Updated document: April 28, 2020

Below are some timely comments, headed up by OVC veterinary parasitologist Dr. Andrew Peregrine, about heartworm testing and heartworm preventive administration during this period of “urgent care only” veterinary services.  You can also click here for a pdf version of the comments Please note that this was created for the Ontario context, considering the provincial directives for urgent care and the epidemiology of heartworm in this province.  It may not apply equally to areas that are approaching social distancing differently or where heartworm risks are greater. 

COVID-19 Recommendations for Heartworm Testing and Prescribing Heartworm Preventives for Dogs in Ontario


ELECTIVE SERVICES AND SOCIAL DISTANCING (REVISED)’ dated April 17, 2020 (OVMA 2020), the recommendations for “Heartworm prophylaxis” are as follows:

“If pets have been on prophylaxis in previous years, dispensing without a heartworm test is reasonable. Maintain social distancing (see below), arrange delivery or use eCommerce. If there are concerns about owner compliance or if prophylaxis history is unclear (or absent), risks should be discussed with the owner but preventives can be dispensed without testing, with owner consent. Postpone visits to clinics for heartworm testing.”

As such, heartworm testing is considered an elective procedure and should only be carried out if there is a high likelihood of infection, e.g. a dog that has been imported in to Canada from a high-risk area with an unknown prevention history. For dogs that reside permanently in a low risk area (i.e. almost all of Canada) and have previously received a heartworm preventive with reasonable to good compliance, testing is not required prior to prescribing a heartworm preventive. For Canadian dogs with an unknown heartworm prophylaxis history or a history of poor compliance with the use of heartworm preventives, a test should only be carried out if there are clinical signs consistent with heartworm. For dogs >6 months of age for which a baseline heartworm antigen test has not been obtained, the test should be delayed until COVID-19 testing recommendations no longer apply; until that time, a heartworm preventive should be used that is safe to give to a heartworm-positive dog.

It should be recognized that:

  • Some heartworm preventives are approved for use in Canada with label text that states “may be safely administered to heartworm infected dogs” (e.g. Revolution®, Advantage Multi®). However, safety data are available for other products and should be discussed with the respective company.
  • All heartworm preventives are approved with a requirement to carry out an annual heartworm antigen test, prior to prescribing the product. As a result, not carrying out the test means the product is being used off label; historically, pharmaceutical companies have typically not covered heartworm adulticide costs if their product is used off label. As of April 22, 2020, no pharmaceutical company in Canada has indicated whether they will cover heartworm adulticide costs for dogs where testing was delayed because of COVID-19. However, such cases are uncommon.
  • These recommendations differ from those provided by the American Heartworm Society (AHS 2020) and reflect the lower risk of heartworm in Ontario compared to the USA.
  • Since there is a legal requirement for rabies vaccination of dogs in Ontario, yet rabies vaccination has not been classified as an urgent procedure, it is hard to position heartworm testing above this.
  • Guidelines for heartworm adulticide treatment during the COVID-19 era are provided by the American Heartworm Society (AHS 2020).

Please note: These guidelines are a compromise between the risk of heartworm causing problems in Canadian dogs and the risk of COVID-19 infecting people. At the end of the day, a veterinarian always has the right to test a dog for heartworm if they believe the test is warranted. The COVID-19 recommendation is to only test if there is a high risk of infection.


AHS (2020) ahs-covid-19-recommendations-20200406.pdf

OVMA (2020) OVMA COVID-guidelines-essential-elective_social-distancing_April-17.pdf

Andrew Peregrine & Scott Weese, Ontario Veterinary College, April 28, 2020