Heartworm Disease in Ontario

Information compiled by Julia Michitsch B.Sc., M.Sc. (Zoology)

About Heartworm Disease

Heartworm (Dirofilaria immitis) is a parasite transmitted by mosquitoes that can cause potentially fatal health concerns in both dogs and cats. Heartworm is present in all 50 U.S. States with Mississippi, Louisiana, Arkansas, Texas and Tennessee being the states with the highest recorded incidence1. It has slowly made its way up to Canada, with Ontario being named as the province with the greatest risk of infection2. In 2019 alone, IDEXX laboratories (one of the companies that perform heartworm testing) reported that there have been 91 positive test results in Ontario 3

image retrieved from: https://www.petdiseasereport.com/content/prevmap.aspx

Prevalence of Heartworm Disease in Ontario

 According to a study conducted between 2007 and 20162 looking at blood samples from dogs, there were 762 positive test results for heartworm in Ontario, with 722 of these occurring in Southern Ontario. Three clusters of highest prevalence were determined based off of these results: the North-Western Region (Rainy River county, west of Thunder Bay), the Southern Ontario Region (encompassing the counties of Haldimand-Norfolk, Oxford, Elgin, Chatham-Kent and Lambton) and the Eastern Ontario Region (Lanark and Renfrew counties, just west of Ottawa). Whereas the total prevalence for all of Ontario was calculated to be 0.12%, Haldimand-Norfolk was recorded at 0.64%, Chatham-Kent at 0.59% and Rainy River at 0.50%. Interestingly, the counties of Prince Edward, Kawartha Lakes, Dufferin, Muskoka, Haliburton, Parry Sound, Manitoulin, Sudbury, Timiskaming, Cochrane and Algoma all had zero positive test results from 2007-2016.

A similar study was conducted by another group of researchers looking at the prevalence of heartworm specifically in cats in both the U.S. and Canada4. Positive test results were found in cats in 35 of the U.S. states (more so in the Southern states), but not in Canada. This study found a 3-fold increase in positive results in outdoor cats, and a 2.5 increase in cats that were unhealthy when tested (immune systems were already weakened by other conditions). The increase in positive test results for heartworm observed in dogs in Ontario from 2007-2016 could be related to an increase in frequency of screen tests being conducted2. The heartworm detection test is part of the screening panel that also includes tick-borne diseases such as Lyme, Anaplasma, and Ehrlichia – all of which became more of a concern in Ontario over the time that this study was conducted, and as such led to an increase in the number of screen tests completed.

Climate change is also considered to be a potential cause for the increased prevalence of heartworm in Ontario1,2,5. The increasing seasonal temperatures have allowed mosquito populations to travel farther north than they ever have before, and have allowed them to remain active for a longer season as well1. Generally, the higher the temperature, the faster mosquitoes will develop from egg to adult, and the faster the heartworm parasite can progress through its lifecycle1

The lifecycle begins when adult heartworms living in an infected animal mate to produce microscopic baby worms called microfilariae (singular:  microfilaria)5. Microfilariae circulate in the blood stream of the animal until they are picked up by a feeding mosquito5 . The baby worms continue to develop within the tissues of the mosquito until they become third stage larvae (L3), the stage at which they can be transmitted by the mosquito to a new host6. The rate at which the baby worms develop into the L3 larvae is temperature dependant, taking as little as 10-14 days when the weather is warm6. The L3 larvae are deposited on the skin of the new host while the mosquito feeds6. They enter the new host’s body through the bite wound and continue to develop as they make their way through connective tissue and into the blood stream6. After 6-7 months of living in the new host, the larvae become sexually mature adults and the cycle starts anew5

 Adult heartworms can be up to a foot in length and usually live in the heart or lungs of the host animal, or in the blood vessels associated with these organs5. Dogs are a natural host for heartworm as their bodies provide a suitable environment for the worms to grow and reproduce5. Cats can become infected with heartworm, but generally, the worms do not survive to adulthood5. Adult worms can live up to 5-7 years in a dog or 2-3 years in a cat5. An infected cat will typically have only 1-3 worms  living in their body (usually juveniles) whereas a dog can have up to several hundred as the adult worms continue to produce offspring over their lifespan5. As adult worms can survive for multiple mosquito seasons, the likelihood of transmission from an infected animal to a new host is high5

 Symptoms of Heartworm Disease

Typical symptoms of heart worm disease in dogs include a mild persistent cough, reluctance to exercise, fatigue after moderate activity, decreased appetite and weight loss, with advanced disease leading to heart, liver or kidney failure5. Life-threatening events such as blockage of the blood vessels in the lungs (pulmonary thrombosis), and Caval Syndrome (cardiovascular collapse caused by sudden blockages within the heart) may occur as the worms increase in number and block blood flow through vessels and organs5. Whereas dogs normally don’t show symptoms until there are many worms present, having just 1-2 worms in a cat can make them very ill5.  Symptoms in cats can include coughing, asthma-like attacks, periodic vomiting, and lack of appetite or weight loss.  Difficulty in walking, fainting, seizures or fluid build-up in the abdomen can occur occasionally, and in some cases the first sign of disease in a cat is sudden collapse, or even sudden death5. Severe complications such as blood clots in the lungs can occur when adult worms die in the cat’s body5. Heartworms in cats can also migrate to other areas of the body such as the brain, eye and spinal cord5

