National health researchers have recently identified a series of mosquito-borne viruses floating around Montana that local physicians may not know how to spot.
A study just published by National Institute of Health scientists from Rocky Mountain Labs shows a sampling of Montanans have tested positive for antibodies of the Jamestown Canyon Virus and its related serogroup of viruses–all carried by a common Montana mosquito.
Researchers say most of the time, the viruses cause mild encephalitis, or brain swelling, present mild, flu-like symptoms, or no symptoms at all.
In some cases, the symptoms can be more extreme, causing memory loss, paralysis or even seizures.
Researchers started looking for the presence of the viral strains in the state after a Montanan was misdiagnosed with West Nile Virus and tested positive for Jamestown Canyon Virus in 2009.
Rocky Mountain Labs researchers teamed up with a retired neuroscientist from Missoula to obtain and test 900 random blood samples from six Montana hospitals against the serogroup of mosquito-borne illnesses. Along with Jamestown Canyon virus (JCV), researchers also tested the samples for La Crosse virus (LACV), California encephalitis virus and Trivittatus virus.
The findings, published in the April 2025 issue of Emerging Infectious Diseases, show 21%-40% of blood samples had neutralizing antibodies to at least one of those viruses, meaning the person’s immune system had faced the viruses at some point.
These viruses are found in several other parts of the US. JCV was first found in Colorado in 1961 and most cases are found in the Midwest. Doctors diagnose about 40-70 cases of JCV in the US each year.
Around the same time JCV was found in Colorado, La Crosse virus was isolated in Wisconsin. Now, the virus is widespread through the Appalachian Mountain region, and Mississippi and Ohio river basins. About 30-90 severe hospitalized cases are reported each year.
Rocky Mountain Labs says it’s important that local physicians know the viruses are present in Montana and what the symptoms may look like.
Along with being able to identify and properly diagnose cases, researchers hope identifying the viruses in yet another state can help drive momentum for the development of therapies to treat the viral illnesses.
