Should You Give Your Pet Heartworm Prevention in the Winter? · The Wildest

Skip to main content

Should You Give Your Pet Heartworm Prevention in the Winter?

Here’s why it’s best to stay on top of things, even when the temps are low.

Cozy at home with tabby cat, woman with her pet on sofa at home in evening.
Olezzo / iStock

Winter has pet parents thinking about cold-weather essentials like warm beds, cute sweaters, and extra cuddle sessions, but there’s another lingering question: Does my pet still need heartworm prevention in the winter?

A common assumption is that mosquitoes go on hiatus this time of year so pets shouldn’t be at risk, right? It’s not that simple.

What is heartworm disease?

Heartworm disease, or dirofilariasis, is a parasitic disease caused by the organism Dirofilaria immitis. Heartworm larvae are transmitted through mosquito bites. When a mosquito carrying heartworm larvae bites a dog, the larvae enter the dog’s bloodstream.

Over time, these larvae can grow into adult worms that can cause inflammation and obstruction in a dog’s heart and lungs — unless the dog is on a reliable heartworm preventative. Obviously, mosquitoes that carry heartworms are more common in the summer months, but that doesn’t mean dogs are safe from the disease in the winter.

Can dogs be diagnosed with heartworm disease during the winter?

It takes about six months for heartworm larvae to mature to adulthood, so dogs that were infected earlier in the year may not show signs until winter. Some dogs infected with only a few heartworms have no symptoms at all. That is why vets recommend yearly testing: to better detect infections early. Coughing is the most common sign of heartworm disease and the first sign that many dog parents notice.

As heartworm disease progresses, additional symptoms can develop, including:

  • Productive cough

  • Exercise intolerance

  • Rapid breathing

  • Labored breathing

  • Weight loss

  • Ascites (fluid in the abdomen)

  • Discolored urine

  • Weakness

  • Syncope (fainting)

Treatment for heartworm disease

Treating heartworm disease in dogs involves a comprehensive approach. After confirmation of a positive blood test, veterinarians will recommend additional diagnostics to determine the severity of the heartworm disease. This may include full blood work, urinalysis, and chest X-rays to evaluate the impact on the heart, lungs, and blood vessels. 

The key points of heartworm treatment are:

  • Addressing immediately life-threatening symptoms: This may include administering oxygen or other treatments associated with congestive heart failure. 

  • Killing adult worms: Melarsomine is the only drug available that kills adult heartworms. Veterinarians may recommend a two-dose or three-dose protocol. Melarsomine is given via injection and can cause dogs discomfort. 

  • Killing larvae: Continued administration of heartworm prevention is vital to kill larvae and prevent them from maturing. 

  • Supportive treatments: Standard treatment protocols often include antibiotics and steroids. Pain control may also be warranted for melarsomine injections. 

  • Exercise restriction: A dog’s cardiovascular system is especially vulnerable while undergoing heartworm treatment. Strict activity restriction (often for six to eight weeks) is essential to reduce the likelihood of complications. 

Heartworm treatment can be an arduous and expensive process, so it’s best to focus on year-round prevention.

Does my cat need heartworm prevention?

Yes, cats need to be on heartworm preventatives. While infection rates are lower than in dogs, cats are still susceptible to heartworm disease and should be on preventative year round. Because cats are not a natural host for heartworms, adults do not survive long in a cat’s body. Despite this fact, immature heartworms can still do significant damage to a cat’s lungs and vessels. 

Immature heartworms combined with the death of adult heartworms causes significant inflammation in a cat’s lungs, which can lead to a condition called heartworm-associated respiratory disease (HARD). Cats with HARD can develop coughing, wheezing, labored breathing, and respiratory distress. There is no approved treatment for heartworm disease in cats, so prevention is crucial. 

Should you be giving your dog heartworm medicine all year?

You should give your pet heartworm prevention all year. Even in the winter. Here’s why:

Heartworm prevention kills heartworms

So, you may read that and say “duh,” but let me explain. Heartworm preventatives don’t prevent mosquito bites — they kill the larvae. Heartworm larvae that are transmitted from a mosquito bite can remain in the larval stage for about two months. So, that nice day in early winter was an opportunity for infection.  And those larvae can continue to grow and develop well into winter. Continued doses of heartworm preventatives will kill the larvae and stop this development.

Many preventatives do many jobs

Many preventatives serve multiple purposes. They not only shield your pet from heartworms but also provide protection against other parasites like intestinal worms, fleas, ticks, and ear mites. Depending on which product you use, dropping the heartworm protection might also mean dropping defense against other parasites.

“Mosquito season” is not set in stone.

As the climate changes and weather becomes less predictable, simply entering the winter months does not mean that the climate is not amenable to mosquitoes. Mosquito eggs are hardy and can survive low winter temperatures, waiting for the weather to warm up just enough so they can hatch.

Consistency is key to keeping routines.

Routines are easier to keep if you’re consistent, and giving preventatives should be part of a pet care routine. It’s easier to just give your pet the medication every month rather than suddenly realize in the middle of spring that you forgot to restart it after winter. Pet parents who are already giving heartworm prevention year-round won’t need to factor it into their trip planning if they choose to escape the freezing weather with their pups. 

Some products come with a guarantee.

Even when pet parents are diligent, things can go wrong. Although rare, some dogs on heartworm prevention still contract the disease. Many manufactures will provide financial assistance with treatment — but only if you’ve used their product as directed, which means providing it to your pet year round.

What if it gets hella cold in the winter where you live? 

Heartworm disease tends to be less prevalent in areas that experience long, hard freezes. Research conducted in Ontario, Canada, spanning the 1950s to the 1980s, revealed no heartworm infections diagnosed after October. This led to the recommendation of seasonal heartworm prevention for the region (June through November). In extremely cold climates, some veterinarians still adhere to a seasonal protocol, while others, along with the American Heartworm Society, recommend year-round coverage.

If you live in an area with harsh winter freezes, your vet can guide you through the best protocol. Factors that may impact the recommendation include:

  • The preventative products you use

  • The prevalence of heartworms (and other parasites) in your area

  • Your pet’s history

  • Your lifestyle

  • Your risk tolerance

Annual heartworm testing is recommended no matter what. And nobody wants to feel the dread of a heartworm test coming back positive. 

More things to consider:

Can any other pets get heartworms?

(Not so) fun fact: Ferrets can get heartworm disease, too. Ferrets (even indoor ones) should be on year round heartworm prevention. Like cats, a single heartworm can do a lot of damage, and there’s no approved treatment for heartworms in ferrets. 

What if I miss a dose of heartworm prevention (HWP)?

If your pet missed a dose of HWP, don’t panic — it happens. If it’s been less than two weeks, give the dose and get back on schedule. Longer than that, contact your vet. Next steps will depend on your pet’s history, your location, and other factors. 


alycia washington, dvm

Dr. Alycia Washington, DVM, MS

Alycia Washington, DVM, is a small animal emergency veterinarian based in North Carolina. She works as a relief veterinarian and provides services to numerous emergency and specialty hospitals. Dr. Washington is also a children’s book author and freelance writer with a focus on veterinary medicine. She has a special fondness for turtles, honey bees, and penguins — none of which she treats. In her free time, Dr. Washington enjoys travel, good food, and good enough coffee. 

Related articles