Treatment of Heartworm Disease

Heartworm disease can be successfully treated in dogs, but treatment is expensive, long in duration and quite complex, usually consisting of several steps5. Symptomatic dogs are stabilized, and then given melarsomine (the only drug approved for heartworm treatment) in hospital which kills both the adult and juvenile worms5. While treatment is going on, the dogs must be restricted in their activity, as physical exertion can increase complications and further the damage done to the heart and lungs by the worms5. Most of the worms die quickly, within 1-3 months of treatment being administered, and follow-up blood tests will confirm whether any further treatment is required5. Unfortunately, while melarsomine is effective and safe to use in dogs, it is not safe for use in cats5. With no approved drug therapy available, treatment of heart worm disease in cats involves stabilizing the patient followed by the implementation of a plan for the long term management of any lung or heart symptoms that arise5. In some cases, heartworm infections in cats can resolve on their own – perhaps owing to the fact that cats are not ideal hosts for the worms5. Even in cases such as these, the worms still leave permanent damage in the respiratory system of the cat5

The Importance of Heartworm Disease Prevention


The earlier heartworm disease is detected, the better the chances of treating it successfully and minimizing the damage caused by the worms5. As the symptoms of a heartworm infection can take months to become noticeable in pets, regular screening via blood test is extremely important in order to catch the infection early5. As a general rule, pets should be tested for heartworm yearly – prior to the start of the mosquito season, and prevention medication. The test works by detecting proteins specific to the adult female worm in the blood.  As discussed earlier, it takes about 7 months after the mosquito transmits the L3 larvae into the host for adults to develop. As such, a test administered prior to the presence of adults will come back negative – masking the fact that the pet is actually infected. This is why puppies under 7 months do not need a blood test in order to start prevention medication, and why pets that have not been on prevention medication before should be tested not only prior to starting medication, but also 6 months later. In cats, since adult worms do not tend to develop at all, an additional type of test may be used which detects antibodies made by the immune system in response to the presence of heartworm larvae in the blood5. A chest x-ray or ultrasound may also be used as a tool to screen for heartworm in cats5. As mosquitoes can live both outside and inside our homes, pets that are kept indoors are also at risk and should be tested5. Pets that are on prevention year-round should be tested yearly as a precaution, to ensure the efficacy of the prevention product5


There are a variety of medications available to prevent heartworm infection in both dogs and cats. They can be administered orally (as a chew), or topically (applying liquid to the skin to be absorbed) on a monthly basis and all act to prevent the larval stage worms from growing into adults. To date, no vaccine for heartworm has been developed5. The heartworm season in Southern Ontario is generally agreed upon to be from May until November – the months during which the temperature is warm enough for mosquitoes to be out and feeding. In order to keep pets safe from contracting heartworm disease, they should be given prevention medications monthly during this time. Remember that mosquitoes can enter and live in our homes as well, so monthly prevention medication is equally important for indoor animals. These medications are highly effective, as long as they are given correctly. If the medication is missed, given late or if the dog vomits or spits out the chew they will be unprotected from infection5


There are several ways that we can help to reduce the number of mosquitoes present in our environment. Mosquitoes can lay 100-400 eggs at a time on the surface of still or slow moving water7. The eggs hatch into larvae after 1-2 days, and larvae develop into adults in less than 10 days if the temperature is ideal (between 22-27°C)7. In order to prevent mosquitoes from breeding it is important to remove standing water from around the yard. Some ways that this could be done include:  keeping gutters clear, storing flower pots, watering cans, boats and wheelbarrows upside down, replacing water in bird baths, wading pools and outdoor pet dishes at least twice a week, covering garbage, recycling and compost bins – or drilling a drainage hole in the bottom of the bin to allow water to escape, emptying rain barrels if water is over 1 week old (unless the barrel is protected by a fine screen), repairing leaking pipes or hoses promptly,  keeping swimming pools chlorinated and removing water collecting on pool covers, and aerating ornamental ponds7. Also, keep natural predators of mosquitoes around – fish, frogs, beetles, dragonflies, waterbugs and birds – as they will help keep the adult mosquito population under control as well as the larvae7


  1. Carrozza, A. (2018, April 18). Heartworm: Where are we today? Retrieved November 11, 2019, from https://www.americanveterinarian.com/journals/amvet/2018/april2018/heartworm-where-are-we-today?p=1
  2. McGill, E., Berke, O., Peregrine, A.S., & Weese, J.S. (2019). Epidemiology of canine heartworm (Dirofilaria immitis) infection in domestic dogs in Ontario, Canada: Geographic distribution, risk factors and effects of climate. Geospatial Health, 14(1). Doi: 10.4081/gh.2019.741
  3. IDEXX – Real-time Pet Disease Reporting. (2019). Retrieved November 18, 2019, from http://www.petdiseasereport.com/content/prevmap.aspx.
  4. Levy, J.K., Burling, A.N., Crandall, M.M., Tucker, S.J., Wood, E.G., & Foster, J.D. (2017). Seroprevalence of heartworm infection, risk factors for seropositivity, and frequency of prescribing heartworm preventatives for cats in the United States and Canada. Journal of the American Veterinary Medical Association, 250(8), 873-880. doi: 10.2460/javma.250.8.873
  5. Heartworm Society. (1970, August 2). Heartworm Basics. Retrieved November 11, 2019, from https://www.heartwormsociety.org/pet-owner-resources/heartworm-basics.
  6. Stilwell, N. (2019, July 24). Get a handle on heartworm disease. Retrieved November 11, 2019, from http://veterinarymedicine.dvm360.com/get-handle-heartworm-disease.
  7. Health Canada. (2016, May 13). Mosquitoes. Retrieved November 18, 2019, from https://www.canada.ca/en/health-canada/services/pest-control-tips/mosquitoes.html