The Case for Year Round Heartworm Prevention

Heartworm infection is common in dogs throughout the United States (cats get them too).  They have the ability to do a lot of damage and can be fatal.  Luckily, it is almost 100% preventable.

Mosquitos transmit heartworm infections from animal to animal.  Once the infant stages mature they live in the heart and large blood vessels of the lungs. An adult heartworm can measure to over one foot in length and can live as long as 5 to 7 years in the dog.

Northern Climates are Not Immune

Heartworm disease has been diagnosed in dogs in all 50 states and many areas of Canada. The Companion Animal Parasite Council (CAPC) tracks parasites and forecasts trends for the upcoming year.  An increased risk for heartworm infection was predicted by CAPC for the past four years. Nationally, prevalence rates have risen each of the last five years and are now up 20% from 2013 levels 1

The largest number of cases are found in the southeastern United States and the Mississippi River Valley but don’t let that lull you into a sense of security.

Approximately 1.3% of dogs who are tested for Heartworms test positive (many more are not tested).2 This percentage seems low but if one of the nearly 130,000 dogs were yours, that one would be too many. While in practice I’ve helped treat many dogs that were heartworm positive but never one of my own.

Heartworm in My Home

Year round heartworm protection
Foster Molly after heartworm treatments.

We are fostering a former puppy mill dog who was surrendered to Angels Left Behind Rescue.  She is a Pug named Molly (we call her Biggie Molls) and came to the rescue after breeding non-stop for almost six years.  She has almost no viable teeth, and the longest nails I’ve ever seen (for a Pug that’s saying something).  Last but not least she was Heartworm positive. Molly has lived with us through her treatment.  As a veterinary nurse I knew it is not easy but didn’t realize how emotional it would be for me, maybe because I know too much.

During treatment, we are poisoning the parasites and killing them off.  Protocols vary but Molly went through three injections. The time frame, start to finish for treatment can last three to four months.  It is crucial to keep a dog quiet post treatment. Cage rest and only leashed walks to eliminate and return inside.  Luckily Molly is a quiet, always ready to take a nap kind of dog.  High energy dogs can find this restricted activity challenging.

As a pet parent (even an experienced one) it is stressful not to mention expensive.  I’m thankful she handled treatment well but she didn’t need to go through it.  A simple monthly preventative would have kept her from this and eliminated her as a reservoir for mosquitos to transmit to other animals.

Year-Round Prevention is Best

Although it’s basically winter, heartworm should be on pet parents’ minds.  Most veterinarians, supported by the American Heartworm Society, CAPC, recommend year-round prevention.

Heartworm can be a threat to your pet anytime of year, due to the disease’s six-month incubation period.  Mosquitoes are associated with the summer months; however, weather can be unpredictable, warm weeks aren’t uncommon here in the Midwest in winter.  Don’t forget those who travel with their pets to warmer weather from colder northern climates.  Pets coming back and forth serve as reservoirs for mosquitos to pass on the disease.

Monthly preventatives are easily dosed at home through tablets or chewables or topicals.  They work by eliminating the infant stages of the heartworm passed from the mosquito and have slightly different claims against other external and internal parasites.

Even in Winter

You may have shoveled a foot of snow off your driveway this morning or chipped ice from your windshield and are thinking;

“It’s OK I forgot to give Paul Anka (the dog, any Gilmore Girls fans out there?) his heartworm prevention this month, I’ll do it tomorrow.”  Then tomorrow it’s forgotten.

Well, stop what you are doing and give it to him.  Heartworm prevention not only protects your pet from this potentially deadly disease it also prevents some types of intestinal parasites (worms).

It is rare for pets to become infected with heartworms during the dead of winter in a northern state or Canada mosquitoes can survive in standing water in heated garages or sheds. “Deworming” monthly is a key feature of heartworm preventatives that can be overlooked.  Intestinal worms are a bit hardier than mosquitos and can be acquired through exposure to animal feces even after a freeze.

It is unfortunate that many pet parents do not understand the severity of heartworm, and what a diagnosis means to them and their pets. As I’ve noted, it is treatable but the treatment is not only risky it is very expensive.  You could buy eight years of heartworm preventative for the cost of one post-infection treatment.

Cats & Heartworm

year round heartworm prevention
Photo: @jessicadvega

Cats that live in endemic areas should be given year-round heartworm prevention. Always ask your veterinarian for their recommendation. The fact that many cats are “indoor” only offers a false sense of security. Spending time outdoors does place cats at higher risk for acquiring a heartworm infection.

One study at North Carolina State University College of Veterinary Medicine indicated that more than one-fourth of the cats diagnosed with heartworm disease were strictly “indoor-only” cats according to their owners. 3

The consequences of feline heartworm disease are severe, and approved medical treatment for cats exists.  It’s not worth the risk.

Like the old saying, an ounce of prevention is worth a pound of cure (and a few thousand dollars when it comes to heartworm treatment).  Year-round prevention is something we can do to help keep our pets healthy.  The unbreakable human-animal bond is my case for year round prevention.

Happy Pet Parenting,







Cover Photo by Nathan Hanna on Unsplash



3 Atkins CE, DeFrancesco TC, Coats JR, et al. Heartworm infection in cats: 50 cases (1985–1997). J Am Vet Med Assoc 2000 Aug 1;217(3):355–8.


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