through their first year. Their mating behaviors of spraying, fighting, and howling often cause them to become a public nuisance. Constant reproduction adds to the problem. Getting the cats spayed or neutered virtually eliminates these problems. Once neutered, males will no longer spray to mark their territory and fighting over females stops, eliminating the problems of smell and noise. The colony size gradually diminishes with no kittens being born. Cats in a managed colony can live healthy, happy lives without causing problems for neighbors.

    “Started as a grassroots effort in the U.S., Trap-Neuter-Return (TNR) is now becoming accepted all across the country. TNR is the only proven humane method that solves the overpopulation problem while allowing the cats to live out healthy lives. The San Antonio Humane Society rents humane traps for those looking to find a way to transport feral cats for the TNR program that is safe for humans and cats alike. Rental fee is $15 for ten days, with a $100 deposit refundable upon return of the trap. Call (210) 226-7461 for more information on renting a humane trap.

    “The San Antonio Feral Cat Coalition also offers free TNR training classes to ensure your safety and the safety of the feral cats throughout the TNR process. For more information, view the San Antonio Feral Cat Coalition class schedule here.”

    Thank you Dr. Nancy for getting the ball rolling! We have scheduled TNR / and vaccinations for these children of God.

    Have a blessed day!

    Bless you and your work for the animals. We all love you.

  3. Thank you for the feline heart worm info. I read your newsletters regularly even though there are only cats in my family. I care deeply for dogs, just do not live with any. I find that much of the canine issues also relate to felines. Am so happy to be recognized by you directly this week. Fur persons united. !!