New Mosquito-Borne Virus Flying Under the Radar in Montana

BUTTE, MT - A mysterious illness is spreading through Montana—but most doctors aren’t even looking for it.

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In a newly published study from the National Institutes of Health’s Rocky Mountain Labs, researchers revealed that a group of mosquito-borne viruses—some capable of causing memory loss, seizures, and even paralysis—are circulating silently through Montana.

The twist? Many healthcare providers may not recognize the symptoms, meaning Montanans are getting sick...and misdiagnosed.

As our state gears up for another mosquito season, let's take a look at this newly-emerging player in the hot seat.

More Than Just a Bug Bite

For years, most Montanans have only needed to worry about West Nile Virus when it comes to mosquito-borne diseases.

But a deeper look has revealed something far more complex.

Thanks to a collaborative effort between Rocky Mountain Labs and a retired neuroscientist from Missoula, researchers tested 900 anonymous blood samples from six Montana hospitals.

The results were shocking: between 21% and 40% of the samples showed evidence of past infection by viruses most people—and even many physicians—have never heard of.

The culprits? Jamestown Canyon Virus (JCV), La Crosse Virus (LACV), California Encephalitis Virus, and Trivittatus Virus—all members of a family of viruses known to cause swelling of the brain (encephalitis).

While most infections may cause only mild symptoms—or none at all—the more severe cases can be life-altering.

And these aren’t imported diseases. They’re spread by Aedes mosquitoes—common throughout Montana.

The Diagnosis Doctors Aren’t Making

The alarming truth is that these viruses are not new to the United States.

Jamestown Canyon Virus was first discovered in Colorado in 1961 and now pops up most commonly in the Midwest. La Crosse Virus, originally identified in Wisconsin, is typically found in the Appalachian and Mississippi River regions.

But Montana? That’s a recent and under-the-radar development.

In fact, it took a misdiagnosis in 2009 to bring any of this to light, when a Montanan initially thought to have West Nile Virus tested positive for JCV.

That sparked curiosity—and concern—that there might be more going on beneath the surface.

This isn’t just a case of mistaken identity. It’s a widespread blind spot.

Doctors aren’t testing for these illnesses. Many don’t know what to look for. And patients with lingering symptoms—headaches, confusion, even memory loss—might never connect the dots to that one mosquito bite last summer.

What This Means for You—and for Montana

The study’s authors are clear: this is not a cause for panic, but it is a call for awareness.

Most people infected with these viruses recover without serious issues. But a small percentage develop serious neurological symptoms, and without a proper diagnosis, they’re left without answers or treatment options.

Rocky Mountain Labs hopes their findings will raise awareness among local physicians, allowing for more accurate diagnoses and, ultimately, better care.

On a national level, the data could push forward much-needed research into antiviral treatments.

But here in Montana, the most immediate step is simply knowing these viruses exist—and that they’re closer than we thought.

The Invisible Outbreak Sweeping Montana

When we think of outbreaks, we imagine headlines, lockdowns, and rising case counts. But not all viruses make themselves known that way.

Some, like these mosquito-borne illnesses, drift silently through communities, infecting without fanfare, without alarms—sometimes without symptoms at all. That’s what makes them so dangerous.

Montana isn’t on the CDC’s radar for Jamestown Canyon or La Crosse Virus. But the viruses are here. They’ve been here. And the people getting sick aren’t even aware of it.

This isn’t just about mosquitoes. It’s about the diseases we don’t see coming—and the systems that aren’t yet equipped to spot them.

So this summer, as the mosquitoes return and we reach for our bug spray, remember: some of the most dangerous things in Montana don’t come with a warning.