Study shows increased cardiovascular risks associated with canine heartworm infection

American Veterinarian®American Veterinarian® July 2023
Volume 5
Issue 7
Pages: 7

A research abstract presented at 2023 ACVIM Forum emphasized the importance of parasite prevention as treatments don’t prevent long-term complications

Mary Lynn Strand /

Mary Lynn Strand /

Heartworm is a widespread disease with cases continuously increasing in number and geographic distribution, according to the Companion Animal Parasite Council.1 “While we consider this a very preventable disease, the rates of infection continue to increase,” said JoAnn Morrison, DVM, MS, DACVIM (SAIM), director of veterinary science at Banfield Pet Hospital, at the 2023 American College of Veterinary Internal Medicine (ACVIM) Forum in Philadelphia, Pennsylvania. 2

To address this epidemic, Morrison presented a research abstract entitled “Outcomes of Naturally Occurring Canine Heartworm Infection”,2 a co-investigation between Banfield and Zoetis that focused on longitudinal outcomes in dogs that tested positive for heartworm disease via an antigen test. Dogs included in the study were older than 1 year, privately owned, and were presented at Banfield Pet Hospital. Along with a population of heartworm positive dogs, there were negative controls.

The dogs with heartworm infection received adulticide therapy in primary care hospitals. “We wanted them to have a negative heartworm test 6 to 12 months after the last adulticide treatment to help ensure that they cleared the infection,” Morrison said.

Dogs were identified in this retrospective, case-control study through a search of medical records, allowing access to longitudinal data for long-term analysis with each dog, which was a major strength of the study. “We did have a large dataset that's in a consistent data structure, which helps. We have a single hospital operating system and operational models,” explained Morrison, adding that licensed veterinarians made the medical diagnoses and there was robust criteria surrounding inclusion to ensure the cleanest dataset possible. Meanwhile, some drawbacks of the study were that there was no boarded cardiologist or internist involved and the necropsy findings were not available.

Analysis on structured diagnostic codes corresponding to right- and left-sided cardiac disease and relative risk of outcomes were performed. The results found heartworm positive dogs had an increased relative risk (RR) for right heart failure (RR 3.59, 95% confidence interval (CI) 2.64–4.86), left heart failure (1.83, 95% CI 1.51–2.22), and cardiomyopathy (2.79, 95% CI 1.71–4.57) in comparison to negative controls. All p values were significant at p<0.0001.2

This research demonstrates the potentially devastating cardiovascular effects of heartworm disease and consequently, reiterates the importance of parasite prevention. “Even if we have all the evidence that we can gather that pets are successfully treated and there's no long-term implications, there are increased risks of negative cardiac outcomes. And that is likely indicating to us that even with successful therapy, prevention is a superior approach to this condition,” Morrison said.

Based on this study, further measures to take when presented with a heartworm positive patient can include incorporating a routine follow-up, along with taking an individual approach to monitoring and treating the pet. Morrison added that future research can focus on the reasons for the relative risk increase and looking into if it’s from the live infection, the impact of the treatment itself, or something else.


  1. Heartworm. Companion Animal Parasite Council. Accessed June 15, 2023.
  2. Morrison J. Outcomes of naturally occurring canine heartworm infection. Presented at: American College of Veterinary Internal Medicine Forum; Philadelphia, Pennsylvania. June 14-17, 2023.
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