10 Myths About Heartworms

It only takes one bite from a heartworm-infected mosquito to jeopardize the health and welfare of your dog or cat. And if your pet becomes infected, heartworm disease in dogs and cats can be fatal or cause lifelong damage.

To help you sort out heartworm facts versus fiction, let’s look at 10 of the most common heartworm myths and what you can do to protect your pet from heartworm disease.

Myth 1: Heartworms Are Contagious

Your pet can’t catch heartworms directly from another animal.

Heartworms are spread when mosquitoes that have previously fed upon heartworm-infected dogs, coyotes, wolves, or foxes bite your pets and infect them with tiny heartworm larvae.

If your dog or cat is not kept current on their heartworm preventive—in other words, given on time year-round—the tiny larvae will mature within a matter of months and cause damage to your pet’s heart, lungs, and arteries.

Keep in mind that a single infected mosquito can infect multiple pets, so it’s critical to have your pet on a routine heartworm preventive and to give it to them on time as instructed by the manufacturer.

Myth 2: Only Dogs Get Heartworm Disease

When we think of heartworms, many of us think of dogs as the intended target. However, cats can also get heartworm disease and so can ferrets.

Cats and ferrets are smaller than most dogs, so just one or two adult heartworms can be fatal for them. That’s why the American Heartworm Society recommends year-round prevention for all three species.

Heartworm disease in cats differs from heartworm disease in dogs in several ways. Because dogs are highly susceptible to heartworm disease, almost any dog bitten by an infected mosquito will likely develop adult worms that can grow to a foot long inside of them.

Cats are less likely to develop adult heartworm infections because their immune systems kill most heartworms before they fully develop. This may sound like a good thing, but it really isn’t, because the dead immature worms can still damage the cat’s respiratory system and cause lifelong issues.

Treatment for heartworm disease in cats is also limited compared to treatment for dogs. There is no medication that can safely eliminate adult heartworms in cats or ferrets once the worms grow, which is why heartworm prevention is critical.

Myth 3: Indoor Pets Aren’t at Risk for Heartworms

Our dogs spend time outdoors playing in the backyard and taking walks. Even a brief bathroom break can expose them to mosquitoes—which is why it’s important to keep your pup up to date on their heartworm preventive.

However, indoor pets are at risk, too. Think about how often you’ve found a mosquito inside your home. That’s a common way for your indoor cat or ferret to be bitten as well.

In fact, a study in North Carolina found that approximately 1 in 4 cats diagnosed with heartworms were considered indoor-only cats. Even if the closest your cat gets to the outdoors is hanging out in their catio, heartworm infection is possible.

Myth 4: Heartworms Are a Seasonal Risk

Knowing that heartworms are spread by mosquitoes, some pet parents assume they can just give heartworm prevention when the weather is warm and mosquitoes appear most active. But heartworm disease is a serious and year-round threat to pet health. Here’s why:

• Heartworm preventives don’t work by preventing pets from becoming infected by heartworms. Instead, most work retroactively, eliminating infections that occurred the previous month. A dog or cat infected in September, therefore, must receive their next scheduled dose of heartworm preventive to be protected.

• Weather is unpredictable! While the risk of heartworm transmission is greatest for pets from spring through fall, it’s impossible—and risky—to try to guess when the first or last mosquito of the so-called “season” will be active, given the volatility of weather conditions and climate change.

• Mosquitoes are great at seeking shelter. Buildings, parking lots, and other structures in city and suburban communities can create “heat islands” with significantly warmer conditions than surrounding open areas. Mosquitoes also survive and thrive by relocating indoors or taking refuge in garages and crawl spaces when temperatures drop.

• If you’re traveling with your pet, they can face a totally different set of health risks on the road, especially if their time away is spent in an area with more mosquito activity than their home environment.

Giving pets heartworm preventive on a regular schedule is habit-forming in a good way, because it’s easier to remember when done year-round.

Myth 5: I Don’t Live in an Area with Lots of Mosquitoes, so I Don’t Need To Worry About Heartworms

Heartworm disease has been reported in all 50 states—including states with cold winters and desert conditions. Mosquitoes are highly adaptable and find places to breed, even during droughts. While some mosquitoes breed and hatch in lakes and streams, others prefer mating in small amounts of trapped water in tires, birdbaths, or watering cans.

