651-224-3038   •   1140 Grand Avenue, St. Paul, MN 55105

Heartworm Disease and Prevention in Dogs

November 2, 2016

In the last year, we have seen a significant increase in the number of dogs with heartworm disease here at Grand Avenue Veterinary Center. While these still represent a small number of dogs, the increase is concerning and not a trend we want to see continue.

A comprehensive review of heartworm disease can be found in a previous blog entry (click here) but today I primarily want to outline some of the potential reasons for the increase we are seeing.

  • Our local shelter system is relatively well funded and successful, so it has become very common practice for rescue groups to pull dogs from overcrowded and underfunded shelters in the southern United States and transport them here to find homes. Due to their year-round mosquito populations, these states have a much higher incidence of heartworm disease. Also, there is up to a six month lag between initial infection and a positive test, so some heartworm positive dogs may initially test as negative and slip through the cracks. These dogs later test positive and until this is discovered can serve as a reservoir for heartworm disease.
  • Our cold weather has traditionally given us a break from heartworm in the winter, but changing weather patterns make it more difficult to predict when mosquito season begins and ends in any given year. Also, the role of microenvironments where mosquitoes may survive the winter (in/under porches, crawlspaces, garages, or other sheltered spaces) is becoming better understood. For these reasons (and also for the added benefit of intestinal parasite control), we recommend using monthly heartworm preventative year round instead of seasonally.
  • Because heartworm preventative has been so successful at reducing the incidence of heartworm disease, there can be a perception that heartworm disease is no longer a risk or problem and preventative is unnecessary.
  • Fear of giving unneeded medication or chemicals sometimes drives the decision to skip giving preventative in favor of treating heartworm disease if it occurs. Unfortunately, heartworm disease and its treatment are much riskier and costlier than simply giving the preventative each month.
  • Some breeders claim their breed is resistant to heartworm disease and preventative is not needed, which is false. Also, several herding breeds may be affected by a mutation called MDR1 and there is a perception that heartworm preventatives are unsafe for these dogs. At prescribed doses, heartworm preventatives are safe even for MDR1 affected dogs. (See more information at WSU’s website, click here.)


The bottom line is that we may have a bigger pool of potential carrier dogs coming from other states combined with people deciding for various reasons to give heartworm preventative inconsistently or not at all. Together, these factors can increase the risk of heartworm in our patients which is something we hate to see happen. Please remember to give your heartworm preventative monthly and if you have any questions or problems please contact the clinic.

Karen Christopherson DVM CVA




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