Our newly-published randomized clinical trial demonstrated high safety and efficacy for a shorter, more accessible protocol for treating heartworm disease in dogs 

At Toronto Humane Society, our commitment to improving the lives of animals includes not only providing care to the animals who come through our doors, but also asking how we can improve the way animals are cared for across the sector and in the community. 

That work is at the heart of recent research led by Dr. Linda Jacobson, Toronto Humane Society’s Director of Science and Advancement, in partnership with Lucky Dog Animal Rescue, the University of Florida Shelter Medicine Program and other research partners. 

Three landmark studies published by our group in 2026 have examined accessible heartworm treatment in dogs, including the safety and effectiveness of a shorter two-dose melarsomine (Immiticide, Diroban) protocol. 

The latest study, published in the Journal of the American Veterinary Medical Association (JAVMA), is the largest randomized clinical trial in the heartworm literature. Until now, no study has ever directly compared two-dose and three-dose melarsomine protocols in clinical patients.  The Lucky Dog study compared compressed two-dose and three-dose melarsomine protocols in 168 dogs with Class 1 or Class 2 (asymptomatic to moderate) heartworm disease.  Dogs in the study received the same adjunctive treatments — doxycycline, moxidectin and prednisone — with the difference being whether they received two or three melarsomine injections. 

What Did The Study Find? 

Both compressed protocols had high treatment success, and survival was 100%. Treatment efficacy (negative antigen test by 9 months post-melarsomine) was 95.3% in the two-dose group and 98.7% in the three-dose group. There was no statistically significant difference in efficacy between the two groups. 

The study also found no significant differences between the two groups for post-treatment cough, increased respiratory effort, or weakness. Very few dogs had post-treatment complications and none needed intensive treatment. 

What Are the Savings? 

The two-dose Lucky Dog protocol saves time, money and resources, and shortens exercise restriction, which is a challenge for many dogs and guardians. Dogs can be treated and ready for adoption in two weeks, compared with three months for the traditional American Heartworm Society (AHS) protocol. 

Why Does this Matter for Shelter Dogs?

Shelter dog populations are much more likely to have heartworm disease than dogs in homes, but heartworm treatment is especially challenging in these settings. Traditional treatment requires multiple injections, extended periods of exercise restriction, significant veterinary resources, and considerable cost. 

For organizations caring for these dogs, these factors create barriers to getting heartworm-positive dogs through treatment and into adoptive homes. 

The compressed protocol studied by the research team allows us to complete melarsomine treatment in a much shorter timeframe. The two-dose protocol substantially reduced the time, resources and cost required for treatment while remaining safe and effective in the dogs included in the study. 

For shelter medicine, research like this can help build the evidence needed to explore approaches that make treatment more manageable — ultimately helping more dogs access the care they need. 

Building the Evidence

The JAVMA publication is the latest of three recent studies from our research collaboration. 

Our retrospective study examined outcomes from 1,246 shelter dogs treated with two-dose or three-dose melarsomine protocols, plus a preventive and doxycycline, at Miami Dade Animal Services. The findings were similar to the Lucky Dog study, with no significant differences between the groups for safety or efficacy. The companion study assessed safety and complications of spay or neuter surgery in 1,011 heartwom-positive dogs.  and found low rates of perioperative complications when standard anesthesia and surgical protocols were used. 

Together, these studies provide support for a spectrum of acceptable ways to treat heartworm disease in dogs, and provide a blueprint for how heartworm-positive dogs can receive rapid and effective care in shelter and rescue settings. 

For Toronto Humane Society, this work is an important part of our commitment to Animal Welfare Science Advancement & Leadership — using research, evidence and experience to help advance animal welfare beyond our own organization. 

“The Lucky Dog study has been a dream study. Randomized clinical trials are the highest level of evidence, and the findings of this study were even better than expected. There needs to be a spectrum of options available for complex veterinary conditions, rather than the “gold standard only” approach that is unfortunately too common these days. Now there are multiple options for dogs with heartworm disease, and many more dogs will have access to lifesaving treatment as a result.” 

— Dr. Linda Jacobson, Director of Science and Advancement, Toronto Humane Society 

We are incredibly proud of Dr. Jacobson and all the researchers, veterinary teams, shelter staff,  and partners who contributed to this work. We are grateful to the ASPCA for partially funding the Miami Dade study, to Idexx Laboratories for discounted testing and especially to PetSmart Charities for funding the Lucky Dog study.  

Research like this reminds us that improving the lives of animals isn’t only about the care we provide today. It’s also about asking better questions, building the evidence and sharing what we learn so that more animals can benefit tomorrow.