Assuming that your pet is protected because you live in the desert is also false security. The American Heartworm Society incidence surveys, which have been conducted for the past 20+ years, report that heartworm infections in desert areas like the American Southwest have increased in recent years.

Meanwhile, if pets in so-called “dry” regions are less likely to be protected from heartworms due to misperceptions about risk, the presence of just one heartworm-positive dog or coyote in a neighborhood can be a serious concern.

Remember, if a mosquito bites a heartworm-positive animal, the mosquito can then transmit heartworms to other animals.

Myth 6: Heartworm Disease Is Rarely Fatal in Dogs

Heartworm disease is a devastating and potentially fatal disease that damages the heart, lungs, and pulmonary blood vessels of infected animals.

Fortunately, heartworms in dogs can be treated. If diagnosed and treated early, before signs that the disease has progressed—such as cough, fatigue, difficulty breathing, and weight loss—many dogs can go on to live normal lives.

However, untreated heartworm infection in dogs can lead to heart failure, damage to other organs, and death, and the longer the infection is allowed to progress before treatment, the more likely it is that the dog will be left with permanent damage.

Myth 7: Heartworm Disease Is Rarely Fatal in Cats

Cats with heartworm disease typically develop lung disease, which can mimic asthma and cause respiratory distress with chronic coughing and vomiting. In cats with adult heartworms, the greatest risk is not from the live worms inside the cat but from the severe inflammatory reaction that is triggered when the worms die.

In fact, the death of even a single adult heartworm in a cat can cause sudden death in the cat. For this reason, cats diagnosed with adult heartworms should be monitored with frequent veterinary checkups, and pet parents should be alert to signs of respiratory crisis so that prompt veterinary care can be provided if needed.

Myth 8: If My Pets Are on Preventives, They Don’t Need To Be Tested for Heartworms

The American Heartworm Society recommends that heartworm preventives be given year-round to give pets the best chance of staying free of heartworm disease.

But dogs and cats still need to be tested annually for heartworms to ensure their prevention program is working. Heartworm preventives are highly effective but not perfect.

Even the best pet parents can get busy and forget to give a dose of heartworm medication—or give it late. Pets might spit out heartworm pills or rub off topical medications. And a pet who has put on weight recently might be inadvertently underdosed.

For these reasons and more, annual testing is recommended. Fortunately, heartworm tests are safe and can be conducted with a small sample of blood during a dog’s or cat’s annual checkup.

Myth 9: Natural Heartworm Preventives Are a Good Alternative to FDA-Approved Medications

Homeopathic and herbal products marketed as “natural” heartworm preventives don’t provide the protection pets need.

Medications approved by the U.S. Food and Drug Administration and prescribed by veterinarians for heartworm prevention are the only proven and effective options. There are many products and formulations to choose from that have gone through scientific studies to prove they are safe and effective.

While there are no substitutes for FDA-approved heartworm preventives, there are “natural” heartworm prevention strategies that can be used in conjunction with preventives to help provide extra protection for your pets.

• Heartworms are spread by mosquitoes, so keeping your pet indoors during peak mosquito times—often at dawn and dusk—can be a helpful practice.

• You can also take steps to rid your yard and property of standing water sources where mosquitoes can breed—or have the water treated to reduce mosquito survival.

• The American Heartworm Society recommends the use of EPA-approved mosquito repellents specifically formulated for pets who spend time outdoors in areas known to have lots of mosquitoes.

However, pet parents should remember that while some mosquito repellents are safe and effective for dogs, most of these products aren’t considered safe for use in cats. Instead, mosquito-control strategies for cats should be targeted toward the environment, not cats themselves.

Myth 10: Heartworm Prevention Is Costly

It’s much less expensive to prevent heartworm disease than it is to treat the disease—in fact, the cost of prevention in a dog is a small fraction of the medication and veterinary costs required for heartworm treatment. Meanwhile, there are NO treatments approved for cats and ferrets.

There are many preventive options on the market, from broad-spectrum preventives that treat a wide variety of parasites—from heartworms to fleas, ticks, mites, and intestinal worms—to products that focus primarily on heartworm prevention. You can work with your veterinarian to determine the best heartworm preventive based on your pet’s individual health and lifestyle.

Finally, the cost to a pet's health can be significant if they develop heartworm disease. Prevention is the easiest and least expensive way to ensure your pet is getting the protection they need.