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Heartworm infections are increasingly common in pups—in 2022, more than 1.2 million dogs in the United States were documented with the condition, and infection rates have risen over the past two decades.
These spaghetti-like nematodes, formally known as Dirofilaria immitis, grow up to nearly a foot long. They can make their way into an animal’s blood vessels in the heart and lungs and prompt heart failure and severe lung disease, among other serious consequences, if left untreated. The disease affects cats, dogs, wolves, foxes, and ferrets, among other animals, but dogs are the natural host. In dogs, the worms can grow and reproduce, multiplying into hundreds of individuals.
Dog owners are often advised to prevent heartworm infections with drugs called parasiticides, but their widespread use has triggered some resistance among heartworms that make these medications less effective.
To combat drug resistance, it’s important to understand where and when these pesky worms came along. And, according to recent research, their story seems to stretch far further back in history than previously thought.
Researchers have long thought that heartworms came from Europe or Asia and traveled the world over the past two to four centuries by way of dogs carried around by people. Past studies have supported this theory with evidence of low diversity among heartworms, but these hints came from small samples that weren’t representative of populations around the globe.
Instead, it’s possible that heartworms dotted the globe long before people domesticated dogs. For one, heartworms tend to take up residence in a wide range of meat-eating mammals, especially canids, the broader dog family that emerged around 40 million years ago (our species emerged much later, around 300,000 years ago). And mosquitos, which are crucial in the spread of heartworms, have been around even longer.
To trace heartworms’ journey around the world, an international team of scientists examined more than 100 heartworm genomes taken from dogs and their wild canid kin in Australia, the U.S., Central America, Europe, and Asia, enabling them to learn how these parasites diverged over their evolutionary history.
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Overall, the team found “distinct genetic differences between heartworms from different continents, indicating a more ancient dispersal in canid hosts than previously recognised,” according to a paper published today in Communications Biology.
“What we can say with confidence is that heartworm evolution is far older and more complex than a simple story of parasites hitchhiking with modern dogs,” said study co-author Jan Slapeta, a veterinary parasitologist at the University of Sydney, in a statement. Instead, it appears that heartworms wandered the world and evolved along with their canine hosts, long before humans turned pups into pets.
Their genetic evidence supports certain canid (and heartworm) migration scenarios during interglacial periods, when canids could move more freely, and bouts of isolation during ice ages. For example, some wolves and coyote-like dogs may have first left North America, where they originally evolved, by traversing the Bering Land Bridge up to around 7 million years ago. As these dogs roamed the globe and formed connected communities, heartworms may have done so in tandem.
More recently, heartworms could’ve hitched a ride to far-flung Australia with the first dingoes to arrive on the continent, around 4,000 years ago—the genetic analysis suggested that Australian heartworms have shared ancestry with parasites from Asia, where dingoes originally lived. Still, the authors noted, their small sample size means that they can’t yet discount the theory that heartworms spilled into Australia after European colonization. To make more definitive conclusions on heartworm movement and evolution overall, they’ll need to gather more samples.
Now, new heartworm treatments and monitoring strategies can take these geographic distinctions into consideration. While human influence may not have sparked these parasites’ global parade, the authors noted in the paper that climate change and the increasing movement of people and their pets “will all influence the distribution and adaptation of heartworms.”
“Understanding where heartworms come from and how different populations are related helps us respond more effectively to disease and drug resistance,” Slapeta said. “Heartworms are not the same everywhere, and local history matters.” 
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Lead image: Pexels / Pixabay
