10 Myths About Heartworms

PetMD Editorial
By PetMD Editorial on Apr. 11, 2013

10 Myths About Heartworms


By Paula Fitzsimmons


It takes just one bite from a mosquito infected with heartworm larvae to jeopardize your pet’s health and welfare. Heartworm disease is often debilitating and can be fatal if not treated. That’s why the stakes are too high to listen to myths like only dogs are susceptible to heartworms and heartworm disease is just a summer issue.


We asked the experts to debunk some of the most common myths surrounding heartworms. Once you are armed with the facts, talk with your veterinarian about how to reduce the chances of your dog or cat (or yes, even ferret!) becoming infected.

Myth 1: Heartworm Disease Is Just a Summertime Issue


We all know mosquitoes thrive in warmer weather, but “mosquito season” can fluctuate from one region to another, and even from one year to the next, says Dr. Laura Hatton, a veterinarian with BluePearl Veterinary Partners in Overland Park, Kansas. “Generally, mosquito activity will begin when the temperature reaches the 50 degrees Fahrenheit level and typically tapers off as the temperatures cool,” Hatton says.


However, “it’s not unheard of for mosquitoes 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 mosquitoes can emerge in the winter during unexpected warm spells, adds Hatton.


If you live in warmer climates, you’re prepared to see mosquitoes, but even in colder climates, it’s impossible to predict when the last mosquito will appear, says Dr. Chris Rehm, president of the American Heartworm Society (AHS). “Mosquitoes 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.”

Myth 2: Heartworm Disease Doesn’t Occur in Dry Climates


Heartworm disease has been reported in all 50 states, Hatton says. “Mosquitoes are highly adaptable and will find other places to breed, even during a drought. While some mosquitoes breed and hatch during rainfall, others prefer tires, birdbaths, or tin cans to reproduce. They will search out watered lawns, gardens, or underground storm systems during times of drought.”


Other areas with standing water, including ponds, lakes, and swimming pools, can provide optimal breeding conditions for mosquitoes, says Jeffrey, whose professional interests include preventative care.


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,” Rehm says.

Myth 3: Only Dogs Can Get Heartworm Disease


Dogs may be the companion animal most at risk for heartworms, but cats and ferrets are vulnerable, too, which is why the AHS recommends year-round prevention for all three species, Rehm says.


“Cats are more resistant than dogs as a heartworm host, but 75 percent of cats exposed to infective heartworm larvae do become infected, as opposed to nearly 100 percent of dogs that become infected when exposed to infective heartworm larvae,” Hatton says.


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 few treatment options for dealing with heartworms in cats (or ferrets), so prevention is the best way to keep them healthy.

Myth 4: Indoor Pets Are Not at Risk for Heartworms


Don’t assume an animal is protected because she’s a homebody. About one-quarter of cats diagnosed with heartworms are considered indoor cats, says Hatton, who is board certified in veterinary internal medicine-cardiology. Disease-carrying mosquitoes can get inside the home and transmit heartworm disease.


“If you’ve ever seen a mosquito flying inside your house, you know that mosquitoes—and heartworms—can find their way inside,” Rehm says. “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.”

Myth 5: Heartworm Disease Is Rarely Fatal


Heartworm disease is a devastating and potentially fatal disease, impacting the heart, lungs, and pulmonary blood vessels, 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,” 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, Jeffrey says. “Some dogs can carry a very low worm burden and be OK, but the majority of dogs who go untreated will not survive.”

Myth 6: An Annual Heartworm Test Is Not Necessary If an Animal Is on Preventives


In addition to a year-round heartworm prevention regimen, the AHS recommends annual testing to ensure the prevention program is working, says 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 2 cases of dogs with heartworm who were on monthly preventatives and didn't miss doses,” says 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. 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 check-up,” advises Rehm.

Myth 7: Natural Remedies Work as Well as FDA-Approved Preventives


Taking steps to reduce mosquito (and heartworm) exposure makes sense, Rehm says. 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.” Natural repellents such as neem oil (which should be used with caution in cats) and insecticides made with all-natural ingredients can also help reduce the number of mosquito bites a pet receives, Rehm adds.


“However, no repellant or avoidance strategy can take the place of heartworm preventives,” Rehm says. Experts stress that repellants and avoidance should be used in addition to preventives, not instead of them. “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.”

Myth 8: It’s OK to Miss a Month of Heartworm Preventives


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,” 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,” Hatton says.


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

Myth 9: Heartworms Are Contagious


Heartworm disease doesn’t spread like a cold or flu. In other words, your companion can’t catch it directly from another animal, even one of the same species.


“Heartworm is spread through a mosquito bite, which acquires the heartworm larvae from other infected dogs, coyotes, wolves, or foxes,” Hatton describes. “Mosquitoes become infected by biting an animal that has the disease. 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?


Heartworms in humans is considered to be extremely rare. “Humans are considered a dead-end host (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,” she says. 

Myth 10: Heartworm Prevention Is Costly and Inconvenient


It’s less expensive to prevent canine heartworm disease than it is to treat it, Hatton says. “Not only is monthly prevention more cost effective, but it will provide you and your pet with a better quality of life,” she says.


Prevention is one of the best investments you can make in your pet’s health, Rehm adds. “It can cost less than the price of a pizza a month, depending on the product you use.” In contrast, he says 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, Rehm says. “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,” 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.”