Study finds high prevalence of hantavirus in some areas of the Pacific Northwest

Peer-Reviewed Publication

Washington State University

News Release 21-May-2026

PULLMAN, Wash. — The Sin Nombre virus – a hantavirus that can cause a deadly respiratory disease in humans – may be more widespread among rodent populations in parts of the Pacific Northwest than previously recognized.

A recent study conducted in the Palouse region of Washington and Idaho found that nearly 30% of rodents showed evidence of past infection with the virus. About 10% were actively infected, meaning they were carrying and could potentially shed the virus. The study, which was led by researchers in Washington State University’s College of Veterinary Medicine, was published in the Centers for Disease Control and Prevention’s Emerging Infectious Diseases journal.

“We were surprised both by how common the virus was locally and by how little data existed for the Northwest,” said Stephanie Seifert, the study’s corresponding author and 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. “We’re really just beginning to understand how widespread and complex this virus is in rodent populations here.”

Sin Nombre virus (SNV), which causes hantavirus pulmonary syndrome, was first identified during a 1993 outbreak in the Four Corners region of the United States. From 1993–2022, 864 cases were reported in the U.S., with a 36% case-fatality rate. A total of 109 of those cases occurred in Idaho, Oregon and Washington.

Hantavirus has been in the headlines because of an outbreak of Andes virus on a cruise ship. While rare, Andes virus, which is found in South America and is a different species than SNV, is the only hantavirus known to spread between people.

SNV is primarily maintained in deer mice, which are common around farms, homes and outbuildings. Rodents can spread the virus among themselves through saliva and direct contact, but human infections typically occur when people inhale airborne particles from contaminated rodent droppings, urine or nesting materials.

Activities that disturb rodent droppings or nests can increase the risk of exposure by sending virus particles into the air. Sweeping or using equipment like leaf blowers in enclosed areas can be particularly hazardous. Instead, health officials recommend ventilating spaces and using wet-cleaning methods to reduce the risk of inhalation.

Field work for the study was done in the summer of 2023 when researchers trapped rodents across farms and natural areas in Whitman County, Washington, and Latah and Benewah counties in Idaho. In total, samples were collected from 189 animals, including deer mice, voles and chipmunks.

Once in the lab, the researchers identified active infections and antibodies, indicating a past infection, in both deer mice and voles, suggesting the virus may move between species.

The researchers also used the samples to produce the first full genome sequences of SNV strains from the Northwest. The sequences revealed high levels of genetic diversity and evidence of viral reassortment, or mixing of genetic material. The information can help public health officials and scientists track how the virus evolves in the future, in addition to assisting in identifying sources of infection and improving surveillance and testing.

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 researchers hope to expand their work in the future if additional funding becomes available, including studying how often people are exposed and how human behavior influences risk.

“"People may be exposed more often than we realize, but severe cases are more likely to be tested for hantavirus,” said Pilar Fernandez, a co-author on the study and a disease ecologist in the Allen School whose research focuses on the eco-epidemiology of zoonotic diseases. “Understanding that gap – how exposure translates into disease – is the next big step.”