10 Myths About Heartworms

By PetMD Editorial. Reviewed by Katie Grzyb, DVM on Jun. 23, 2019
dog and little dog on grass lawn

Reviewed for accuracy on June 24, 2019, by Dr. Katie Grzyb, DVM

It takes just one bite from a mosquito that’s infected with heartworm larvae to jeopardize your pet’s health and welfare. And if your pet becomes infected, heartworm disease is often debilitating and can be fatal if it’s not treated.

The stakes are simply too high to believe myths like, “Only dogs are susceptible to heartworms” or “Heartworm disease is just a summer issue.”

To help you sort out fact from fiction, we’ve debunked 10 of the most common heartworm myths.

Myth 1: Only Dogs Can Get Heartworm Disease

Dogs may be the companion animal most at risk for heartworms, but cats and ferrets are vulnerable, too. That’s why the AHS recommends year-round prevention for all three species, says Dr. Chris Rehm, president of the American Heartworm Society (AHS).

“Cats are more resistant than dogs as a heartworm host,” but they are still at risk of becoming infected, says Dr. Laura Hatton, a veterinarian with BluePearl Veterinary Partners in Overland Park, Kansas.

Like dogs, cats can develop adult heartworms, but it’s more common for heartworms in cats to die before they reach full maturity, she adds. There are no known safe drug therapy treatment options for dealing with heartworms in cats, so prevention is the best way to keep them healthy.

Myth 2: Indoor Pets Are Not at Risk for Heartworms

Don’t assume that your pet is protected just because she’s a homebody that doesn’t venture outdoors much. Disease-carrying mosquitos can easily get inside the home and transmit heartworm disease.

About one-quarter of cats diagnosed with heartworms are considered indoor cats, says Dr. Hatton.

“Even if your pampered pooch only goes outside for bathroom breaks or brief walks, remember—it takes just one bite from an infected mosquito to infect a pet,” says Dr. Rehm.

Myth 3: Heartworm Disease Is Just a Summertime Issue

We all know mosquitos thrive in warmer weather, but “mosquito season” can fluctuate from one region to another, and even from one year to the next, says Dr. Hatton.

“Generally, mosquito activity will begin when the temperature reaches the 50 degrees Fahrenheit level and typically tapers off as the temperatures cool,” Dr. Hatton says.

However, “It’s not unheard of for mosquitos to be active in 40-degree temperatures,” says Dr. Susan Jeffrey, a veterinarian with Truesdell Animal Hospital in Madison, Wisconsin.

The first frost is usually a reliable indicator that mosquito season is over, but some hibernating mosquitos can emerge in the winter during unexpected warm spells, adds Dr. Hatton.

If you live in warmer climates, you’re prepared to see mosquitos even in winter months, but even in colder climates, it’s impossible to predict when the last mosquito will appear, says Dr. Rehm.

“Mosquitos also seek out warm, protected places like crawl spaces and decks where they can survive until well after the last leaves have fallen. For these reasons, the AHS recommends year-round prevention for all pets,” says Dr. Rehm.

Myth 4: Heartworm Disease Doesn’t Occur in Dry Climates

Heartworm disease has been reported in all 50 states, Dr. Hatton says. “Mosquitos are highly adaptable and will find other places to breed, even during a drought. While some mosquitos breed and hatch during rainfall, others prefer tires, birdbaths or tin cans to reproduce.”

Other areas with standing water, including ponds, lakes and swimming pools, can provide optimal breeding conditions for mosquitos, says Dr. Jeffrey.

Thinking that your companion animal is protected because you live in the desert is false security. In fact, “The lower likelihood that pets are protected from heartworms in desert regions makes the presence of just one heartworm-positive dog or coyote in a neighborhood a serious concern,” Dr. Rehm says.

Myth 5: Heartworm Disease Is Rarely Fatal

Heartworm disease is a devastating and potentially fatal disease, impacting the heart, lungs and pulmonary blood vessels, Dr. Hatton says. “Heartworms lead to an inflammatory reaction that can cause permanent damage to the blood vessels in the lungs. Apart from the risk of fatality, heartworms can compromise an animal’s quality of life and cause debilitating clinical signs and symptoms, which may improve but not necessarily resolve, even with treatment.”

In dogs, symptoms usually start with a cough, which can worsen as the disease progresses. “Fatigue, difficulty breathing and weight loss are common later in the disease,” Dr. Hatton says. “Left untreated, dogs can go into heart failure and ultimately die.”

Cats with heartworm disease typically develop lung disease, which can mimic asthma and cause respiratory distress, chronic coughing and vomiting, she says. “The death of one adult heartworm in a cat can cause that cat to die abruptly.”

How Long Can a Dog Live With Heartworms?

