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Do dogs need heartworm prevention all year?

Does my dog need heartworm prevention year round?

*Note: I choose to give my dog the minimum amount of heartworm prevention medication I believe is necessary to keep him reasonably safe from heartworm disease. My personal decision should not necessarily be your decision.

To determine whether or not to give your dog a monthly heartworm prevention tablet, I recommend you weigh the risks of the toxic prevention tablet vs. the risk of your dog developing heartworm disease.

For 2013, I have decided to give my dog fewer doses of heartworm prevention medication than I have in the past. 

In previous years, I gave Ace monthly heartworm prevention in the six warmest months of the year, typically May through October. I’ve never believed year-round treatments are necessary for Midwestern dogs, because heartworm disease is transferred by mosquitoes and those Suckers are dead around here by November.

I live in one of the coldest places on earth. In Fargo, N.D., it’s completely normal for the temperature to remain below zero for days. We are unfazed when the temperature dips below -30. 

While I tend to be a worrier when it comes to my dog, I’ve never worried about the possibility of Ace getting infected by heartworms in the winter …

Does my dog need heartworm prevention in winter?

While the vets in our area generally recommend monthly heartworm prevention year round, they don’t seem to mind if dog owners choose to treat for just six months as long as they do an annual heartworm test. (This is not a scientific survey, it’s just my observation.)

Before we get to some of the reasons why I believe this is still too aggressive, I’ll go over some brief details about heartworm disease.

What is heartworm disease?

Heartworm disease is transmitted by infected mosquitoes carrying immature heartworm larvae, according to “Dogs Naturally Magazine.”

When one of these mosquitoes bites a dog, she could inject the larvae into the dog’s body where they could eventually travel to the dog’s heart.

There, the larvae could develop into adult heartworms capable of creating new larvae which could circulate in the dog’s blood. If a mosquito bites the infected dog, she could transfer the larvae to other dogs.

Symptoms of heartworm disease include a loss of appetite and weight loss, coughing, gagging, shortness of breath and a lack of energy, according to “Dog’s Naturally.”

Fear is an effective marketing tool

While heartworm disease can be very serious and deadly, your dog may not face a high risk of contacting the disease in the first place. Let’s remember that fear is an effective and profitable marketing tool.

Is the cost of heartworm prevention a factor?

Heartworm prevention through a monthly medication such as Heartgard is affordable, starting at around $6 per tablet. This is not going to break the bank for most of us.

In addition, a heartworm test is around $40 in my area. Obviously if you have to treat your dog for heartworm disease, the cost will be much more than it would have been to prevent the disease.

So what’s the problem? Why don’t I just spend $100 or so on heartworm prevention and call it good?

Because heartworm prevention tablets are toxic chemicals

Heartworm prevention drugs are parasiticides – neurotoxins designed to kill any larvae that your dog may be harboring, wrote Ted Kerasote (author of “Merle’s Door”) in the book “Pukka’s Promise: The Quest for Longer-Lived Dogs.” 

Just as some veterinarians continue to give dogs unnecessary vaccines every year, some vets also insist on giving dogs a monthly dose of toxic heartworm prevention.

Vets profit from promoting excessive heartworm prevention

Vets are taking advice directly from the American Heartworm Society, a group that studies the disease and recommends year-round chemical preventative treatments for all dogs, regardless of where they live, according to Dr. Karen Becker in the article “Why Haven’t Pet Owners Been Told These Facts About Heartworm?” on the web site The AHS has eight sponsors, and all eight are pharmaceutical manufacturers.

Veterinarians are also permitted to sell the drugs they prescribe (unlike doctors with human patients), so their prescriptions may not always have sound medical justification, Kerasote wrote.

What is a dog’s actual risk of contacting heartworm disease?

Climate matters.

Do dogs need Heartgard year round?

If a mosquito actually bites an infected dog and ingests heartworm larvae, the ambient temperature must remain above 57 degrees Farenheit for about two weeks in order for the larvae to develop into mature infective larvae within the mosquito, wrote Kerasote.

Under those ideal circumstances, these could then be injected into another dog, starting a new cycle.

Because of the unique temperature requirements, heartworm disease is only a year-round risk in warm regions such as the southeastern portion of the United States and the Gulf Coast, Kerasote wrote. 

If the ambient temperature drops below 57 degrees even for a few hours, the heartworm larvae living in the mosquito cannot become infective and the life cycle of the heartworm is broken.

Calculate the temperature in your area to determine actual risk

A healthy way to address the risk of heartworm is to calculate the first date the local temperature consistently stays above 58 degrees and when it once again dips below 58 degrees, Kerasote wrote.

“For the mid-Atlantic states and New England, this would mean giving your dog its first monthly heartworm treatment around June and the last one in November – six months of taking a toxic chemical instead of 12.”

For Fargo, it has not yet remained above 58 degrees consistently in 2013. While it has reached 80 and 90 degrees during the day, the temperature has dipped below 58 degrees on all but a few nights so far this spring.

We haven’t even gone 48 hours with the temperature above 58 degrees, let alone two weeks. I plan to start my dog’s heartworm prevention if and when the temperature consistently remains about 57 degrees.

Treatment every three months should be effective no matter where you live

Even treating your dog monthly for six months is unnecessary, Kerasote wrote. If your dog gets stung by an infected mosquito on June 1, the larvae will not grow into adult heartworms for a couple of months.

You could treat your dog with Heartgard in September and again in December and call it done.

“Even on the Gulf Coast, where the ambient temperature may not ever dip below 57 degrees, dosing your dog with heartworm treatment once every three months should protect it,” he wrote. “For many parts of the United States and Canada, no treatment at all is required.”

He does recommend a yearly heartworm test, just to be sure, especially if you travel to warmer climates with your dog.

While every situation is unique, I hope you consider your dog’s actual risk of developing heartworm disease and make the most appropriate decision for your dog.

What about all those shelter dogs that are heartworm positive?

I know someone is going to attack me, saying how she works at a shelter and sees dozens of heartworm-positive dogs each year and how her shelter spends thousands of dollars to save these dogs.

Yes, this happens, and it is very unfortunate. But let’s remember the majority of these dogs entered the shelter system after being stray or lost.

They would’ve been living outdoors and potentially exposed to more infected mosquitoes than an average dog. They would’ve likely had weakened immune systems due to a poor diet and other health problems.

Others would’ve come from families that lacked education about any heartworm prevention at all or perhaps lacked the money for heartworm prevention or veterinary care in general.

Animal shelter workers see the worst of the worst, and they tend to make inaccurate generalizations about dogs as a whole based on the tiny group of dogs they actually work with. Most dogs will never end up in a shelter.

Heartgard is probably the most popular heartworm prevention medication in the United States.

I was curious what Heartgard had to say about monthly treatments. According to its web site, it recommends monthly treatments for all dogs since it can’t be “determined with certainty when the threat of mosquitoes has passed.”

Um … I’d say the threat of mosquitoes has passed once we reach January in North Dakota. I’d even say so with certainty …

Does my dog need heartworm prevention?

Additional ways to keep your dog healthy

While I do believe heartworm prevention tablets are the most effective way to prevent heartworm disease (if your dog is at risk in the first place), here are some additional ways to keep your dog healthy:

  • Feed your dog a healthy diet. Obviously, I recommend a raw dog diet for dogs, but you could also feed a high-quality kibble.
  • Take your dog in for an annual checkup and heartworm test.
  • Make sure your dog gets out for regular exercise. Fit dogs are generally healthier. Plus, dogs benefit from Vitamin D just like us.
  • Use natural mosquito repellent. “Dogs Naturally Magazine” recommends eucalyptus oil or citronella candles. While dogs supposedly don’t like citronella, we have used citronella candles on our patio while out with our dog and he doesn’t seem to mind the smell. There are also natural bug sprays you can use for yourself and your dog.
  • Limit other toxins your dog is exposed to such as spot-on flea treatments.
  • Only give your dog the minimum amount of vaccines necessary to keep him safe.

What are your thoughts on monthly heartworm prevention tablets?

While I know everyone won’t agree with my decisions, I’d still like to hear your thoughts.

As for Ace, I plan to wait until the local temperature consistently remains above 57 degrees (if that even happens) and then give him a heartworm tablet every three months until the temperature once again consistently drops below 57. I will probably still do an annual heartworm test, just to be sure.

I asked Ace if he had an opinion on all this, and he said, “I frickin’ love those beef flavored tablets!”

I guess I’ll have to feed him more steak.

