Does a new study upend the origin of heartworms in dogs?

A new population genomics analysis of canine heartworms is challenging the long-held idea that Dirofilaria immitis spreads worldwide mostly through modern, human-driven movement of domestic dogs.

Although the researchers caution that the results are not conclusive, the findings could reshape how veterinarians think about surveillance and manage emerging drug resistance.

An international team of researchers led by the University of Sydney sequenced and compared whole genomes from 127 adult heartworms collected from dogs and wild carnivores across Australia, the United States, Central America, Europe, and Asia. Rather than finding a single, relatively uniform global parasite population consistent with a recent “hitchhiking” narrative, the investigators report strong genetic structuring by continent and signals that older canid movements likely helped shape heartworm’s distribution over tens of thousands of years.¹

“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 senior author Jan Šlapeta, MVDr, PhD, from The University of Sydney School of Veterinary Science.¹

The study was published on January 20, 2026, in Communications Biology.

Using more than 100 genomes’ worth of data (and ultimately 127 adult worm specimens), the researchers reconstructed population histories, examined admixture patterns, and assessed genetic diversity across regions.² They found that heartworms from different continents show distinct genetic differences in nuclear DNA, suggesting geographic separation and dispersal patterns that predate the past few centuries of dog trade and travel.²

The study also included a smaller set of parasites taken from nondog hosts, including cats, foxes, a ferret, a golden jackal, a leopard, and a wildcat, which allowed the team to test whether host species or geography better explained the observed genetic patterns.²

Overall, they found that geography, rather than host species, was the dominant signal in the data set, supporting the view that transmission can occur between domestic animals and wildlife when competent hosts and mosquito vectors overlap.²

Lead author Rosemonde Power, PhD, a researcher at Stockholm University, said the findings overturn many prior assumptions about the origins and spread of heartworms.

“For decades, we assumed heartworms were spread mainly through recent human activity,” she said. “What we’re seeing instead is evidence of deep coevolution between heartworms and their canine hosts, even before humans were part of the picture.”¹

Genomic differences across regions may have practical implications as resistance and treatment failures are investigated, Šlapeta said.

“Understanding where heartworms come from and how different populations are related helps us respond more effectively to disease and drug resistance,” he said. “Heartworms are not the same everywhere, and local history matters.”¹

That framing aligns with growing calls to pair clinical vigilance—testing, prevention adherence, and investigation of suspected lack-of-efficacy cases—with better regional surveillance. The American Heartworm Society, for example, continues to emphasize year-round prevention, appropriate diagnostic testing, and careful case management to reduce morbidity and limit opportunities for resistant parasites to spread.³

The new study does not change the fundamentals of prevention and case management for practicing clinicians today. However, it may influence how researchers and public health partners prioritize sampling, interpret regional differences in parasite response, and design future surveillance approaches that account for both wildlife reservoirs and geographic population structure.^1-3

References

1. Ancient ‘spaghetti’ in dogs’ hearts reveals surprising origins of heartworm. EurekAlert! January 20, 2026. Accessed January 29, 2026. https://www.eurekalert.org/news-releases/1112914
2. Power RI, Abdullah S, Walden HS, et al. Population genomics reveals an ancient origin of heartworms in canids. Commun Biol. 2026;9(1):68. doi:10.1038/s42003-025-09250-x
3. American Heartworm Society Canine Guidelines for the Prevention, Diagnosis, and Management of Heartworm (Dirofilaria immitis) Infection in Dogs. American Heartworm Society; 2024. Accessed January 29, 2026. https://d3ft8sckhnqim2.cloudfront.net/images/AHS_Canine_Guidelinesweb03FEB2025.pdf?1738626677