Achey Breaky Heartworms

“My dog can’t get heartworms, he barely ever goes outside.â€

“Fifi doesn’t need to be tested for heartworms – I never see worms in her stool.â€

“I have an indoor-only cat, I don’t need to worry about heartworm disease.â€

These are a few of the most common responses I’ve heard when I discuss heartworm disease with my clients. Unfortunately, all of these statements are incorrect, and today we’re going to talk about the most important things you as an owner should understand about the dreaded heartworm disease. Ready? Here we go!


Heartworms are a parasite that can infect dogs, cats, ferrets, and some wild mammals including coyotes, foxes, and wolves. They have a multi-stage life cycle, with the adult stage being a worm that is approximately 1 foot long that lives in the chambers of the heart, in the lungs, and/or the large blood vessels of the infected animal. These worms cause severe inflammation and permanent damage to the cardiovascular and pulmonary systems, as well as other organs throughout the body. Heartworm disease is often fatal for the infected animal. Heartworms are not generally considered a danger to humans, however, in rare cases humans can become infected with heartworms.


The short answer: mosquito bites.

Here’s a bit more information: When a mosquito takes a blood meal from an infected dog, coyote, fox, or wolf, they pick up “baby heartworms†called microfilaria. These microscopic baby heartworms live inside the mosquito for 10-14 days, and during that time they mature to become infective larvae. Once they are infective, the heartworm larvae will be passed from the mosquito to a new dog, cat, ferret, or other susceptible animal the next time the mosquito bites.

Once the infective heartworm larvae are inside a new host, it takes about 6 months for them to fully mature into adult heartworms. They can then start reproducing, and the life cycle starts again. Mature heartworms can live for 5-7 years in dogs, and up to 2-3 years in cats. Remember that the heartworms are causing severe inflammation and permanent damage to the host dog or cat the entire time they are alive, and even after they die inside the host.


In my opinion, every dog and cat should receive year-round heartworm prevention unless it is medically contraindicated. Mosquitoes are present all over the United States, even if in very small numbers. Mosquitoes can enter houses and expose even indoor pets. All it takes is ONE bite from an infected mosquito to expose your beloved pet to this deadly disease. **As with all medications, supplements, etc, please discuss with your veterinarian before making any changes for your pet.


There are many forms of heartworm prevention, including oral medications (tablets, pills), topical medications, even injectable medications (e.g. ProHeart). All of these heartworm preventative medications work by killing the infective heartworm larvae inside the dog or cat’s bloodstream. However, once a heartworm larva has been inside the host dog or cat for 50-60 days, they are no longer susceptible to heartworm preventative medications. This is why it is so important to continue a regular schedule of heartworm prevention exactly as prescribed and to avoid missing any doses – even 1 missed dose could potentially lead to your dog or cat being infected with heartworms.


Despite our best efforts, sometimes a dose of prevention gets missed. If this happens, you should first call your veterinarian’s office to ask their advice – this will depend on which specific type of heartworm prevention your pet takes. If you don’t have a regular veterinarian, you can also call the heartworm preventative manufacturer directly to ask for guidance. (Sometimes “Dr. Google†can lead you astray, so please consult veterinary professionals directly.)

In most cases, part of the recommendation will be to have your dog retested for heartworms after 6 months to determine if they may have become infected in relation to the missed dose.


For dogs, a heartworm test requires just a few drops of blood and can often be performed in your vet’s office while you wait. The test looks for specific proteins found in adult heartworms – this is called an antigen test.

For cats, the test is a bit different. The feline heartworm test also requires just a small amount of blood, but since cats usually carry a lot fewer adult heartworms than dogs do, this test looks for both antigens (proteins) AND antibodies, which would indicate an exposure to heartworms. In cats, your veterinarian may also use cardiac ultrasound to look for evidence of heartworms if an infection is suspected.


The American Heartworm Society publishes treatment guidelines for dogs and cats with specific protocols based on rigorous scientific research. This approach to treating heartworm disease is considered the standard of care in veterinary medicine. More information can be found here: Heartworm Guidelines

For cats, there is no direct treatment for heartworms – the medication to kill adult heartworms cannot be used in cats. The strategy is one of preventing any new infections and trying to reduce inflammation as much as possible while waiting for the heartworms to die on their own. **For cats, prevention is not just the best medicine, it’s the ONLY medicine when it comes to heartworm disease.

There *is* a treatment for heartworms in dogs. If your dog tests positive for heartworms, your veterinarian will first order a confirmatory blood test to make sure the diagnosis is correct before moving forward. They will likely also need to perform some additional tests such as a full blood panel and chest x-rays to see how severely affected your dog is and whether they need to be stabilized before treatment can begin. Treatment will involve monthly heartworm preventatives, one month of antibiotics, several months of steroids, a series of 3 injections to kill the adult heartworms, and several months of exercise restriction.

The treatment for heartworms in dogs is complicated, costly, and not without risk; however, heartworm disease left untreated is most often fatal.


In the past, it was thought that a “slow-kill†method for treating heartworm disease could be effective. This approach essentially amounts to placing the dog on monthly heartworm prevention to protect against any new infections, and then waiting for the adult heartworms to eventually die on their own. Remember that heartworms can live for 5-7 years, so that means 5-7 years of chronic severe inflammation in the dog’s heart and lungs, causing permanent damage to the heart muscle, blood vessels, lungs, as well as other organs such as the kidneys. Many veterinarians now feel the “slow-kill†method is not a form of treatment, and the in-hospital injections of adulticide are the recommended approach in the majority of cases.


Here are some reputable resources for you to learn more at home. I also suggest speaking directly with your veterinarian to get your questions answered and to decide on a specific type of heartworm prevention for your dog or cat.

American Heartworm Society

US FDA Message on Heartworms

Companion Animal Parasite Council

Veterinary Partner Guidance on Heartworms