Texas A&M Research Sheds New Light On Heartworm’s Ancient Global History

Story by Camryn Haines, Ph.D., VMBS Marketing & Communications

April 1, 2026

By mapping genetic differences among global heartworm populations, the study establishes a baseline that may help researchers track emerging cases and monitor potential drug resistance.

For decades, scientists have assumed canine heartworms spread globally alongside modern dog movement, but new genomic research co-authored by researchers from the Texas A&M College of Veterinary Medicine & Biomedical Sciences (VMBS) suggests that assumption may be incomplete, pointing instead to a far older and more complex evolutionary history.

Heartworm disease remains one of the most significant and potentially fatal parasitic threats to dogs worldwide, particularly in the Southern United States, including Texas, where transmission remains highly prevalent. Despite its clinical importance, its deeper evolutionary history has remained largely unclear.

The international study, led by collaborators at the University of Sydney in Australia and the Wellcome Sanger Institute in the United Kingdom, represents the first effort to compare the full genomes of Dirofilaria immitis — the parasite that causes heartworm disease — from populations around the world. By analyzing samples collected across North America, Central America, Europe, Southeast Asia, and Australia, researchers were able to examine how genetically distinct heartworm populations are and how long they have been evolving.

“We wanted to understand how old heartworm is and how it spread globally,” said Dr. Guilherme Verocai, an associate professor in VMBS’ Department of Veterinary Pathobiology, director of the Parasitology Diagnostic Laboratory, and one of the study’s co-authors. “Did it emerge only after dog domestication and modern transport, or has it been associated with canids for thousands — even millions — of years?”

Looking Back In Evolutionary Time

Studying parasite evolution presents unique challenges. Unlike vertebrate hosts, parasites leave no fossil record, making it difficult to reconstruct their history through traditional paleontological methods.

Instead, the research team relied on whole-genome sequencing — a powerful genetic approach that examines the parasite’s entire DNA sequence rather than a small number of genetic markers.

Verocai said earlier studies relied on small portions of the parasite’s genetic code, often capturing only part of the evolutionary picture. Whole-genome sequencing provides a more comprehensive view of how heartworm populations are related and how they may have diverged over time.  

The genetic comparisons revealed clear differences among heartworm populations in different regions of the world, suggesting the parasite’s global distribution likely predates recent centuries of dog movement associated with colonization and trade.

In Australia, for example, heartworms were closely related to Southeast Asian populations — a pattern consistent with the historical movement of dingoes and early human migration across island chains thousands of years ago.

In North America, distinct genetic patterns suggest heartworm may have been present long before European colonization.

“Dogs were already in the Americas prior to European arrival,” Verocai said. “The genetic differences we see raise the possibility that heartworm was here earlier than we once assumed.”

Rather than revealing a single point of origin, the findings suggest heartworm’s history is intertwined with ancient canid movement, human migration, and ecological adaptation.

Why It Matters Today

Heartworm is one of the most consequential parasitic diseases in companion animal medicine. It infects the heart and pulmonary arteries, can be fatal if untreated, and drives much of modern veterinary preventive medicine.

Because of its impact, heartworm disease has shaped routine testing protocols, prevention strategies, and clinical decision-making in small animal practice. Yet its deep evolutionary history and global spread have remained poorly understood.

“This gives us a baseline,” Verocai said. “If we understand where heartworm populations came from and how they are related, we can better interpret what we’re seeing today.”

Whole-genome data provide insight into population structure, genetic diversity, and historical movement — information that could prove critical as researchers monitor emerging cases or investigate suspected drug resistance.

“If a case appears in a region where heartworm is not established, genetics can help trace where that parasite most likely originated,” Verocai said.

The study also strengthens the foundation for future research into resistance to macrocyclic lactones — the drug class used in heartworm preventives — by mapping genetic variation across global populations.

A Global Collaboration

The project relied on international collaboration, with researchers contributing samples from multiple continents. At Texas A&M, Verocai worked alongside Joe Luksovsky, laboratory manager of the VMBS Parasitology Diagnostic Laboratory, and Dr. Tiana Sanders, a graduate student in the Biomedical Sciences (BIMS) program and veterinary parasitology resident, to acquire and process heartworm samples submitted through the diagnostic laboratory, many of which originated from clinical cases.

The study also included samples from multiple host species, including dogs, cats, and a ferret, with several contributions originating from Texas.

“This would not have been possible without global collaboration,” Verocai said. “No single institution could answer a question at this scale.”

More Questions Remain

While the findings shed new light on heartworm’s ancient origins, they also highlight how much remains unknown.

“As researchers, we often end up with more questions than answers,” Verocai said. “There are still geographic gaps and areas of the world that need to be studied.”

For veterinarians and pet owners, the message is clear.

“Heartworm matters,” Verocai said. “It has endured for millennia, and prevention and routine testing remain essential.”

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For more information about the Texas A&M College of Veterinary Medicine & Biomedical Sciences, please visit our website at vetmed.tamu.edu or join us on Facebook, Instagram, and X.

Contact Information: Jennifer Gauntt, Director of VMBS Communications, Texas A&M College of Veterinary Medicine & Biomedical Sciences, jgauntt@cvm.tamu.edu, 979-862-4216

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