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Cecilia C

Friday 3rd of November 2023

My 3 year old German shepherd gets her preventatives all year long. It is the beginning of November, 2023 and I saw a mosquito outside our breakfast nook window while eating an early dinner. This reassures my decision to keep it up all year round. My neighbor adopted an Irish wolf hound that had heartworms, and the treatment was arduous and time consuming.

Cecilia C

Friday 3rd of November 2023

@Cecilia C, oh btw we live in Chicago


Tuesday 2nd of March 2021

I love this article. I also "Cycle" Heartgard. (I refuse to call it medication or preventative, as it is actually a insecticide) I give it June 1 and September 15 as I read it takes 3 - 4 months for heartworm to develop.


Thursday 29th of August 2019

We live up in Michigan and we give it June through December. Ivermectin is an extremely safe drug. It is also used world wide in humans to kill worms as well. Within 36 hrs the body will have flushed at least half of it (50%) by excreting it in the feces - Ivermectin does NOT stay active in the system like flea preventative does for a month. Ivermectin is a Anthelmintic. Which means they kill helminths. Helminths are worm-like parasites such as flukes, roundworms, heartworm, and tapeworms. It is important that anthelmintics are selectively toxic to the parasite and not the host. Some work by inhibiting metabolic processes that are vital to the parasite but absent or not vital in the host. Other anthelmintics are poorly absorbed through the gut, which means the parasite is exposed to much higher concentrations of the anthelmintic than the host. Starvation or paralysis of the parasite result, followed by subsequent expulsion or digestion. Ivermectin is barely metabolized at all which means it gets excreted almost the same as it went in.

Now there are breeds such as collies that are sensitive to the amount of Ivermectin. If given ivermectin they must be on a more exact amount so usually to prevent a problem those dogs are just on another preventative like Intercepter. So Ivermectin isn’t the drug you should be worried about. It’s the flea and tick preventative that is often sold with it in the same pill that is so toxic. I always strongly suggest people do not give combo heartgard and flea preventative together to our puppies. We have had some puppies end up in the ER who's owners were given a sample of Trifexis. Secondly, ivermectin is a clean up preventative. Not a real preventative in the sense that it stops microfilaria from entering the body. So if an infected mosquito bites a dog and transfers the microfilaria then that microfilaria will stay in the system and developing until you give the next dose of Ivermectin. Yes, it takes 5-6 months for the worm to be mature and to produce more microfilaria but by waiting 3 months, it’s now a half mature worm. Killing the microfilaria early is key. I know dogs who’ve gone through treatment. The treatment is extremely toxic not to mention extremely costly as well. So toxic that the two dogs I’ve seen go through it were never the same after. They both went mental and had to be euthanized. Both dogs were sweet and loving pets that became neurotic dogs. One becoming chronically restless and at times aggressive to its owners and the other also becoming restless and irritable and turning on the other family dog who was her best buddy from puppyhood. Those two dogs were inseparable they loved each other so much. That dog was a well taken care of house dog who’s owner just didn’t get her dog in early enough to start preventative in the summer here in Michigan. I’ve seen mosquitos below 57 especially if it warms back up during the day. In fact we don’t consider Michigan to be mosquito free until it dips below freezing for several days during the day. Some mosquitos can hide on things like firewood and a person can bring them in during the fall and early winter. Your house will warm them up and make them active. So personally telling people they shouldn’t give heartworm preventative every month, especially in areas that are heavily laden with it like the gulf coast etc is horrible advice. The treatment side effects and the cost far outweigh the price of preventative and Ivermectin is extremely safe. Plus if their dog is infected to the point that it is detectable on a test then they can’t get more heartgard from the vet. My advice always is: definitely give heartworm preventative. It’s extremely safe and has been used for decades now in domesticated animals and widely used in humans too. Only use flea and tick treatment if you are in an area that is highly infested with fleas and ticks. Not every area needs flea and tick preventative. Flea and tick preventative IS toxic. It’s far more dangerous than ivermectin.


Saturday 27th of May 2017

I give our yorkies a heartguard every month.

Brooke Limperis

Tuesday 14th of February 2017

Hello! I am very excited to have found your informative article. I also live in San Diego and am looking to lessen the drugs given to my dog. Seems as though every two or three months will be our regimens for heartworm medication.

I was wondering your opinion on Comfortis however. We are very active and definitely need flea protection. What I'm trying to find out is if we can get away with a dose every other month since it kills eggs and adults within hours.

I am curious to know your thoughts on this. Thank you very much!

Brooke L