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Welcome to Spring! It’s Time To Book Your Dogs Annual Heart Worm Test

Mar 25, 2023 | Cats, Disease and Symptoms, Dogs, Parasites, Pet Care, Pet Safety, Pet Tips, Wellness

Heartworm infection can become a life threatening illness very quickly if not rapidly diagnosed and treated, it is arguably one of the most important parasites to protect dogs against.  According to statistics, approximately 100 000 cases of heartworm are reported annually within North America – that is a lot! The great news is that it is preventable! Although before discussing the prevention of heartworm, it is important to understand what heartworm is and why it is so important to protect our canine friends against this havoc wreaking parasite.

What is a heartworm and how is it transmitted to dogs?

The parasite that causes heartworm disease is known scientifically as Dirofilaria immitis, and it takes on the appearance of long hair like worms when fully matured. The only way that this parasite can be transmitted is from the bite of an infected mosquito.

The mosquito is the intermediate host for this parasite, which in simpler terms means that the mosquito is essential in order for the parasite to properly develop into its infective stage, although the mosquito is not the final host. Once the parasite has developed and reached its infective stage within the mosquito, it can be transmitted through the bite of a mosquito into a dog, as the dog is the final or definitive host for heartworm. Once transmission of the infective heartworm parasite has occurred, the parasite will continue to grow and develop within the dog over 2-3 months. After this time period, they will begin to migrate from bodily tissues and into the heart around 3-4 months post mosquito bite, and after 6-8 months the parasite will begin to produce offspring called microfilaria which can be detected in the bloodstream by a heartworm test. Heartworms can only be transmitted from mosquitoes and cannot not be transmitted between pets.

Heartworm parasite on a blood smear

Diagram of basic heartworm transmission

What is the geographic range of heartworm infection?

  • The heartworm parasite can be transmitted all throughout North America, with most cases being diagnosed in the southern states of America. This is important to be aware of as more dogs are being transported from southern regions to Ontario and Canada in search of adoption.
  • Within Canada, Ontario has the highest rates of infection versus other provinces. The latest data suggesting about 75% of cases have been found in Ontario, and most of those cases are being reported in Southern Ontario.
  • Roughly half of the mosquitoes in Canada are potential hosts for the infective stage of heartworms.


Who is affected by heartworm?

Dogs are primarily affected by heartworm infection as they are considered the definitive or final host for this parasite. Cats are also susceptible to heartworm infection, although they have a reduced risk of contraction as they require a much greater level of exposure to the parasite to become infected. Cats are not the ideal final host for heartworm and have developed resistance to the parasite, however, a few cases have been reported in felines. Humans cannot contract heartworm infection.

What are the signs of a potential heartworm infection, and is it treatable?

Depending on the individual dog and severity of the infection the symptoms can range from mild to more severe which are outlined below:

Mild: The infected dog may show no outward symptoms if there is a minimal infection or may display a mild cough.

Moderate signs: The infected dog may display a moderate cough, may have difficulty breathing, and may be slightly exercise intolerant (ex: cannot run, tires more easily than normal on walks).

Severe signs: The infected dog may have difficulty breathing that will become labored, especially during exercise if tolerated. Sometimes an infected dog may not tolerate any exercise due to increased breathing effort. Signs of heart failure are very common

Very Severe Signs / Caval Syndrome: This is the most severe form of heartworm disease – in this stage there is such a heavy burden of worms that blood flow throughout the heart becomes obstructed. The onset of this condition is rapid and life-threatening and may be displayed as extreme breathing difficulties and collapse. If surgical removal is not rapidly completed, death is likely.

As for the treatment of mild-severe heartworm infection, there is an injectable medication that is designed to kill adult worms residing in the heart that is proven to be highly effective when combined with another product designed to kill the microfilaria (baby heartworms). Although this treatment is lengthy, as multiple injections need to be administered and can take quite a toll on your pet as the injection can be painful and A LOT of cage rest is required before and after treatment. This treatment poses its own risks to the patient and may not always be successful depending on the severity of infection.


Overall – prevention is the best treatment!


What is a heartworm test and is annual heartworm testing really necessary?

The short answer to this question is yes! Annual heartworm testing is recommended to be completed every spring around the month of May for every dog that is over the age of 6 months to test for any infection that may have been transmitted from the previous season. An added benefit of this blood test is that it also tests for 3 other tick borne diseases including Lyme disease, Ehrlichiosis and Anaplasma.

A small blood sample is required for the heartworm test to be run and it takes about 10 minutes until results are received in the clinic.

It is recommended that dogs have a negative heartworm test result prior to beginning parasite prevention each season. The reason this is so important is because mature heartworms can survive in the dog for a very long time – up to 5-7 years! If a dog becomes infected with a small burden of heartworms clinical symptoms may not be present, but a heartworm test will detect microfilaria (also known as baby heartworms) in the blood. This test could potentially save your dog’s life before the disease process advances or becomes more severe. Due to the long life span of these worms, each mosquito season can lead to an increasing number of heartworms in an infected pet.

How do you properly prevent heartworm infection?

The current guidelines for heartworm and intestinal parasite preventative medication suggest medication be taken from June 1st until November 1st after receiving a negative heartworm test for pets over the age of 6 months. Most preventative products are given once monthly throughout the duration of the season. These products come in 2 different forms, oral tablets and topical application.

Both methods of prevention display a high level of efficacy if taken properly throughout the duration of the season. The method that works best for your pet mainly depends on your preference and the dog’s lifestyle – with some topical medications having the added benefit of acting as an insect repellant. Dogs that are food motivated often do very well with oral tablets as they are easy to administer. For dogs who will not easily consume oral products, topical application may be considered.

Be careful if you choose to purchase heartworm preventative medication from a pet store or online. A lot of these products are not FDA approved and do not have the scientific research behind them to prove their efficacy. Some of these products may contain toxic ingredients as many of these over the counter products contain pesticides. For more information, discuss with your veterinarian today which prevention plan is the safest and most effective for your pet!

Give us a call today to schedule your dog’s annual heartworm test! At this appointment, a preventive plan can be discussed that best suits the needs of your pet with one of our friendly staff – Call us today at 519-893-8937! Have a safe and happy spring!