Most Americans are aware of the relationship between deer and parasites—specifically the Lyme disease–carrying deer tick. But ticks aren’t the only creepy-crawlies infesting the animals. In a recent study published in Science Advances, researchers report finding malaria parasites in many American deer.

Although human malaria has been eradicated in many parts of the world, including North America, the disease is still a threat elsewhere. "Malaria is a top parasitic disease in humans and wildlife," study co-author Ellen Martinsen said in a press release. "It's important that we gain a better understanding of its diversity and distribution not just across humans but across other species too."

Ellen Martinsen (foreground), Joseph Schall (background). Image Credit: Joshua Brown

There are more than 100 species of Plasmodium (malaria parasite) species, but only five of those are any threat to humans. The other 95 find other hosts in birds, lizards, bats, rodents, monkeys, and, it seems, deer.

Martinsen had not set out to look at deer. The biologist had collected mosquitoes from the Smithsonian’s National Zoological Park and was scanning their DNA in search of a bird-infecting malaria when she saw a parasite she didn’t recognize. More analysis revealed that the mosquito in question had been feeding on white-tailed deer (Odocoileus virginianus)—and that the deer was infected.

This is not the first time Plasmodium has been found in white-tailed deer, but it is the first time it’s been taken seriously. In 1967, a malaria expert reported finding Plasmodium parasites in a single white-tailed deer in Texas. But he could only find one infected deer, and he had no additional proof. At the time, co-author Joseph Schall said in the press release, “malaria wasn't supposed to be in mammals in the New World. It was like the guy was reporting he saw Big Foot."

The nay-sayers may soon change their minds. Martinsen, Schall, and their colleagues collected 1978 mosquitoes (representing 27 species) from sites in Washington, D.C. and San Diego. They examined the mosquitoes’ stomach contents—that is, other animals’ blood—for parasites, sequencing the DNA of any Plasmodium they could find. They also looked at blood samples from nine of the zoos’ ungulates (members of the deer family).

The results were surprising for a number of reasons. First, they found a pretty high prevalence: up to 25 percent of white-tailed deer living in Virginia and West Virginia sites were infected. Deer tested in San Diego were free and clear of Plasmodium, as were the other ungulate species. The researchers also found that even the infected deer had very low levels of the parasites—so low that the actual disease did not seem to be affecting them. DNA tests revealed two separate but related types of Plasmodium, which suggests an evolutionary split shortly after deer first arrived in the New World 2.3 to 6 million years ago.

"You never know what you're going to find when you're out in nature—and you look," Martinsen continued in the press release. "It's a parasite that has been hidden in the most iconic game animal in the United States. I just stumbled across it."