North America’s most popular game species, the white-tailed deer, harbors a secret: low levels of a malaria parasite that have only now been detected thanks to advanced DNA technology. Though this particular species of parasite poses little risk to humans, researchers say the find could reshape our understanding of malaria’s origins.

There are more than 100 species of malaria parasites, distributed on every continent except Antarctica. Those that infect birds and lizards are widely distributed, even on seemingly isolated ocean islands, and certainly in the Americas. Yet scientists believed that the microorganisms that infect mammals originated in the Old World, mainly Africa and Asia.

The new findings were discovered by chance. Researchers led by Ellen Martinsen, a biologist at the Smithsonian Conservation Biology Institute’s genetics center in Washington, D.C., were searching for the source of malaria parasites in birds at the national zoo. Using polymerase chain reaction (PCR) technology, which amplifies DNA to make it easier to study, they identified a genetic signature of an unexpected malaria parasite, Plasmodium odocoilei, previously unknown in the Americas. The researchers were able to obtain a large enough sample of blood from the mosquito’s enlarged abdomen to trace its origin to white-tailed deer. “We weren’t out there, testing a hypothesis,” Martinsen says. “We serendipitously stumbled upon this weird sequence.”

To find out how common the infections are, the researchers screened more than 300 white-tailed deer with PCR. They found 41 animals harbored the parasite, from 10 of the 17 states surveyed. No infected deer were identified in the west, but in the east, malaria parasites were widespread. Twenty-five percent of the animals tested at sites in Virginia and West Virginia carried them, the team reports today in Science Advances.

No signs of the parasite turned up in the other hooved species they tested, including cows, gazelles, goats, elk, oryx, alpaca, donkeys, and Przewalski horses.

The study marks the first finding of a malaria parasite species of mammals that is native to the Americas. (Scientists had largely discounted the significance of a 1967 report of what appears to be the same parasite in single specimen of white-tailed deer in Texas.)

The new research suggests that these malaria parasites have a long evolutionary history in the New World, dating back to the ancestors of white-tailed deer that made their way to the continent across the Bering Land Bridge 2.3 million to 6 million years ago Other as-yet-to-be discovered malaria parasites of mammals may be “hidden in plain sight,” the researchers write.

The findings are especially surprising in light of how intensively the popular game species has been studied, says David Hewitt, a wildlife biologist at Texas A&M University, Kingsville, who was not involved in the research. “This story suggests there is still much we don’t know about the natural world.”

Martinsen says future research should examine whether low-level parasite infection has caused disease in deer that previously has been undetected.