White-tailed deer are some of North America's most familiar megafauna, frequenting parks and yards across the continent. But they've been hiding something from us, a new study finds: In some places, up to a quarter of them harbor malaria parasites.

Earth has more than 100 species of malaria parasites, all in the genus Plasmodium. They infect an array of reptiles, birds and mammals, but aside from a lone Texas white-tail in 1967, they hadn't been documented in any deer species. And no other native Plasmodium species is known to infect any mammal in the Americas.

Malaria is notorious, of course, for its effects on humans. Five species are known to infect people, killing 600,000 per year and sickening 200 million. Spread by Anopheles mosquitoes, these parasites are a huge public-health threat in many tropical and subtropical regions, especially in sub-Saharan Africa, South Asia and Oceania.

The newfound parasites probably don't pose a risk to humans, according to the scientists who discovered them, and it's not even clear yet if they're dangerous to deer. But since they're in up to 25 percent of Eastern U.S. white-tails, they do highlight how many bizarre ecological secrets are still lurking under our noses.

"We were just completely surprised to find malaria in a backyard mammal species that people see on a daily basis," says Ellen Martinsen, a research fellow at the Smithsonian Conservation Biology Institute (SCBI) who made the discovery. "We may find that malaria parasites are common in other mammals, too."

white-tailed deerDespite a high prevalence of the parasites, individual deer have low levels in their blood. (Photo: Larry Smith/Flickr)

'Hidden in plain sight'

Martinsen and her colleagues found at least one, possibly two Plasmodium species in white-tailed deer. They've published a new study about them in the journal Science Advances, but it all started by accident, explains co-author Robert Fleischer, head of the SCBI's Center for Conservation and Evolutionary Genetics (CCEG).

"Ellen was screening for bird malaria in mosquitoes at the National Zoo, and she got this strange sequence," Fleischer tells MNN. "We expected everything we got to be bird malaria, because there isn't really any mammalian malaria here."

While studying mosquitoes with this strange Plasmodium sequence, the researchers realized one still had a blood meal from a recent victim. Hoping to identify that host, they took a closer look at some genes from its blood.

"And bam, it was a perfect match to white-tailed deer," Fleischer says. That was a shock, so they started testing white-tailed deer at the National Zoo and other nearby locations. "We found this parasite was prevalent in these areas, so that's when we decided to look farther and expanded the survey across North America."

map of Plasmodium in white-tailed deerThis map shows where the researchers sampled potential hosts for Plasmodium. Red sites are where infections were found; the star is where the only previous deer Plasmodium was reported in 1967. (Image: Martinsen et al. Science Advances 2016)

Blood, sweat and deers

The team screened 308 white-tails from 45 counties in 17 U.S. states, and found infected deer at more than a third of the sites. "Overall prevalence at the Plasmodium-positive sites was 18%" in white-tails, they report, "but reached approximately 25% at sites in Virginia and West Virginia."

Yet despite such a high prevalence, they also found a low density of parasites in each deer — about one for every 65,000 red blood cells. That's reminiscent of the 1967 deer, found in Texas, which had one parasite per 30,000 red blood cells.

Low-level infections aren't uncommon among Plasmodium in mammals, the researchers note, and this concentration is apparently still high enough for the parasites to be picked up by mosquitoes and find "what are likely rare mates" in the insects' blood meal. It's not clear what effect it has on the deer, though.

"Given that white-tailed deer are the most intensively studied wild vertebrates in the U.S, and nobody has picked up on any pathological effects, it's unlikely that it has significant effects on adults," Martinsen says. "But it may have some effects on fawns, or some subclinical effects that we don't know about yet."

That kind of subtle harm has been seen in other hosts, Fleischer adds. "There are studies where you don't see any major effect of a parasite on a host, like a bird, but if you look at its life span, it might have lower reproductive success or a shorter life."

Anopheles mosquitoThe spread of malaria parasites depends largely on the mosquitoes that inject them into new hosts. (Photo: Shutterstock)

Passing the buck

Some of the newly discovered parasites match the 1967 case, Fleischer says, but others seem to be a previously unknown species. A genetic analysis shows the two groups diverged 2.3 million to 6 million years ago, suggesting they may have been carried across the Bering Land Bridge from Asia by ancestors of white-tailed deer.

The parasites weren't found in Western U.S. white-tails, which are less abundant than their eastern relatives. One positive test came from Louisiana, and the rest were all east of the Mississippi River. The researchers can't be sure they're totally absent from western white-tails, but Martinsen points out eastern deer occupy wetter habitats.

"It might be a matter of vectors, since there's better mosquito habitat in the East," she says. "But we do need to do more screening in the western part of white-tail range."

The team also tested 151 wild elk, mule deer and pronghorn in areas that overlap with white-tail range, plus nine species of domestic ungulates. No sign of Plasmodium was found, suggesting this infection doesn't easily spread beyond white-tailed deer.

What about humans? One of the five Plasmodium species that can infect us is a natural pathogen of wild macaques in Southeast Asia, and other types of zoonotic parasites are also known to spread between humans and animals. Could these newly discovered parasites make an even larger jump from white-tailed deer to people?

"It's pretty unlikely," Fleischer says. "Malaria parasites can make leaps, but we don't expect any major leaps in this case. Especially since we don't even seem to find it in any other ungulate species in the white-tailed deer range or at the zoo. So we don't know it can't happen, but it's not likely."

white-tailed deer in grassA white-tailed deer hides in tall grass at Seney National Wildlife Refuge in Michigan. (Photo: Seney NHA/Flickr)

Our big backyard

These parasites are still shrouded in mystery, and it will take much more research to reveal their secrets. In the meantime, rather than worrying about malarial deer, the study's authors say the main takeaway should be an appreciation for how much we still have to learn about the ecosystems around us. If we missed malaria parasites in white-tailed deer until now, who knows what else we've been missing?

"This just shows the power of surveillance," Martinsen says. "We were out surveying mosquitoes and just happened to stumble upon mammalian malaria parasites. It shows the power of going out into nature and turning over stones and looking for things. And in the medical realm, if we study more hosts and invertebrates, are we likely to find more parasites like this hidden in plain sight?"

"You never know what's in your backyard," Fleischer adds, "until you investigate."

Russell McLendon ( @russmclendon ) writes about humans and other wildlife.