Every pet owner has heard of heartworm disease. Most likely, you have stared at a model heart containing a fistful of spaghetti-shaped worms in the veterinary exam room. Maybe you thought, “That won’t happen to my pet, right?” Maybe you had had questions, but were too embarrassed to ask. 

Our team at Wales Animal Clinic believes that informed pet owners are the best pet owners, and that any question about your pet’s health and well-being is a question worth asking. Let’s start with the myths about heartworm disease and prevention—and don’t worry, you only have to look at the heartworm model if you want to. 

MYTH: Heartworm disease is not common here so I don’t need to give my pet heartworm prevention.

FACT: Heartworm disease has been diagnosed in all 50 states.

While Wisconsin may not be classified as a high-risk state, over the past two years one in every 200 dogs has tested positive for heartworm disease, according to the Companion Animal Parasite Council. This data does not include the many thousands of pets who do not receive heartworm tests, so the frequency of heartworm disease is almost certainly much greater. Additionally, hot spots of positive cases exist across the border in northeastern Illinois, where veterinarians have reported an average of 26 to 99 positive patients per clinic. At the national level, the United States continues to experience a wave of heartworm cases that are originating in the South and moving northward and along the East Coast. If one mosquito is all it takes to infect an unprotected dog or cat, and your neighbor’s dog is heartworm positive, it’s likely your pet will be infected as well.

MYTH: Heartworm disease only occurs in pets kept outside. My pet lives indoors, and their risk of developing heartworm disease is low.

FACT: Mosquitoes are present indoors, and so is heartworm disease.

Heartworm disease occurs in both indoor and outdoor pets. For instance, indoor dogs who go outside to use the bathroom and to exercise are vulnerable to mosquito bites and can also carry feeding mosquitoes back indoors. 

One bite from a mosquito carrying Dirofilaria immitis—the microscopic, early stage of heartworms—and your pet can be infected. As the mosquito feeds, the juvenile heartworm enters through the bite and beneath the skin, beginning the journey toward the lungs and heart. Mature heartworms can reach up to 12 inches in length—and that’s only one worm. Infected dogs can have hundreds of adult heartworms at a time, causing inflammation and irreversible damage to vessels and tissues.

Mosquitoes typically are most prevalent during the summer, and are present in wet seasons and droughts. They can gain access to homes through holes in screens, open doors, garages, or basements. Female mosquitoes will lay eggs indoors if a humid, dark location is available. 

MYTH: If my dog is receiving heartworm preventives, they do not need an annual heartworm test.

FACT: Annual heartworm testing is recommended for all pets.

No medication is 100% effective 100% of the time. Pets on preventives occasionally test positive for adult heartworms. A positive result in a “protected” pet may be because of missed doses, stopping and resuming preventives without a heartworm test, and, rarely, a resistance of the heartworms to the preventive. The reason for such heartworm resistance is unclear, but these “breakthrough” infections are rare. Consistent use of prescription heartworm preventives is still your pet’s best defense.

MYTH: It is not necessary to give your pet heartworm preventives in the winter.

FACT: Year-round prevention is key to a successful heartworm prevention plan.

Oral heartworm preventives work retroactively by killing any circulating juvenile heartworms—known as microfilariae—that may have developed in the previous month. Discontinuing monthly preventives leaves your pet vulnerable to an infection that will grow during the subsequent months. Another practical reason for continued dosing is simply that once you stop something, it can be difficult to begin again. If you forget to restart their preventives in early spring, your pet may be exposed and unprotected.

MYTH: Cats do not get heartworm disease.

FACT: Cats are susceptible to heartworm disease, but it is different.

Cats are fortunate—they are not the preferred blood meal for some mosquitoes, and they are not an ideal host for heartworms. However, they can be infected. While heartworm infections in cats typically contain only a few adult worms and rarely contain microfilariae, adult worms migrating to the lungs can cause initial signs that resemble asthma or bronchitis—leading to misdiagnosis. 

Indoor and outdoor cats should be given year-round heartworm preventives and tested annually. Testing in cats may result in a false-negative result because heartworm tests look for female heartworms—and a cat may have an all-male infection. For the best feline protection against heartworms, preventives are key.

Understanding how to provide the best care for your pet can be a challenge. A lot of information is available—especially on heartworm disease and prevention—but much of it is contradictory. At Wales Animal Clinic, we pride ourselves on being your trusted resource for veterinary care and information. Call us to schedule your next appointment—and bring all of your questions, concerns, and, of course, your pet!_