Should I Give My Dog Heartworm Medication in the Winter?


Most dog owners know about Heartworm, and how it’s spread by Mosquitos. Most probably think mosquitos outdoors will die after the first winter frost. Why provide heartworm preventatives during the Winter?

What if there is more to this heartworm tale, and your dog is in danger all year round?

Because temperatures can vary during any time of the year, and especially between indoor and outdoor environments, not all mosquitoes will die after the first frost of winter. Your dog can still contract heartworm!

Remember to always treat your pet with a monthly heartworm preventative, even during the winter, and get a heartworm test at least once annually!

What is Heartworm

Heartworm is classified as a very serious, often fatal disease occurring in many animals (not just dogs). A blood-borne parasite called Dirofilaria immitis, shaped just like a long, thin worm causes this. Mosquitoes pick up heartworm larvae from infected animals, transmitting that larva to other animals.

Though they can manifest differently in various animals, heartworms will collect in the heart, pulmonary arteries, and surrounding blood vessels of dogs. Female heartworms can reach a whopping 6-8 inches long and ⅛ thick, while males reach about half the length.

Dogs can often have around 300 worms when finally diagnosed! Blood vessels inside the body (even the heart) are very small, yet absolutely vital to an animal’s survival. Imagine 300 of these ⅛ inch thick, 8 inch long worms clogging your pet’s circulatory system.

This is exactly why so many dogs (and other animals) die from heartworm, and why constant treatment is vital for prevention. Heartworms also cause dangerous inflammation to the heart, lungs, and arteries.

*Heartworm removed from surviving dog’s pulmonary artery. Courtesy of Cornell University Veterinary Specialists

Heartworm Symptoms

There are actually four classes, or stages, of heartworm infection in dogs. As the parasites grow and multiply, symptoms will become worse and your pet will progress in these stages.

Stage 1: No symptoms at all to a mild cough

Stage 2: Persistent cough with mild lethargy (lack of desire for physical activity)

Stage 3: Unusual lung sounds with increased lethargy, lack of blood flow to the brain, fainting, poor balance, lack of appetite, weight loss, swollen belly caused by heart failure

Stage 4: Life-threatening cardiovascular collapse called Caval syndrome, eventual death due to complete organ failure

How is Heartworm Spread

Heartworm requires mosquitoes as intermediary hosts and isn’t spread from dog to dog directly. Infection rates are worse during mosquito season, but a Heartworm infection can still be a problem if conditions are right during colder months of Winter.

The heartworm larvae will enter a new host when its mosquito carrier stops for a blood meal. The larvae aren’t in the blood itself, but residue covering the mosquito’s mouthpieces. About two months after traversing a newly infected host’s bloodstream, the worms will begin to settle on the right side of the host’s heart and grow.

Heartworm can reach maturity in about six months and live for a total of seven years inside the infected animal Spring, Summer, Winter and Fall. These worms are producing offspring that entire time. Though dogs can harbor hundreds after just one year, the normal limit for this time is about 15.

It will normally take several years before a dog begins to show symptoms of infection, allowing plenty of time for the parasite to grow and multiply inside your pet. Because the disease is so advanced by the time a dog actually begins to show symptoms, it can be much harder to treat and often fatal.

Prevention vs. Treatment

Heartworm parasites in dogs are pretty simple to prevent! All you have to do is grab a prescription and monthly heartworm medication from your veterinarian. With great tasting chewable, administering them to your dog is even a sinch.

On the other hand, the nasty parasites can be much, much more difficult to treat! Unless you can afford the standard $12,000-$16,000 costs for extremely invasive open-heart surgery, heartworm in dogs is commonly treated with an injectable drug series.

This series of injections is also costly and is no guaranteed cure. All too often, dogs will still pass even after the injections. Complications can arise as the worms break up, becoming lodged in the small blood vessels of the lungs, and the treatment itself can be toxic.

There is a possibility continuous treatment will still be required for lifelong heart failure in dogs that do recover.

Do I Need to Get a Blood Test First?

The American Heartworm Society recommends all dogs are tested annually in general, but veterinarians will usually want to see these negative test results before prescribing any heartworm preventative. You need prescriptions for the medication in the United States.

Offering a heartworm preventative to a dog already infected with the parasites can be very dangerous, even deadly. If microfilaria (early-stage worms) are in the dog’s bloodstream, the preventative can cause them to die suddenly and send your dog into shock.

Heartworm preventative won’t kill the adult parasites. Annual testing is also important to ensure any kind of preventative plan is working like it is supposed to.

Why it is important year-round

A heavy frost will kill all exposed mosquitoes outdoors. They become sluggish at temperatures below 50 degrees Fahrenheit. Of course, this only covers the mosquitoes that are exposed outside.

Certain females can even enter a kind of hibernation. Any of the up to 500 eggs a single female can lay in a single brood won’t die off.

There still is a very real risk your dog can contract heartworm during the colder months of the year! Your dog can still become infected, so you should never stop administering any preventative.

“Because weather is unpredictable and hardy mosquitoes can survive indoors as well as outdoors in protected areas, so-called “seasonal” usage creates ample opportunity for animals to unintentionally become infected (American Heartworm Society).”

Also, consider using FDA approved repellents for mosquitoes, and do your best to limit the breeding grounds available to mosquitoes, and decreasing your dog’s activities outdoors at both dusk and dawn.

In Conclusion: All Year Round!

Yes, it is important for pets to be tested and treated for heartworm.

Does my Pet need identification?

Sources Cited

Barnette, Catherine. DVM. Heartworm Disease in Dogs. VCA Hospitals. Retrieved from

“Lucky’s Heartworm Removal and Orthopedic Surgeries” Cornell University Veterinary Specialists,

Accessed 28 October 2020.

Heartworm Testing: What Every Dog Owner Needs to Know. American Heartworm Society.

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