A dangerous tick-borne virus that first surfaced in humans in Missouri in 2009 appears to be common in wildlife across the central and eastern United States, according to a new study. Researchers at the Centers for Disease Control and Prevention (CDC) in Fort Collins, Colorado, found evidence of the so-called Heartland virus in deer, raccoons, coyotes, and moose in 13 states. Doctors should be on the lookout for human patients, the researchers say, because they might go undetected. But other researchers say such warnings are premature.

The first known human cases of the Heartland virus were two farmers in northwestern Missouri. Both became very sick in 2009, with symptoms including fever, fatigue, headache, lack of appetite, nausea, and diarrhea. Both had also recently been bitten by ticks, but tests for the usual tick-borne suspects—such as several Ehrlichia species, a group of intracellular bacteria—came back negative. Both patients recovered, although one of them kept suffering from fatigue and headaches.

It wasn't until 3 years later, in 2012, that scientists reported that the patients had most likely been infected by a hitherto unknown agent. The virus, named after the Heartland Regional Medical Center in St. Joseph, Missouri, where the farmers had been treated, is a species of the genus Phlebovirus. That group also includes the mosquito-borne Rift Valley fever virus and several other agents linked to human disease.

Since then, seven more Heartland cases—including two fatal ones—have been reported in Missouri, Tennessee, and Oklahoma, and researchers have begun to unravel the virus's ecology and geographical spread. For a study published in 2013, they collected more than 50,000 ticks at the Missouri farms and elsewhere in the region; they found that the lone star tick, Amblyomma americanum, was the only species carrying the virus and thus most likely the culprit. Another study in northwestern Missouri, published last year, fingered white-tailed deer and raccoons as likely hosts for the virus. Ticks probably feed on these animals as tiny larvae in the late summer and fall, they concluded, and infect humans during the following spring and summer when they're nymphs.

In the new study, CDC's Nicholas Komar and Kasen Riemersma—who's now at the University of California, Davis—used existing blood samples from white-tailed deer, raccoons, moose, and coyotes collected between 2009 and 2014 in 19 states, both in the center of the lone star tick's range and its periphery, and tested them for antibodies against the Heartland virus. Out of 1428 animals, 103 tested positive, the researchers report in a paper posted this week in Emerging Infectious Diseases: 55 deer, 33 raccoons, 11 coyotes, and four moose.

The positive samples came from 20 geographical clusters spread out over 13 different states, from Maine to Texas. "These findings should encourage clinicians and public health officials to consider [Heartland virus] as a potential source of illness throughout the eastern United States," the duo writes.

But not everyone is convinced. The lone star tick doesn't occur in northern New England, so it's not clear why the Heartland virus would circulate there, says medical entomologist Durland Fish, a professor emeritus at Yale University. It may well be that the antibodies found by the team weren't directed against the Heartland virus but some other agent; it's not uncommon for cross-reactivity to occur in serological tests. In fact, Fish isn't convinced that the Heartland virus is tick-borne; the fact that it was found in ticks doesn't mean they also transmit the virus, he says. "The whole thing is very speculative.”

Komar acknowledges that the team was surprised to find evidence of the virus outside the lone star tick's range. There could be another tick species acting as host in the northern United States, he says. It's also possible that the antibodies were directed against another virus, as Fish suggests, he acknowledges—but if so, it would be a closely related and unknown phlebovirus, which still would be something to watch closely. "We wanted to raise awareness about this in the medical community," Komar says. But Fish says it's too early for that. "There's enough paranoia about tick-borne diseases out there right now.”