The chances are that your vet recommends a heartworm test and preventive medication every time you take your dog in for his annual exam. Canines are a prime target as a carrier, including dogs, foxes, wolves, and coyotes. However, cats and ferrets are also susceptible to the disease. While it may seem like a pain to get your pup tested first, it’s imperative to protect your pet.

Over 70 species of mosquitoes can spread this parasite. It occurs in all 50 states to varying degrees. If you live in a northern area, you may wonder, do I need to give my dog heartworm medicine in the winter? The short answer is yes, no matter how counterintuitive it may sound. Our primer will tell you everything you need to know about this devastating condition.

dog walking through the snow in winter

Heartworm 101

Heartworm is appropriately named for the organ that it attacks. That fact is enough to convey its seriousness. However, it can also affect the circulatory system and lungs. Mosquitoes infect other pets by biting an animal that has the parasite. It matures in the insects before being transmitted to a new host. The condition develops slowly, which comes into play with the testing recommendations.

The results aren’t pleasant, with foot-long mature worms affecting the aforementioned systems. An infected pet can have hundreds of parasites in its body. Left untreated, it can cause permanent damage, making it potentially fatal.

Heartworm Season

Heartworm is most prevalent during the warmer times of the year when mosquitoes are active. While it’s found throughout the country, the states where it occurs most include:

  • Mississippi
  • Louisiana
  • South Carolina
  • Arkansas
  • Alabama

Looking at those stats, we can understand the confusion about whether to give your pup heartworm preventive year-round.

Symptoms of Heartworm

It can take several months before you see any noticeable symptoms. That’s one reason why testing is necessary before starting the preventive. Unfortunately, giving it to your pet if it has an undiagnosed condition is equally as dangerous for your pup. The FDA is the regulatory agency that makes it a requirement before your vet can dispense the meds. Symptoms include:

  • Exercise intolerance
  • Coughing
  • Lethargy
  • Labored bleeding
  • Nose bleeds
dog and owner going for a walk in the winter


Treating heartworm typically involves the use of only one approved medication, injectable melarsomine dihydrochloride. The cure is often difficult, with side effects common. Even the treatment has complications. Part of the problem exists with the dead worms in the wake of the drug used to kill them. Needless to say, prevention is preferable, making a solid case for year-round dosing.

Heartworm Preventive Medication

Heartworm preventive is an oral chewable given once a month. Popular brands include Heartgard, Simparica Trio, and Interceptor Plus. You can buy them from your vet or online with a prescription given after a negative test. The essential things to remember are that your dog should chew instead of swallowing them. He must also eat the entire chewy since the dosage goes by weight.

The medication isn’t cheap. However, you must balance that cost with what you’ll pay to treat an infected pet. Adding to the confusion is that it’s often sold in a 6-month supply package. That fuels the idea that you only need to give it to your dog for half a year and not year-round. However, other benefits support this use.

Benefits for Year-Round Protection

Heartworm preventives are highly effective against this parasite. It’s vital to understand that nothing is 100 percent effective. Nevertheless, these meds offer additional protection against intestinal parasites, such as tapeworms, roundworms, and hookworms. Some are zoonotic conditions or disorders transmittable to humans. It’s worth noting that people can get heartworm from their pets, too.

dog owner holding out his hand and petting his dog's face in the snow

Lifestyle Changes

One way you can help protect you and your pet from heartworm is to repel mosquitoes. That includes simple things, such as reducing standing water. That’s a breeding ground for these pests. You can also add mosquito dunks to ponds, rain barrels, or other water sources to help control them. Adding mosquito-repellent plants like catmint to your landscaping is another option.

However, it’s imperative to understand that these measures won’t prevent heartworm 100 percent. They may make being outside more comfortable for you and your dog, but it’s not going to deter every mosquito. Limiting your pet’s walks to daytime hours isn’t a solution, either, since some species, such as Tiger Mosquitoes, are active during this time. Also, no so-called natural preventives exist.

The Case for Year-Round Heartworm Prevention

The last paragraph makes a strong case for keeping your dog on heartworm meds during the winter, especially if you have kids. If COVID-19 taught us anything, it’s that prevention is a moving target. The American Heartworm Society recommends giving your dog a preventive, even during the winter. The argument is sound, even if the risks seem low.

Mosquitoes have a short generation time of only 14 days. That means they can evolve quickly to new environmental pressures. The possibility exists that they can adapt to winter conditions with each subsequent generation. We can’t dismiss climate change and its impact on insects and their life cycle.

The winter kill that we may think wipes out the insects during this time of year may become less relevant in the years to come. You also have to keep the possibility of the mosquitoes being able to overwinter in your house on the table. Traveling with your pet to a warmer clime is another possibility that you shouldn’t discount. We can’t forget that mosquitoes can become stowaways from overseas goods.

Perhaps the strongest reason the American Heartworm Society and the American Veterinary Medical Association (AVMA) is that it may affect how well the medicine performs if you stop and start it. After all, it’ll take some time for the drug to kick in while your dog is vulnerable in the interim. Remember that it only takes one bite from one mosquito to infect your pup with heartworm.

Final Thoughts

Giving your dog heartworm prevention is the single best way to keep your pet safe from this devastating parasitic condition. While it isn’t cheap, it’s a lot cheaper than the risk of infection. If you live in a warmer climate, it’s a no-brainer to give your pup the med year-round. However, climate change has turned the tables. It doesn’t matter where you live. Using 12-month prevention is the right thing to do.