Improving the Lives of Heartworm Dogs

Featuring an interview with Dr. Karen Ward, Chief Veterinary Officer

Heartworm is a serious disease in which parasitic worms live in the blood vessels of a dog’s lungs and sometimes in their heart. The best way to protect your dog against heartworm is with a topical or oral preventative treamtment. . Once a dog is infected with heartworm, it is a complicated disease to treat because it presents in a spectrum – but whether a dog has a few worms or a lot, heartworm can be fatal if left untreated.

Dr. Karen Ward, Chief Veterinary Officer at Toronto Humane Society, has been treating heartworm for over 15 years. “It’s not a one-solution-fits-all type of treatment”, she explains. “If you think about the animal, the pet parent, and the environment, sometimes you have to make specific decisions for a specific individual, so we offer two types of heartworm treatments at Toronto Humane Society, but we almost exclusively use the melarosmine treatment in the shelter.”

Using melarosmine is the American Heartworm Society recommended treatment. It is slow, expensive, and potentially dangerous. It involves an antibiotic, a heartworm preventive to kill the baby heartworm called microfilariae, and three injections of a medication called melarsomine.

“Melarsomine is an arsenic-based drug, so it’s pretty serious and has a narrow margin of safety. After the melarosmine injection, you’re going to get a big worm die off – we never really know how many worms each animal has – so that first month, you’ll get a worm die off and maybe that’s half of the worms, and then the second month, the other half of the worms die,” Dr. Ward explains. “But we have to think about where the worms live. They actually live in the blood vessels that supply the lungs more than in the heart. Unlike other worms, like roundworm, the dog cannot poop out heartworms. The dead worms are in the blood vessels of the dog and then the dog’s own immune system is going to eventually nibble away at the dead worms, and those white blood cells are going to get rid of it. As that happens, the little bits of dead worm break off and get pushed forward in the blood vessels where it narrows and they’ll cause little thromboembolism, which are little clots. We know that’s going to happen. Sometimes it’s asymptomatic, and at other times it’s mildly symptomatic – or it can go all the way to being catastrophic.”

Coupling the difficult treatment for heartworm with limited exercise for a dog further complicates the treatment process. It is no walk in the park, quite literally, as strict exercise restriction is an essential component of the treatment process. If there is a large heartworm die off and a dog continues to exercise, there will be an abundance of worms moving into their blood vessels and the consequences are serious.

“We had big 40 kg dog, and he was living with his foster parents during treatment. They lived in a walkup apartment that was above a retail store. For him, after his melarsomine injections, walking down the flight of stairs to go to the bathroom, and then walking back up, caused him to become clinical. He presented with coughing and a bit of difficulty breathing, so that was too much exercise,” Dr. Ward shares. “He was too big for his foster parents to carry, and there was no elevator, so they ended up setting up a toileting area on their patio.” Luckily, with their new bathroom approach and some additional medication, the dog got better and eventually became heartworm free.

Toronto Humane Society conducts outreach trips where we send a group of volunteers to help treat animals that have no or very limited access to veterinary care. Dr. Ward shares, “Because they’re in areas where it’s difficult to access veterinary care, when pet parents are presented with the different treatment options, they almost inevitably choose the alternative, or slow kill treatment. It’s a much longer treatment (anywhere from 6 to 18 months) and does not use melarsome, but the dog still gets to be reasonably active for the most part. While it’s very efficacious, it’s not as efficacious as the melarsomine treatment.

With years of experience of treating heartworm, our experts have developed a highly effective system to care for and treat animals with the disease. We estimate that we’ve treated more heartworm cases than almost any other veterinary clinic in Canada thanks to the generosity of our donors. Not many other animal welfare organizations have the resources necessary to provide treatment for heartworm because it can be very costly.

The standard treatment, according to the American Heartworm Society, is that on day one of diagnosing the animal, they’re given a month’s worth of doxycycline, then after one month the melarosmine treatment is given. This means that their melarosmine treatment begins 60 days after they’ve been diagnosed.

“Doxycycline is an antibiotic and there is a bacterium that is a symbiotic bacterium with the heartworm – symbiotic means that they need each other. This bacterium, Wolbachia, and the heartworm don’t thrive without each other,” Dr. Ward explains. “As veterinary research and knowledge of heartworm evolved, it came to be understood how important the Wolbachia is in the lifecycle of the heartworm and how starting animals on this antibiotic decreases inflammation, decreases some of the inflammatory factors that this parasite produces, makes the worms a little weaker, and makes them easy to kill. The animal receives more effective treatment and has fewer side effects."

With new research and information available, Toronto Humane Society’s Dr. Karen Ward and Dr. Linda Jacobson modified the standard treatment to a more efficient and shorter treatment process. “We start doxycycline on day one – all treatment plans include doxycycline – and we give our first melarsomine injection on day 14. So, we start melarsomine 45 days earlier than the American Heartworm Society,” Dr. Karen Ward shares. “That means our treatment protocol is 45 days shorter, and our dogs have 45 days less of exercise restriction. We’re really shortening that treatment interval, which is better for the welfare and emotional wellbeing of the animal and allows us to help more animals.”

Our experience and desire to improve access to veterinary care in underserved communities led us to publish the first comprehensive review in 2021 of alternative heartworm treatments. This peer reviewed article piqued many conversations in the veterinary world. “Because of Dr. Linda Jacobson’s academic strengths and her skills in research and professional medical writing, we were able to share our learnings along with those of many other groups, so that folks working in the field, pets, and the people who care for them can also benefit,” Dr. Ward shares.

Thanks to their hard work, the review serves as a guide for veterinarians, detailing doses, duration, and efficacy. It has served as a go-to resource for veterinarians all around the world to provide pet parents with more options as we collectively work together to save more lives.