Rodent study finds Sin Nombre in Pacific Northwest

A study of 189 rodents across eastern Washington and western Idaho found about 10% carried Sin Nombre hantavirus at the time samples were taken, while nearly 30% showed signs of past infection—highlighting how people’s exposure to rodents and their excretions

By May. the Pacific Northwest had already been placed in the path of a hantavirus outbreak that began on a cruise ship in early May. Three people have died, and many more have been sickened. But long before that cluster became visible in the news. researchers were out in eastern Washington and western Idaho collecting samples from the region’s own small wildlife—looking for clues about what may be circulating quietly on farms and at the edges of rural homes.

In a study published in April in Emerging Infectious Diseases. Stephanie Seifert. an assistant professor at Washington State University and a co-author. reported results that suggest more of the local rodent population may carry Sin Nombre hantavirus than previously suspected. The work was conducted in the summer of 2023, predating the current outbreak.

The researchers collected fecal and tissue samples from a total of 189 rodents of various kinds. including several types of voles. mice and chipmunks. on farms and in other areas in eastern Washington and western Idaho. They tested those samples for hantavirus antibodies—evidence an infection was occurring at the time—and for viral RNA. which can indicate if a rodent has ever carried the virus.

The numbers they found landed with weight. Around 10 percent of the rodents had Sin Nombre at the time the samples were taken. Almost 30 percent showed signs of past infection.

Sin Nombre belongs to the same family as the Andes virus that has been at the center of the cruise ship outbreak. but the two behave differently in how they spread. Sin Nombre is not transmissible from human to human, unlike the Andes virus. Instead, people become infected when they are exposed to rodents and their excretions, such as their feces and urine.

That limitation has kept Sin Nombre cases scarce, but it has never made the disease harmless. The virus was first identified in 1993 after 11 people died and almost two dozen more got sick in the Four Corners region of the U.S. Its mortality rate is between 35 and 50 percent.

Hantavirus cases are relatively rare across the United States, with most cases occurring in the South West. Even so, the Pacific Northwest has accounted for a disproportionate share of the total. Of the 864 cases reported in the United States between 1993 and 2022, 109 were in Idaho, Oregon or Washington, according to the study.

Seifert says the field still lacks the kind of baseline data that would make it possible to say whether rodent carriage in the area has grown over time. “We know tilling is disruptive to rodents which flee the croplands to surrounding refuge. including rural homes and outbuildings. ” she said. The question hanging over the new findings is what happens when land use changes.

Climate, too, may be part of the story. Seifert says wetter winters can lead to increased vegetation, which can support a larger rodent population. Warmer winters can also cause prolonged breeding seasons and improve the odds of surviving the cold—again boosting population numbers. And the way humans use the land can increase exposure to animals.

In the region, farmers have begun using techniques that don’t employ tilling. That could change how often rodents move and where they settle—potentially shaping how frequently people encounter them indoors and around outbuildings. Seifert put the dilemma plainly: “Will conversion to no-till lead to fewer human-rodent interactions or support more robust and diverse rodent communities that continuously expand outward to neighboring homes and support a higher baseline prevalence in [Sin Nombre virus]?. I don’t know the answer.”.

More research could help narrow that uncertainty, she added—but the ability to study these systems depends on funding that can’t be assumed to last forever. Seifert said her team’s current funding has run out.

“If there is anything folks in the U.S. should take from this. it’s that expertise on infectious disease systems is not like a faucet that you can turn on and off when convenient. ” she said. “If you want hantavirus or Ebola virus experts to be here. ready to jump into action with answers and solutions. then we need to fund our public health research and basic science.”.

The immediate takeaway from the summer 2023 sampling is simple and uncomfortable: in places where people and rodents overlap—on farms and in rural surroundings—Sin Nombre appears to be present at levels that were not fully expected. The longer-term takeaway sits in the unanswered questions: how climate shifts and changes in farming practice affect rodent populations. how that translates into human exposure. and whether the scientific capacity to track it will be sustained.

hantavirus Sin Nombre Pacific Northwest rodents Emerging Infectious Diseases Stephanie Seifert Washington State University Andes virus cruise ship outbreak public health research