Our pets can carry many different parasites, from ticks and fleas to various worms. But where did these parasites come from and how did they get here? A common assumption is that parasites have travelled alongside humans by hitchhiking with our various animal companions throughout history, such as dogs, cats, and livestock species. This has become the general default dogma, reflecting a natural human tendency to place ourselves in the centre of… well, everything. Although human movements are undoubtedly behind the geographical spread of many parasites, this may not necessarily be the case for all of them.
Among all the parasites infecting dogs, heartworms are perhaps the most gruesome. Heartworms are long, spaghetti-looking parasites that live inside the hearts of dogs, wolves, jackals, coyotes, foxes, and many more. The adult female worms are about 30 cm long. Imagine a strand of spaghetti in your bowl. The males are half this length. Living in the heart, or more precisely, in the artery that carries blood from the heart to the lungs, heartworms can cause quite serious problems for dogs. This is why veterinarians in many countries recommend that dogs be on year-round prevention against heartworms.
Imagine having this “spaghetti” in your bloodstream… No thanks! For the parasite, though, this is the perfect home – an evolutionary masterpiece. After all, parasites only need two things: food and shelter. And what could be better than a constant supply of blood for nourishment, combined with an artery for physical protection? It’s the perfect environment for them.
Heartworms have managed to spread to nearly every corner of the globe. But where did they come from?
At the University of Sydney, Australia, we wanted to determine how old this “perfect environment” in the bloodstream was, and where heartworms actually came from. We teamed up with parasitologists from all over the world and gathered over 100 of these heartworms. Next, we sequenced their entire genomes to see what their DNA could reveal about their history.
What first drove us to this question was the growing problem of drug resistance in heartworms in the USA. Here, the drugs we currently rely on are unfortunately becoming less effective. Understanding where heartworms came from isn’t just an academic curiosity, it’s critical for tackling drug resistance. Tracking and monitoring resistance is therefore one of the most important tools veterinarians have.
For a long time, it was widely believed that heartworms in the USA were the same as those found in Europe, Australia or Asia. This view was primarily based on the assumption that heartworms spread globally only recently, mostly through human movement. But this assumption has never been rigorously tested. We had a hunch that there was much more to the story, especially since canids have been around far longer than humans. Canids originated ~40 million years ago, while hominids are only ~5-6 million years old, and humans emerged ~300,000 years ago in Africa. Dogs were domesticated much later, sometime between 40,000 and 15,000 years ago. Canids roamed the Earth long before humans appeared, and could have potentially carried heartworm parasites long ago. This prompted us to question whether the genomes of heartworms today might contain any evidence of a deep co-evolutionary history with ancient canids.
Our hunch was correct: heartworms are indeed ancient, and they dispersed across the world much earlier than previously thought. In our genome-wide study, we found that each continent harbours its own distinct heartworm population. Their population size histories show periods of similarity during interglacial times, when ancient canids likely moved freely between North America and Eurasia, and clear separations during ice ages, when ancient canids populations became isolated.
Not all heartworm translocations were old, as humans have affected their distribution too. When we first sequenced some heartworm genomes from Europe and Central America, we did not believe it at first. For our own sanity, we went back and redid it again. The result was the same – heartworms from Central America were carrying a clear genetic signal from Europe, and this signal was rather recent. What could explain this? One possibility is the European colonisation of the Americas, as there are firm records of European dogs being carried on Columbus’ voyages.
So, where did heartworms come from? Just as our own DNA carries echoes of Neanderthals and Denisovans, the genomes of heartworms carry the footprints of those from when ancient wolves roamed the worlds during and between ice ages. Their story reminds us that origins matter, not just for us, but for the parasites that have been evolving alongside canids for millions of years. Understanding that deep history isn’t just fascinating; it’s key to protecting our dogs today.
With this new knowledge, we are better equipped to address the emerging issue of drug resistance in heartworms. Because heartworm disease affects dogs globally, local expertise is essential; we can no longer assume that heartworms are the same in all countries, and that what works in one region will work equally well in another. Heartworms have been outsmarting dogs for thousands of years, but with genomic insights, we finally have a chance to stay ahead.