Toronto researcher touts 'game-changing' heartworm treatment clinical trial for dogs
· Toronto Sun
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A Toronto researcher says a landmark clinical trial could change how heartworm disease is treated in dogs, making treatment faster and more affordable for pet parents, shelters and rescues.
Dr. Linda Jacobson, director of science and advancement at the Toronto Humane Society , was part of an international research team that included Lucky Dog Animal Rescue in Florence County, S.C, the University of Florida Shelter Medicine Program and PetSmart Charities.
The study, conducted between April 2024 and December 2025, published in September 2026 in the peer-reviewed Journal of the American Veterinary Medical Association, followed 168 heartworm-positive dogs from rural shelters.
It found that a streamlined, two-dose treatment achieved 95.3% efficacy, with all 168 dogs surviving compared with a more intensive three-dose treatment.
“We really went back to an older treatment for heartworm that was initially used in the 1990s,” said Jacobson.
“In the intervening 30 years, a number of other treatments have been added into the traditional heartworm treatment protocol. So are question was with these additional treatments could we go back to the original two injection protocol and would the dogs do as well as they do with the current three injection protocol?”
Jacobson said the injections of Melarsomine, which quickly kill the parasites that lead to heartworm, are really expensive and the overall treatment is time consuming.
Dogs also have to be kept very quiet after treatment with very limited exercise “because of these dead and dying worms in the lungs, which cause complications,” she said.
“The recommended protocol requires three injections that are spread out over a month and it also requires a two-month waiting period for pre-treatment, so that’s a three-month period until the third injection is given,” said Jacobson.
“And we actually tested two shorter protocols where we decreased the waiting period from two months to two weeks and then with the two dose protocol the injections were given one day apart at two weeks. So, essentially shortening the treatment time from three months to two weeks. And our results were really good.”
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Vets can start using two-dose protocol immediately
Jacobson said vets can immediately start using the new protocol “if they chose to. That’s just a decision that each veterinarian will make.”
The two-dose versus three-dose protocol will also “reduce costs by about 60%,” said Jacobson.
“It also reduces time. So, for a shelter dog, we can get them in and out in two weeks. And that’s really huge for shelters because a lot of shelters don’t have the resources to keep dogs for three months. So we really anticipate that particularly in shelter settings that this could really be game-changing.”
“And there are big welfare aspects to this as well . So the dogs’ stay in the shelter will be shorter and they’ll also have a much shorter period of exercise restriction,” she added.
Heartworm disease is spread through mosquito bites and can cause serious damage to a dog’s heart and lungs.
Jacobson said at the Toronto Humane Society , each dog that comes into the shelter is tested for heartworm.
“Not all shelters test but we do,” she said. “It’s just a blood test.”
Almost non-existent in Toronto
Jacobson said there’s “a surprising amount of heartworm in Canada and Ontario that we know of and that’s in communities that historically have not had access to veterinarian care.”
“It’s interesting, so less than 1% of dogs have it in Canada,” she said. “It’s very, very low actually. But in communities that are underserved historically, haven’t had access to care, we’re seeing numbers like 10, 20, 30% of dogs testing positive.”
It’s almost non-existent in Toronto but still remains an important health concern for dogs.
“It can cause, over time, severe lung damage, it can damage to the heart and in severe cases, it can cause death,” said Jacobson.
But there are heartworm preventative treatments available and dog owners should get their pets on one of those in early spring.
“Most commonly it’s tablet once a month, it can also be a topical treatment once a month, and there’s also a long-acting shot,” said Jacobson.