Handling a Dog's Positive Heartworm Test

As part of our canine patients’ annual wellness care, our veterinarians recommend having pups tested for heartworm infection each year in order to prescribe heartworm prevention to be used over the mosquito season (June till November).

The reason our veterinarians send these preventive medications?

FACT: Although specific medication is available for treating a heartworm-infected dog, the treatment regimen and need to restrict the patient's exercise can be challenging for the dog, owner, and veterinarian. Drugs are administered multiple times over a 4-month period, during which the dog must be kept quiet. If activity is not restricted, treated dogs are at risk of severe complications or death as decomposing worms in the bloodstream cause inflammation and blockage.

BEST STRATEGY: Preventing heartworm disease is much less expensive and far safer than treating it.

Picture source: The American Heartworm Society"

What happens when a dog tests positive?

After a dog receives a positive result on the heartworm antigen screening test, further testing is required to confirm actual heartworm infection.

These tests include additional blood collection with the samples subjected to any of the following analyses:

  • A different manufacturer’s heartworm antigen test
  • Recovering microfilariae (heartworm larvae) circulating in the bloodstream
  • Complete Blood Count (CBC)
  • Chemistries to assess organ function (liver, kidneys)

Other tests include chest x-rays or urinalysis depending on the dog’s symptoms. Most dogs with the early infection do not display any symptoms.

As the disease progresses, the heartworm-infected dog may show any of these signs:

  • cough
  • exercise intolerance
  • rapid breathing
  • fluid in the abdomen
  • pale mucous membranes with a blue-coloured tinge
  • incoordination
  • heart murmur
  • rapid heartbeat

Your veterinarian will consider all of your pet’s symptoms when combined with a positive antigen test result. Because of the risks involved in heartworm treatment, the doctors must be certain of their diagnosis. In a subsequent blog, we will be presenting the details of heartworm treatment in diagnosed dogs.

If you have any questions or concerns about heartworm disease, please contact us! We are happy to help.


  1. American Heartworm Society. (2018). Heartworm Basics. Retrieved from
  2. Dryden, M. W. (2017). Canine Heartworm Antigen Tests. Retrieved from
  3. VetStreet Inc. (2014). Heartworm Disease in Dogs. Retrieved from: