March 19th marks the first anniversary of a significant research study led by Dr. Linda Jacobson, Director of Science and Advancement at Toronto Humane Society, in collaboration with the University of Florida Shelter Medicine Program, Lucky Dog Animal Rescue, and PetSmart Charities. This large-scale study may transform how animal shelters across North America treat heartworm-infected dogs.
The study aims to improve veterinary care access and enhance health outcomes for dogs diagnosed with heartworm in shelters, and it has already shown promising results. Dr. Jacobson, who serves as the lead investigator on this project, shared her optimism: “The study is going really well, with all the data so far supporting our hypothesis that a shorter, less resource-intensive treatment course is safe and effective. We believe that this research could significantly impact heartworm treatment in shelters and beyond.”
The study follows Toronto Humane Society publications about heartworm disease in Canadian dogs and a highly impactful review of alternative treatments.
Heartworm occurs in dogs across the world and is most common in dogs without access to veterinary care, including shelter populations. Heartworm disease is a serious and potentially fatal condition caused by parasitic worms that live in the heart, lungs, and associated blood vessels of affected dogs. These worms are transmitted through the bite of an infected mosquito, and over time, they grow, reproduce, and cause severe damage, particularly to the lungs. The disease can lead to heart failure and chronic debilitating lung disease if left untreated. Using conventional protocols, dogs with heartworm require months of care and exercise restriction while undergoing treatment, limiting their ability to be adopted from shelters.
Unfortunately, many shelters, due to limited space and resources, struggle to provide the extensive care necessary to treat heartworm-positive dogs, making this study especially important for improving the accessibility of treatment options.
In recent years, supportive medications that target the adult worms, prevent reinfection, and reduce inflammation are administered to increase the safety and efficacy of the treatment. Current guidelines are based on studies that were done before these added medications. We hypothesized that with these added medications, a shorter, modified protocol will be as safe and effective as the current protocol recommended by the American Heartworm Society (AHS).
Our Protocol:
The American Heartworm Society Protocol:
The recommended treatment protocol is a lengthy and resource-intensive process involving a 60-day wait period, three injections of melarsomine over a month, and a total of up to five months of strict exercise restriction. Our modified treatment protocol involves only two injections, two weeks after diagnosis, followed by one month of kennel run restriction and scheduled exercise periods – a total of six weeks. Dogs could even be adopted after two weeks, if adopters can continue to restrict exercise at home. “This is an important study,” Dr. Jacobson said. “With the help of the incredible team at Lucky Dog and the generous support of PetSmart Charities, we are rigorously testing this hypothesis in a large population of dogs. This is a randomized controlled study, the highest level of evidence. We are optimistic that this study will majorly impact heartworm treatment in shelters and beyond.”
As of March 17, 2025, 143 dogs (84% of the goal enrollment) have already received lifesaving treatment through the study. Each dog is retested 6 to 9 months after the final injection to determine if and when they test negative for heartworm.
The early results of the study have been extremely encouraging. Of the dogs who have completed their final test, 48 out of 48 have tested negative for heartworms!
“This study could change how we think about the options for heartworm-positive dogs,” said Dr. Jacobson. “A positive heartworm test result would no longer be a sentence to a poor outcome or a barrier to adoption. This research could pave the way for treatment to become a viable option for large numbers of heartworm-positive dogs, both in and out of shelters.”
With the continued commitment of shelter staff, veterinary professionals, and adopters who help track test results, this study has the potential to improve outcomes for heartworm-infected dogs. This study offers hope not only to the dogs currently battling heartworm disease in shelters but to countless others who may face this diagnosis in the future. Toronto Humane Society is exceptionally proud to be at the forefront of this groundbreaking work.
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