WSU Study Finds Hantavirus Widespread Among Rodents Across Idaho and Washington Palouse
A Washington State University research team has found that the virus responsible for a deadly respiratory illness in humans is far more common among rodent populations in the Palouse region than public health officials had previously understood — raising new questions about exposure risks for residents of Latah County and neighboring areas of Idaho and Washington.
The study, led by researchers in WSU’s College of Veterinary Medicine, found that nearly 30% of rodents sampled in the region showed evidence of prior infection with Sin Nombre virus, the hantavirus strain that causes hantavirus pulmonary syndrome. Roughly 10% of animals tested were actively infected at the time of sampling, meaning they were capable of shedding the virus into their surroundings.
The findings were published in the Emerging Infectious Diseases journal, a publication of the Centers for Disease Control and Prevention. Field sampling was conducted during the summer of 2023 across farms and natural areas in Whitman County, Washington, and Latah and Benewah counties in Idaho — putting the study’s footprint squarely in the communities of the Palouse. A total of 189 animals were collected, including deer mice, voles, and chipmunks. Researchers detected both active infections and antibodies indicating past infection in deer mice and voles, suggesting the virus may circulate across multiple rodent species. You can read additional coverage of the regional findings here.
Virus Poses Serious Risk Despite Rare Human Cases
Sin Nombre virus was first documented during a 1993 outbreak in the Four Corners region of the American Southwest. Between 1993 and 2022, 864 confirmed human cases were recorded nationally, with a case-fatality rate of 36%. Of those cases, 109 occurred across Idaho, Oregon, and Washington combined.
Despite those historically low human infection numbers, researchers say the virus’s prevalence in the local rodent population may mean people are exposed more frequently than the data reflects. Stephanie Seifert, the study’s principal investigator and leader of the Molecular Ecology of Zoonotic and Animal Pathogens lab, noted the gap between how common the virus appears among rodents and how rarely severe human illness is reported.
“We were surprised both by how common the virus was locally and by how little data existed for the Northwest,” Seifert said. “We’re really just beginning to understand how widespread and complex this virus is in rodent populations here.”
Co-author Pilar Fernandez, a disease ecologist in the Allen School, echoed that concern. “People may be exposed more often than we realize,” Fernandez said, “but severe cases are more likely to be tested for hantavirus.” She described understanding the relationship between exposure and disease progression as the critical next step for researchers.
Deer Mice Are the Primary Carrier — and They’re Common on Palouse Farms
Sin Nombre virus is primarily carried by deer mice, a rodent species that commonly inhabits farms, outbuildings, barns, and rural homes throughout the Palouse. The virus does not spread person-to-person — human infections occur almost exclusively when people breathe in airborne particles from contaminated rodent droppings, urine, or nesting material.
Health officials consistently warn that activities which disturb rodent-contaminated areas — sweeping, vacuuming, or using leaf blowers in enclosed spaces — sharply increase the risk of inhaling viral particles. The recommended approach is to ventilate affected spaces thoroughly and use wet-cleaning methods, such as damp cloths or mop-based cleaning with disinfectant, rather than dry sweeping that can launch particles into the air.
For agricultural communities in Latah County, where grain storage facilities, machine sheds, and older farm structures often harbor deer mouse populations, awareness of these precautions is especially relevant heading into summer when residents are more likely to be cleaning out barns and outbuildings.
The research team also produced the first complete genome sequences of Sin Nombre virus strains from the Pacific Northwest. That genetic data revealed significant diversity among local viral strains and evidence of reassortment — a process in which viral genetic material mixes — which could help scientists track how the pathogen evolves over time and assist public health authorities in identifying sources of human exposure.
What Comes Next
Researchers say expanded study is needed, particularly to understand how frequently people in the Palouse region are actually being exposed and what behavioral factors may increase or reduce risk. Seifert and Fernandez have indicated they hope to pursue additional funding to extend their fieldwork. Local residents with concerns about rodent activity in or around their homes and agricultural properties are encouraged to contact their county health districts for guidance on safe cleanup procedures.