By June 18, 2015 December 3rd, 2015 Uncategorized

Spring time is notoriously known as “heartworm season” for many pet owners as well as “flea and tick season”. There is truth to this! Springtime is when all of the creepy crawlies begin to come out of their hibernation and start infecting our pets (and sometimes us!) to start their lifecycles. The main illnesses we worry about in the springtime (and really all summer long) are :

  • Heartworm
  • Fleas
  • Ticks (which can spread Lyme disease and other diseases as well)
  • Leptospirosis (a bacteria shed in the urine of wildlife, especially raccoons)
  • Intestinal Parasites (roundworm, hookworm, whipworm, etc)

Probably the most well known and the topic of hottest debate out of these is HEARTWORM. I wanted to write something to put the myths to rest and let pet owners know the TRUTH about heartworm disease because there are so many conflicting views out there on the subject. Putting all personal opinion aside, I wanted to share the facts and educate people to allow pet owners to make up their own minds on whether or not they wish to test and prevent heartworm in their pets.


Heartworm is a parasite that is spread by the bite of a mosquito. Basically, the mosquito will bite an infected animal (fox, wolf, coyote, other dog, cat etc) and pick up the larval stage of the parasite. The mosquito then will bite our pets and deposit the parasite into our pets’ blood stream. The parasite will then make its way to the heart and mature into a worm within the vessels and chambers of the right side of the heart. This ends in heart failure or sudden death of the worms break free and embolize to the lungs.  There is treatment for heartworm if they do contract it, but there is approximately a 50% survival rate and the treatments are very expensive, very painful for your pet, and are done over a very long period of time.


Many people don’t choose to prevent heartworm disease in their pets because they read that the prevalence of heartworm in Ontario is very, very low. It is true that if you look at the statistics of the heartworm testing for Ontario for pets, the prevalence is exceedingly low. AND IT SHOULD BE! Don’t forget that this statistic is taken from peoples’ pets that they bring into the vet to have them tested yearly, and more often than not, they also have their pets on heartworm prevention too! OF COURSE IT’S GOING TO BE LOW!!! What we are not taking into account with these statistics are the people who don’t bring their pets in to be tested AND wildlife that are likely harboring the disease. If we can’t get a blood sample on these animals we can’t test them, and I can assure you that if we could test them, the statistical prevalence that is tabulated and presented every year would be much higher than it currently is!

So, overall, when someone says that the risk of heartworm is so low in Ontario based on stats, they are just plain WRONG. The truth is that the prevalence is likely much higher than we are aware of, and so is the risk, as we cannot test wildlife/animals that don’t come in to see us each year that may well be harboring the disease allowing it to be spread to your pet!


Heartworms need specific conditions in the environment to develop into its’ infective state within the mosquito before it can be transmitted to our pets. The current data suggests that the temperature must be at least 14 degrees Celsius for at least 4 consecutive days for heartworm to become a threat. This is a real problem for people and animals that live in warm climates, like Florida for example, where the temperature is often above this all year round. For us in Canada, the situation is different as we have our freezing cold winters to stop the parasite from being an issue during the winter months. For this reason, we only need to protect our pets during the summer months, providing that they don’t travel to anywhere warm during the winter months.  We protect our pets from May or June 1st – November 1st each year to keep them heartworm free.


YES! The answer to this question is always YES, regardless of if your pet is on heartworm medication the year before. There are a few good logical reasons for doing this! First of all, it’s good to know that they are negative before starting their prevention. If they are negative on their test and you have them on prevention for the season, and the next year they come back positive, you have proof that they were negative before starting the preventative. If you have this proof, the company that makes the preventative you are giving your pet will help you out in most cases to investigate why their medication didn’t work.  No medication ever created works 100% of the time, so it is possible (although improbable) that you pet may become infected with heartworm while on medication. Also, if the preventatives are not given correctly and at exactly the right time each month this may leave them open to infection as well. Think of the test in this case as a mini insurance policy for you! 🙂

Also, our 4DX test that we are using now does much more than just Heartworm testing! It also tests for Lyme disease, Anaplasma, and Erlichia which are all tick spread diseases that you pet is at risk for, so it’s good to test them annually for these diseases as well.

