Heartworm Season

Bay Cities Animal Hospital recommendations for heartworm prevention in 2015, as adapted by the American Heartworm Society:

  • All dogs should be tested annually for heartworm.
  • All dogs should be on a monthly heartworm preventive medication.
  • If your dog is on a year-round monthly preventive, it can be tested at any time during the year.  Click here to find out why we recommend year-round monthly preventives.
  • Dogs that are on monthly preventives from June 1 - November 1 should be tested during the months of April, May and June before they resume their summer heartworm preventives.
  • We also recommend the use of flea and tick preventives from June - November for all dogs and cats who spend time outdoors.  Flea prevention can easily be combined with your monthly heartworm prevention.  For more information on fleas click here.

Call Bay Cities Animal Hospital at 905-639-5414 to book your dog's heartworm appointment or email us at baycitiesanimalhospital@coegco.net

Heartworm and your Dog in 2015


Did you know that the first published evidence of disease in the dog in the United States was in The Western Journal of Medicine and Surgery in 1847? 

168 years have passed and heartworm disease is still a serious threat to dogs in North America, including Southern Ontario.  Each year, over 1 million dogs in the United States are infected with heartworm parasites.  Since 2005 our region has actually seen an increase in the number of dogs that test positive for heartworm disease.  This is in part due to the large number of dogs that have been brought across the border recently from New Orleans after hurricane Katrina left these dogs homeless. 

The Ontario Veterinary Medical Association website states: "a private laboratory that conducts heartworm tests for Ontario veterinarians reported 676 heartworm positive pets in 2008, up 280% from 2005."  "Many of the "hurricane rescue dogs" were imported only having had one negative heartworm test."

So do you need to be worried?  The answer is no, but you do need to ensure that your dog is on a proper heartworm preventive during the summer months.  You should also test your dog annually for heartworm disease, even if it is on the heartworm preventive medication.  This is because often times an owner may accidentally miss a dose of the preventive medication, or the dog may spit out the pill without one knowing.  Although today's heartworm preventives are remarkably effective and safe, companies still recommend testing dogs annually.  Giving a heartworm preventive to an already positive dog can cause further complications. 

Heartworm is a thread-like worm that is transmitted by mosquitoes. These worms can grow up to 14 inches in length.  When a mosquito carrying infective heartworm larvae bites your dog, the microscopic larvae goes through several life stages before they become adult worms infecting the heart and pulmonary arteries.  It takes between 6-7 months for these life stages to occur in what is called the pre-patent period.   It is not possible to diagnose heartworm during the pre-patent period.  This is why we test for heartworm in the spring, 6-7 months after the end of the summer mosquito season. 

Symptoms of heartworm disease are not normally evident until the parasites are adults and the infection has progressed.  Many dogs will show little or no sign of infection even after the worms become adults, especially if they live a fairly sedentary lifestyle.   However, active dogs and those with heavier infections may show the classic signs of heartworm disease. Early signs include a cough, especially on exercise and early exhaustion upon exercise. In the most advanced cases where many adult worms have built up in the heart without treatment, signs progress to severe weight loss, collapse, coughing up blood and, finally, congestive heart failure. 

Should your dog test positive for heartworm disease, it can be treated.  However, the treatment involves 2-3 deep muscle injections of melarsamine, an arsenic-containing drug that can have many undesirable side effects.  It is much easier to prevent heartworm than to treat it! 

People are not natural hosts for heartworm infection, however dogs of all breeds and ages are highly susceptible, as well as foxes, wolves, coyotes, and raccoons.  In Ontario, cats are not known to be a major source of heartworm infection.  However, we encourage cat owners to discuss heartworm disease, risks and preventives with our veterinarians.   

For more information on heartworm disease please visit:
The American Heartworm Society
The Companion Animal Parasite Counsil
Novartis Animal Health
Pfizer Animal Health

The following Pet Tales articles by Dr. Barry Burtis also address Heartworm Disease:

Mosquitos and Heartworm

Heartworm Blood Tests

Heartworm Life Cycle