A new study finds nearly 30% of rodents in Washington and Idaho’s Palouse region had been infected with the Sin Nombre virus, with about 10% actively carrying it—raising the likelihood of exposure for people in the Pacific Northwest where deer mice are common.
For people living in Washington and Idaho’s Palouse region, the warning isn’t about a new outbreak—it’s about how much of the virus is already quietly circulating.
A new study found nearly 30% of rodents tested—189 mice. voles. and chipmunks—had been infected at some point with the Sin Nombre virus. or SNV. a hantavirus strain that can cause a deadly respiratory illness in humans. About 10% of the rodents were actively infected, meaning they were carrying the virus and could potentially shed it.
Researchers also reported that the findings point to one rodent species being able to infect another, a detail that matters because it widens the number of animals that could help keep the virus moving through local ecosystems.
SNV is the most common hantavirus strain in the United States. It is different from the Andes strain of hantavirus that recently caused three deaths among passengers on the cruise ship MV Hondius earlier this month. Andes is rare but can be spread from person to person. while SNV is more common—but people are infected only through exposure to rodents.
The study’s lead author, Stephanie Seifert of Washington State University’s College of Veterinary Medicine, said the results were both surprising and revealing for the region.
“we were surprised both by how common the [SNV] virus was locally and by how little data existed for the Northwest,” Seifert said.
Sin Nombre virus is the most common cause of hantavirus pulmonary syndrome in the United States. It was first identified during a May 1993 outbreak in the Four Corners region of the United States. From 1993 to 2022, at least 864 cases were reported in the United States, with a 36% case-fatality rate.
How the virus gets from animals to people is also specific—and unsettling. Rodents spread the virus among themselves. Humans typically get infected by inhaling airborne particles from contaminated rodent droppings, urine, or nesting materials.
SNV is primarily found in deer mice, which the study described as common around farms, homes, and outbuildings. But the research team collected samples from 189 animals that included deer mice, voles, and chipmunks, suggesting the virus may be transmitted between species.
“Despite the relatively high prevalence of the virus in rodents, reports of human infections remain rare, which could indicate some infections are going unnoticed,” the study reported.
The work was conducted by Washington State University’s College of Veterinary Medicine and published in the Centers for Disease Control and Prevention’s Emerging Infectious Diseases journal. Field work took place in the summer of 2023. with researchers trapping rodents across farms and natural areas in Whitman County. Washington. and Latah and Benewah counties in Idaho.
The study adds detail to an ongoing question: if the virus is present at high rates in local rodent populations. why are confirmed human cases still relatively uncommon?. The authors didn’t answer that directly—but the possibility that some infections are being missed sits at the center of the new findings.
Seifert is also principal investigator of the Molecular Ecology of Zoonotic and Animal Pathogens lab in the College of Veterinary Medicine’s Paul G. Allen School for Global Health.
hantavirus Sin Nombre virus SNV Pacific Northwest Palouse region deer mice Washington Idaho Emerging Infectious Diseases CDC rodent-borne disease