More than a quarter of mice on the Palouse show signs of previous Hantavirus infection, WSU study finds

May 24-Unusually high hantavirus levels were found in mice across the Palouse, according to a recent Washington State University study.

The WSU study found 29% of deer mice had signs of past hantavirus infection across farms and natural areas in Whitman County, Washington, and Latah and Benewah counties in Idaho. About 10% were actively infected, meaning they were carrying and could potentially shed the virus.

Study co-author Stephanie Seifert said she was "surprised" by how common the virus is in Eastern Washington and how little data there was in the Pacific Northwest.

"We're really just beginning to understand how widespread and complex this virus is in rodent populations here," said the WSU veterinary medicine professor.

A Virginia Tech study published last year found 13.4% of rodents across the United States showed signs of previous hantavirus infection. It is unclear why hantavirus levels in this region are elevated. Previous research of rodents' spread of the disease in Eastern Washington are more than 20 years out-of-date, said fellow WSU researcher Pilar Fernandez.

Human infection of hantavirus is rare, but with the level of infection within the rodent population, it will remain "endemic" among the deer mice of the region, she said.

"It's not one or two rodents. It's nearly a third of the rodents. It is circulating and it's not going away, Fernandez said. "It's here, it's being transmitted, and it's going to be a constant threat to the human population."

The hantavirus infection is spread by different species of rodents through their urine, saliva and droppings. Mice and other animals carrying the disease typically do not get sick, but humans are at high risk of death if they contract hantavirus.

The dominant form of hantavirus in North America is the Sin Nombre strain, which is spread by the deer mouse. There are no documented cases of human-to-human transmission of Sin Nombre hantavirus.

The Sin Nombre strain is different than the Andes strain, which has been the subject of intense scrutiny this month following the deaths of passengers on a Dutch cruise ship. The Andes strain is the only hantavirus strain known to be spread from person-to-person.

The Sin Nombre strain of hantavirus disease was first identified in a 1993 outbreak in the Four Corners region of the U.S. There have been 890 cases of hantavirus reported in the U.S. since then, mostly in the Western U.S. There have been 61 cases of hantavirus in Washington since 1993. Twenty of those Washington residents died from the disease after contracting the virus. Five or fewer cases of the virus are reported in Washington each year.

The researchers hope to expand their study in the coming years to see if the high levels are similar in other areas of Eastern Washington, including central Washington and rural areas around Spokane.

It is unclear how the levels of hantavirus infection may translate to human infection. There may be some mild human cases that go undiagnosed, said Fernandez.

"People may be exposed more often than we realize, but severe cases are more likely to be tested for hantavirus," she said. "Understanding that gap - how exposure translates into disease - is the next big step."

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This story was originally published May 25, 2026 at 8:09 AM.