"[Heartworm] life expectancy depends on the size of the dog, the relative health of the dog, if the dog has a reaction to the worms, and how many worms the dog has," says Dr. Sarah Wooten, DVM at West Ridge Animal Hospital in Greeley, Colorado.

However, left untreated, heartworm disease is usually fatal, Dr. Jeffrey says. “Some dogs can carry a very low worm burden and be okay, but the majority of dogs who go untreated will not survive.”

Myth 6: You Can Skip the Annual Heartworm Test If Your Pet Is on Preventatives

In addition to a year-round heartworm-prevention regimen, the AHS recommends annual testing to ensure that the prevention program is working, says Dr. Rehm. Experts say that although heartworm preventives are highly effective, nothing works 100 percent of the time.

Even dogs on strict preventive regimens can become infected. “I've had two cases of dogs with heartworm who were on monthly preventatives and didn't miss doses,” says Dr. Jeffrey.

“The best of pet owners can be forgetful, and missing just one dose of a monthly medication—or giving it late—can leave a dog unprotected. And even if you do everything right and on time, it’s no guarantee,” says Dr. Rehm.

“Some dogs spit out their heartworm pills when their owners aren’t looking. Others may vomit their pills or rub off a topical medication. Fortunately, heartworm tests are safe and can be conducted during your pet’s annual checkup,” advises Dr. Rehm.

Myth 7: It’s Okay to Miss a Month of Heartworm Preventatives

Heartworm disease is a year-round threat. “Heartworm preventives work retroactively, so a dog or cat that is infected one month must receive heartworm preventives in subsequent months in order to be protected,” Dr. Hatton says.

Changing weather patterns coupled with mosquito hardiness make it difficult to predict the timing of infection. “Rather than guessing when it’s safe to stop prevention, it’s best to keep your pet on year-round prevention,” Dr. Hatton says.

Plus, skipping a month can lead to infection down the road, says Dr. Jeffrey. “If a month is missed, a dog should be tested for heartworm six months later.”

Myth 8: Natural Remedies Work as Well as FDA-Approved Preventatives

“At this time, nosodes [a type of homeopathic preparation] and herbal preventives are not recommended as alternatives to FDA-approved preventives, because these remedies do not have proof of effectiveness,” says Dr. Rehm.

“No repellent or avoidance strategy can take the place of heartworm preventives,” Dr. Rehm says. Experts stress that repellents and avoidance should be used in addition to preventives, not instead of them.

Natural repellents such as neem oil (which should be used with caution in cats) and insecticides made with all-natural ingredients can help reduce the number of mosquito bites a pet receives, Dr. Rehm adds.

According to Dr. Bianca Zaffarano of Iowa State University, "Drug-free strategies, such as avoiding mosquito exposure and eliminating standing water that serves as mosquito breeding grounds, can help reduce heartworm transmission.”

Myth 9: Heartworms Are Contagious

Heartworm disease doesn’t spread like a cold or flu. In other words, your pet can’t catch it directly from another animal.

Heartworm is spread through a mosquito [that bites and] acquires the heartworm larvae from other infected dogs, coyotes, wolves or foxes,” Dr. Hatton says. “The infected mosquito then bites a dog or cat and transmits the immature worms to them. If not on heartworm preventive, the larvae mature and multiply, causing damage to the heart and lungs.”

Can Humans Get Heartworms?

Finding heartworms in humans is considered to be extremely rare. “Humans are considered [to be] dead-end hosts. It’s extremely rare for humans to get heartworm disease, but they can be exposed to heartworm disease through the bite of mosquito and end up with lung pathology and granulomas in various organs,” Dr. Hatton says. 

Myth 10: Heartworm Prevention Is Costly and Inconvenient

It’s less expensive to prevent canine heartworm disease than it is to treat it, Dr. Hatton says.

“Not only is monthly prevention more cost-effective, but it will provide you and your pet with a better quality of life,” says Dr. Hatton.

Prevention is one of the best investments you can make in your pet’s health, Dr. Rehm adds. “It can cost less than the price of a pizza a month, depending on the product you use.” In contrast, treating a dog with heartworm can cost more than 10 times the annual cost of heartworm prevention.

Prevention is also convenient. A number of options are available to accommodate different lifestyles.

“Does your dog love treats? If so, it may make sense to give him a monthly chewable medication. Does your cat hate pills? There are several spot-on options that provide comprehensive parasite protection. Are you a forgetful dog owner? A twice-annual injection may be your preference,” Dr. Rehm suggests.

“Because no two pets—or pet owners—are alike, it’s good to know that you have options. The important thing is to find a product that’s convenient for you and your four-legged friend,” says Dr. Rehm.

Be sure to talk with your veterinarian about how to best reduce the chances of your dog or cat (or yes, even ferret!) becoming infected with heartworms.

By: Paula Fitzsimmons

Featured Image: iStock.com/photo_Pawel

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