Ancient 'Spaghetti' in Dog Hearts Uncovers Surprising Heartworm Origins

Research led by the University of Sydney is reshaping scientific understanding of one of the world's most widespread canine parasites, suggesting heartworm disease has a far deeper and more complex evolutionary history than previously believed—including a possible ancient origin of Australian heartworms linked to dingoes. The findings have significance for developing treatments given the rise in drug resistance to the disease. Global study challenges old assumptions In a global genome-wide study of canine heartworm parasites, researchers analyzed more than 100 heartworm genomes collected from dogs and wild canids across multiple continents. The findings challenge the long-held assumption that heartworm spread largely through recent, human-mediated movement of domestic dogs. Instead, the study points to a new evolutionary paradigm, where ancient canid hosts such as wolves and dingoes played a pivotal role in shaping the global distribution of heartworms over tens of thousands of years. The paper is published in Communications Biology. Heartworm disease, caused by the parasite Dirofilaria immitis, is transmitted by mosquitoes and can be fatal for dogs if left untreated. Adult worms—often described as looking like strands of spaghetti—can grow up to 30 centimeters long and live in the blood vessels of the heart and lungs. One of the most intriguing findings relates to Australia. Genetic signatures in Australian heartworms suggest they may share ancestry with parasites from Asia. This raises the possibility that heartworm could have arrived in Australia with the continent's first dingoes, which are believed to have migrated from Asia thousands of years ago. However, the researchers caution that the evidence is not conclusive. An alternative explanation—that heartworm was introduced to Australia more recently, following European colonization—cannot yet be ruled out. Expert perspectives and future directions "While our data suggest an ancient link between Australian and Asian heartworms, the sample size means we need to be careful about drawing firm conclusions," said senior author Professor Jan Slapeta from the University of Sydney School of Veterinary Science. "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." The international team used whole-genome sequencing to compare heartworms from different regions, allowing them to reconstruct population histories and track how parasites diverged over time. By examining patterns across continents, the researchers identified distinct regional heartworm populations, shaped by the movements and isolation of ancient canids during ice ages and interglacial periods. Lead author Dr. Rosemonde Power, who completed her Ph.D. at the University of Sydney and is now based at Stockholm University, said the findings overturn long-standing assumptions in parasitology. "For decades, we assumed heartworms were spread mainly through recent human activity," Dr. Power said. "What we're seeing instead is evidence of deep co-evolution between heartworms and their canine hosts, even before humans were part of the picture." The research also has important implications for animal health today, particularly as drug resistance to heartworm treatments emerges in parts of the world. "Understanding where heartworms come from and how different populations are related helps us respond more effectively to disease and drug resistance," Professor Slapeta said. "Heartworms are not the same everywhere, and local history matters." The authors emphasize that while the study analyzed over 100 genomes, further sampling—particularly from under-represented regions—will be needed to refine timelines and test competing hypotheses, including the origins of heartworm in Australia. Still, the study marks a significant step forward in understanding the ancient, global journey of one of dogs' most dangerous parasites—and the long evolutionary history hidden within that unsettling spaghetti in a dog's heart.