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Cardiac outcomes of heartworm disease in dogs

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Heartworm disease can severely damage pets’ lungs, heart, and other organs. Banfield Pet Hospital – part of the Mars Veterinary Health family of practices – took a closer look at the incidence of specific long-term cardiac outcomes of heartworm disease in dogs. We initiated a retrospective cohort study of more than 4 million dogs, a subset of which tested positive and underwent therapy for heartworm infection within our network of primary care hospitals. Compared with dogs that tested negative, those that tested positive and received treatment for heartworm infection had higher risks for right-sided heart failure, left-sided heart failure, and cardiomyopathy1. The analysis provides real-world evidence of the potential for heartworm disease to cause long-term cardiac damage.

What this means for veterinary teams
With accurate information about the clinical consequences of heartworm disease, veterinary teams can educate dog owners regarding the importance and benefits of heartworm prevention to protect their own pets and the canine population overall.

Study details
We compared the long-term risks of right- and left-sided heart failure and non-breed-related cardiomyopathy in dogs that were successfully treated for heartworm disease versus dogs with no history of heartworm disease.

For the analysis, we used data from electronic medical records of adult dogs that received a heartworm antigen test at any of the >1,000 hospitals in the Banfield Pet Hospital network from 2005 to 2014.

Dogs included in the analysis had no history of the cardiac outcomes of interest and had tested positive for heartworm infection, received adulticide treatment for heartworm disease, and subsequently tested negative for heartworm.

Risks of cardiac outcomes
We found that dogs that recovered from heartworm disease (n = 6,138) had higher risks of receiving a clinical diagnosis of right heart failure, left heart failure, or cardiomyopathy than dogs with no history of heartworm disease (n = 4,022,752). The increased relative risk and 95% confidence interval for each of the three outcomes was 3.59 (95% C.I. 2.64–4.86) for right heart failure, 1.83 (95% C.I. 1.51–2.22) for left heart failure, and 2.79 (95% C.I. 1.71–4.57) for cardiomyopathy1.

Importance of heartworm prevention
Our findings of greater risks of heart damage among dogs that contracted heartworm disease, even after they were successfully treated and reverted to negative heartworm status1, highlight the importance of preventive measures. The highest relative risk was seen for right heart failure, which aligns with our current understanding of heartworm infection: the right side of the heart and the lungs are most severely affected.

The American Heartworm Society (AHS) recommends administration of heartworm prevention products for all dogs in the US to prevent infection of individual animals and to reduce the overall prevalence of heartworm disease at the population level2. The AHS recommends 12 months of protection and a heartworm test every 12 months. However, compliance with AHS guidelines remains low; pet owners do not always purchase or administer heartworm prevention products consistently2–4. As a result, canine heartworm disease continues to occur in the US, with about 1% of dogs testing positive for the disease in 2022 and thus far in 20235. Heartworm infections have been identified in dogs in all 50 states, and the risks of infection continue to increase with climate change.

As the leading provider of preventive veterinary care in the United States, Banfield has a scientific focus on reducing preventable diseases. We carried out this study as part of that focus, and the results suggest that prevention is superior to (even apparently successful) treatment. We hope the findings will be useful to veterinary teams when counseling clients about heartworm disease prevention.


  1. Mwacalimba, K. et al. 2023. Prevention and long-term outcomes of naturally occurring canine heartworm infection in primary care settings. Frontiers. https://www.frontiersin.org/articles/10.3389/fvets.2023.1334497/full
  2. American Heartworm Society. 2018. https://www.heartwormsociety.org/images/pdf/2018-AHS-Canine-Guidelines.pdf
  3. Atkins, C.E. et al. 2014. Heartworm ‘lack of effectiveness’ claims in the Mississippi Delta: Computerized analysis of owner compliance – 2004–2011. Vet Parasitol.
  4. Drake, J. & Wiseman, S. 2018. Increasing incidence of Dirofilaria immitis in dogs in USA with focus on the southeast region 2013-2016. Parasites & Vectors.
  5. Companion Animal Parasite Council. 2023. https://capcvet.org/maps/#/2023/all-year/heartworm-canine/dog/united-states