Heartworms in Dogs: Myths and Facts

Medically Reviewed by Amy Flowers, DVM on January 21, 2024
5 min read

Heartworm -- a worm that can take up residence in a dog’s heart, blood vessels, and lungs -- might seem like an unlikely threat to your pet. But it’s not uncommon.: It's been found in all 50 U.S. states. Cases have risen in recent years ccording to  the American Heartworm Society.

“They look like spaghetti in the heart and blood vessels,” says veterinarian Wendy Mandese, DVM, clinical assistant professor at the University of Florida College of Veterinary Medicine. “Enough worms will cause potentially irreversible damage to the heart and vessels, and can eventually lead to heart failure and pulmonary hypertension,” which is high blood pressure in the arteries that supply blood to your lungs. 

That means protecting your pooch from this parasite is more important than you might have realized, no matter your dog’s age, breed, or where you live in the country. Yet many myths about heartworm persist, and they might lead otherwise responsible dog owners to neglect this aspect of their pet’s health.

Are you falling for a common misconception? Read on and get the facts.

Fact: Heartworm is transmitted by mosquitoes – so warm, muggy areas are certainly hotbeds, but infection has been reported in all 50 U.S. states. If a mosquito carrying heartworm larvae bites your dog – one bite is all it takes! – those larvae will make themselves at home within your dog, travel to their heart and lungs, and develop into mature heartworms and start reproducing.

“These worms can grow to be up to a foot long,” says Dwight D. Bowman, PhD, professor of parasitology at Cornell University College of Veterinary Medicine.

Fact: The best way to deal with heartworm is to prevent it in the first place, and that’s not hard to do.

Heartworm prevention comes in both oral and topical formulas (which are absorbed into the bloodstream), so you and your veterinarian can decide which method is most convenient for you and your pet.

If you think you might forget to give this medicine monthly, ask about an injectable option. Heartworm preventive shots are usually given by a vet only once or twice a year.

You should also ask your vet about the pros and cons of choosing a combination product that prevents heartworm as well as fleas and ticks. These all-in-one formulas might be convenient, but they tend to be expensive, Mandese says. If you decide not to use a combination product, you can compare the costs of products to guard against heartworm, fleas, and ticks. But do protect your dog against all three things.

Treating a heartworm infection, especially a severe one, isn’t as easy as you might think.

Early on, you probably won’t notice any symptoms. But as the number of worms increases, your dog will develop inflammation in their lungs, and they’ll probably start coughing or become very fatigued. If it gets bad enough, your dog can have trouble breathing and die. Heartworm may also lead to heart failure, kidney disease, and liver damage, which can cause bloody or dark urine and a swollen abdomen and become fatal. But it's not the most common cause of a swollen abdomen in dogs.

Assuming you find an infection in time, your dog will need several months of treatment, including two months of injections of a medication called melarsomine. This drug has to be given slowly and carefully because if you kill off the worms too quickly it can be dangerous for your dog, Bowman says. He explains that the worms that die slowly decay in the lungs. (They don’t come out in poop like intestinal parasites would.) Meanwhile, you’ll need to restrict your dog’s activity to help prevent serious (and even life-threatening) complications that can happen if your dog exercises during treatment. This can happen if pieces of dead worms block blood flow in your dog's lungs, creating a blood clot (pulmonary embolism).

“Heartworm treatment requires a minimum of 2 months of strict rest and can be quite expensive,” Mandese says.  

In severe cases, heartworm infection will call for emergency surgery: A veterinarian may have to anesthetize your dog and try to pull worms out of their blood vessels in an effort to save their life. That’s costly, highly invasive, and your pet’s life will be in jeopardy.

Fact: Preventive treatments are incredibly effective, but “no preventive is 100% effective, and since dogs with early infection can be asymptomatic, annual testing is recommended,” Mandese says. “We are also seeing some evidence that there may be resistance developing to our available preventives, making testing even more important.”

Your dog might also need to be tested if you’ve forgotten to give them their regular prevention or if they’re switching to a new type of heartworm prevention.

The good news is that if a dog on a preventive medication does test positive, they are unlikely to be infested with a large number of worms. So  treatment should be far easier than it would be if they weren’t on any preventive.

Getting a new dog or puppy? Ask the shelter or breeder if they’ve recently tested the dog for heartworm; if not, it’s worth having your vet do a check before starting (or continuing) prevention, says Bowman.

Testing typically costs $35-$50, though some clinics may charge more or less. It’s very easy; your vet just needs to take a few drops of your dog’s blood.

Fact: While humans don’t usually get heartworm, most preventive heartworm medications also protect against intestinal parasites like hookworms and roundworms, which are definitely contagious to humans,  Mandese says.