Heartworm - Why I Don't Give My Dog Heartworm Meds

Mar 04, 2022 Dan

Heartworm prevalence in Ontario.

The poop on heartworm and the options available to pet owners.

(There is another blog about Ticks, Fleas and Heartworm.  You can find it HERE.  The reason we are focusing on heartworm in more detail is that many of our customers have chosen a natural route to prevent fleas and ticks, but, for some reason, have felt that a heartworm med was necessary.  So, we wanted to share with you some information about heartworm in Ontario and the options we have to deal with this parasite.)

First things first.  The purpose of this blog is not to shove my personal views into your pet space.  I am extremely sensitive to the fact that decisions about how we choose to deal with the critters is a very personal one.  I just want pet owners to make good and informed decisions.  

The influencers in our lives regarding our pets’ health is numerous: veterinarians, breeders, dog walkers/sitters, next door neighbour, sister-in-law, pet store employees, pharma, big pet food manufacturers and the list goes on.  So much comes at us.  Effective marketing and advertising can also get in the way.  Ever seen the picture of a dog’s heart infected with adult heartworm?  That there is enough to say “Yup, I don’t want my dog to die of this.  Let’s get him/her on the heartworm meds.”  

So let us have a look at what heartworm is, how our dogs (cats are not good hosts for heartworm) contract heartworm, the prevalence of heartworm in Ontario and what our prevention options are.   

What is heartworm?

Heartworm infection is caused by a vector-borne parasite known as Dirofilaria immitis and primarily infects dogs, both wild and domestic. Heartworm infection can become severe if left untreated with life-threatening health conditions and even death.  There is quite a size difference between adult male and female worms, with male worms 20-30 cm long and 0.7- 0.9 mm wide with a tapered, spiral-coiled posterior end, and female worms being 25-31 cm in length and 1.0-1.3 mm in width with an obtuse posterior end. Mature adult worms produce unsheathed motile vermiform embryos known as microfilariae (baby heartworm). These baby heartworms can travel through a dog’s blood system and circulate in the vascular system in large concentrations.  The baby heartworm is not the issue.  The real risk is when the babies are not treated and grow into adults.  This entire life cycle takes place between 184 to 210 days under optimal conditions; cooler temperatures and lower relative humidity result in a longer life cycle (remember this factoid, it is an important piece of info). 


How our dogs get heartworm.

The heartworm lifecycle has 5 stages of which stages 1 through 3 occur in the mosquito host.  During their time in the mosquito, the larvae molt twice (L1 to L2 to L3) and can only survive the molting process when temperatures are 14 degrees Celsius and above (another important piece of info).  This can take anywhere from 8 to 30 days depending on temperature and humidity.  If at anytime the temperature drops below this number, the baby heartworm will not develop.  Only when they get to stage 3 can the baby heartworm be transmitted to a dog host via a mosquito “bite”.  At the end of the day, it will take anywhere from 6 to 7 months for the babies to grow into adults (in optimal temperature and humidity conditions) where settlement into the right ventricle and lungs can occur.

Prevalence of heartworm in Ontario.

In a thesis presented to the University of Guelph by Erin McGill, it was estimated that the rate of positive heartworm infection in Ontario from 2007 to 2016 was 0.12%.  To put this into perspective, that means that if 10,000 dogs were tested during this time, 12 tests for heartworm were positive.  This is a small rate of infection and can be explained partially by the number of dogs tested who were on heartworm meds, but a large part can be explained by average temperatures in Ontario while mosquitos are active and feeding (remember the factoid referred to earlier?).


If you would like to read Mr. McGill’s thesis, you can get it HERE.


Prevention Options

The obvious choice is a heartworm medication that kills the baby heartworm if transmission is successful.  You might be familiar with brands like Revolution, Sentinel, Heartgard, Advantage to name a few.  Some are built into a treat you give your dog monthly while others are administered directly onto the skin (oh yeah, when you apply the spot-on treatment, the instructions tell you to apply wearing gloves because it could burn your skin????).  They have all proven to be effective at killing the baby larva and preventing heartworm from forming.  However, the downside to these meds is that most are really a pesticide you are putting into your dog.  The number of dogs that have had severe negative reactions to these meds is a little scary.  You can look into the number of submissions to the EPA (Environment Protection Agency).  Many of these reactions are neurological.  

My choice of protection is to not give a pesticide to my dog.  I have chosen to test for heartworm every spring (within that time frame of 6 to 7 months after the extreme heat of the previous summer. A time in Ontario that might be warm enough for the baby heartworm to survive from mosquito to pet).  If the test comes back positive, then we can successfully treat since the parasite is still at baby stage and has not had time to achieve full adult and settle in the heart and lungs.  The downside to this is that the treatment is more expensive than the heartworm med prevention.  To me, the extra cost of the treatment is miniscule compared to the risks to my dog of giving heartworm meds over 5 or 6 months.

I also like to use a safe spray of safe essential oils when we are out hiking.  Another important thing to consider is that tight coated dogs have a higher rate of getting heartworm versus heavily coated dogs.  

Let’s wrap this puppy up!!!

What you choose to do is a very personal choice.  Your lifestyle and the environment where your dog spend most of its time can also play a role in your decision.  Before you decide, remember the following:


  1. Heartworm exists and can be deadly when not properly screened by allowing the larva to grow into adults.


  1. Heartworm risks in Ontario are extremely low due to temperature.


  1. Heartworm identified early via a simple blood test every spring can be dealt with and cleared up.


  1. Most heartworm meds are effective at killing off the larva.  Most are also pesticides that can be harmful to your pet.


  1. You have options.