Lastly, some of the preventatives we use for our pets will kill adult heartworms as well as the larval stages of this parasite. This means that if your pet DOES have heartworms, it will kill them too quickly causing them to detach from the inside of the heart and vessels all at once. When this happens the worms travel to the lungs and can cause an embolism blocking the blood from passing. This leads to respiratory distress and death! So, knowing that they are negative before starting prevention can actually save their life.


The easy answer to this is YES, your pet needs protection! If you could prevent your pet from getting a potentially fatal disease, you would! Heartworm is no different! It’s all about preventative medicine rather than treating the problem after it has already happened, especially when the disease is as preventable as Heartworm is. With incidence going up in the USA, the incidence in Canada is going to follow suit, and if people don’t have their pets protected we are going to see the disease become more and more prevalent!

There are many online stores and “pharmacies” now that claim to have the heartworm protection products for your pets at a much cheaper price. BEWARE THESE WEBSITES! Heartworm medications are just that…MEDICATIONS. These can not just be purchased in a store. They need a prescription from a licensed veterinarian to obtain. Anything else that you can buy at the store is NOT medication…it is a pesticide which can seriously harm your pet! Zoetis, the makers of Revolution, have actually informed us that they DO NOT distribute any of their products to ANY source other than veterinary clinics, so anything you buy online is not likely to be the product you think it is. Many of these products are coming from overseas and are fraudulent, and just like knock off purses and shoes and jewelry, the packaging can be very convincing! This means we really don’t know what you are giving to your pet when you buy these products, and we don’t know how they were shipped or stored either, and this can affect how the product works. Think about this critically for one minute…you go to your doctor and they suggest a medication to you and you don’t want to have to pay the pharmacy price. Do you then go onto the internet, search for a site that sells the medication super cheap, and order it to take it yourself? NO! We are all aware of the dangers of doing something like that for ourselves, but we forget that the same applies to our pets!

On top of this, I have had a few clients insist on getting their products online on a pet pharmacy site, so I obliged. I wrote them a prescription for Revolution and they ordered it online. The distribution facility was in the USA, so by the time they got the package, paid duty and other fees on it, and taxes, the final price ended up being MORE expensive than if they had just purchased the medication in the clinic!

Finally, if you order your prevention through a site that is not a veterinary clinic, the makers of the medications will NOT back up their products in any way, because they know that it is likely NOT their product you are actually getting! In addition, the storage conditions and handling as well as the expiry date may be inappropriate, and they have no control over these things if you are not getting your medication from a veterinarian. Typically, if your pet contracts heartworm while on a medication for prevention, the company will do everything they can to help you find out why they contracted it and help you to treat your pet as well! This helps A LOT financially as treating heartworm is extremely expensive. If you get your product from anywhere other than a licensed veterinarian, they will not help you.

So, be critical of these sites selling cheap medications for pets, especially if no prescription is needed, because what you are giving your pet may be very dangerous and there is NO guarantee that the product is going to work or that it will end up saving you money in the end!


Overall, there is A LOT more to this whole heartworm thing that you realize and the statistics that people base their opinions on are not accurate and do not give us anywhere close to an accurate picture of the true prevalence of the disease. There is very likely A LOT more heartworm positive animals than we know about due to the limitations of our testing, and these animals are a direct threat to your pets because they will spread the disease through the mosquitoes.  As veterinarians we are focused on keeping your pets happy and healthy and on practicing preventative medicine to achieve this! When we say that we want to protect your pet against a disease, it’s because we really do want to protect them and save them from having to endure painful and costly treatment for a disease we can prevent in the first place!

Whether or not you choose to test and protect your pet is 100% your decision. All we ask is that you think about your decision based on reliable and scientific FACT, not on propaganda on the internet or advise from a friend. Talk to us about heartworm, or any other health related issues with your pets, because we want the best for you and them and we are always willing to discuss things with you!

Please call us or email us if you have any further questions about Heartworm, and please bring your pets in for their spring exam and 4DX test and flea/tick/heartworm preventative today!

Dr. Christine Nawas, BSc, DVM

(c) Erin Mills Pet Hospital

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