Hantavirus Is More Common in Pacific Northwest Rodents Than Anyone Expected. Here Is What the Data Shows.

Sin Nombre virus does not get a lot of attention in veterinary continuing education. It is not a disease that presents in companion animal patients. It does not generate the kind of clinical caseload that drives conference programming. But it kills more than a third of the humans who develop severe disease from it, it lives in the rodents that share space with farms, outbuildings, and rural homes across the Pacific Northwest, and a new study from Washington State University suggests it is significantly more widespread in that region than the existing surveillance data indicated.

For veterinary professionals working in large animal and mixed practice settings, and for any clinician whose clients live and work in rural environments, this is worth understanding.

What the Study Found

Researchers in WSU's College of Veterinary Medicine conducted a study in the Palouse region of Washington and Idaho, trapping rodents across farms and natural areas in Whitman County, Washington, and Latah and Benewah counties in Idaho during the summer of 2023. Samples were collected from 189 animals including deer mice, voles, and chipmunks.

The results were more striking than the research team anticipated. Nearly 30% of the rodents showed evidence of past infection with Sin Nombre virus, meaning they had antibodies indicating prior exposure. About 10% were actively infected, meaning they were currently carrying the virus and potentially capable of shedding it.

The study was published in Emerging Infectious Diseases, the CDC's peer-reviewed journal, and produced the first full genome sequences of Sin Nombre virus strains from the Pacific Northwest. Those sequences revealed high levels of genetic diversity and evidence of viral reassortment, the mixing of genetic material between strains, which has implications for how scientists and public health officials will track the virus as it evolves.

"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 WSU'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."

What Sin Nombre Virus Is and Why It Matters

Sin Nombre virus causes hantavirus pulmonary syndrome, a severe and potentially fatal respiratory disease in humans. The virus was first identified during a 1993 outbreak in the Four Corners region of the United States. Between 1993 and 2022, 864 cases were reported nationally, with a case fatality rate of 36%. Of those cases, 109 occurred in Idaho, Oregon, and Washington combined.

The primary reservoir host for Sin Nombre virus is the deer mouse, a species that is extremely common around farms, homes, and outbuildings throughout the Pacific Northwest and much of North America. Rodents spread the virus among themselves through saliva and direct contact. Human infections, however, do not typically occur through animal bites or direct contact. They occur when people inhale airborne particles from contaminated rodent droppings, urine, or nesting materials.

That transmission route is what makes certain activities particularly dangerous. Sweeping, using leaf blowers in enclosed spaces, or any activity that disturbs rodent droppings or nesting material without proper precautions can send virus particles into the air. Health officials recommend ventilating spaces before working in them and using wet-cleaning methods rather than dry sweeping to reduce the risk of inhalation.

A New Finding Worth Noting

The WSU study also identified active infections and antibodies in both deer mice and voles, which suggests the virus may be moving between species rather than remaining confined to deer mice as the primary reservoir. That finding has implications for understanding how the virus maintains itself in the environment and for assessing exposure risk across different ecosystems and land use types.

The genetic diversity and reassortment patterns documented in the first Pacific Northwest genome sequences indicate that the viral population in this region has a more complex evolutionary history than a single introduction would suggest. That kind of data is the foundation for better surveillance tools and more accurate detection methods going forward.

The Gap Between Rodent Prevalence and Human Cases

One of the most clinically and epidemiologically interesting aspects of this study is the apparent disconnect between how common the virus appears to be in rodent populations and how rare severe human cases remain.

Nearly 30% of rodents in the study area showed past infection. Human cases in the three-state Pacific Northwest region over nearly 30 years total 109. That gap could mean the virus is less efficient at transmitting to humans than the rodent prevalence alone would suggest. It could also mean that many human exposures are occurring but not resulting in severe disease and therefore going untested and undetected.

"People may be exposed more often than we realize, but severe cases are more likely to be tested for hantavirus," said co-author Pilar Fernandez, 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."

That question has direct relevance for clinical decision-making. A patient presenting with an acute respiratory illness and a history of working in or around farm structures in the Pacific Northwest is a patient whose exposure history matters. Hantavirus pulmonary syndrome can be difficult to distinguish from other severe respiratory illnesses early in its course, and the window for intervention is narrow.

What This Means for Veterinary Professionals

Veterinary professionals working in large animal, mixed animal, or rural practice settings in the Pacific Northwest are among the populations with the most consistent and sustained exposure risk. Farm calls, barn work, handling equipment stored in outbuildings, and any activity that brings a practitioner into close contact with spaces where rodents live and defecate represents a real, if generally low, exposure opportunity.

The practical precautions are straightforward and worth reviewing with clients who live and work on farms as well. Rodent control around farm structures reduces the reservoir. Ventilating enclosed spaces before working in them matters. Wet-cleaning methods when clearing out buildings are significantly safer than dry sweeping. Personal protective equipment, including appropriate respiratory protection, is appropriate when working in heavily contaminated environments.

The study's findings also underscore the value of expanded surveillance. The researchers note that this level of prevalence was not reflected in existing data for the Pacific Northwest because the data largely did not exist. The Palouse region does not have a particular history as a hantavirus hotspot in the way the Four Corners region does. It simply had not been studied at this level before. Where else that same gap exists in regional surveillance is an open question.

The WSU research team hopes to expand this work if additional funding becomes available, including studying how often people in the region are actually being exposed and how human behavior influences risk.

The full study is published in Emerging Infectious Diseases. For information on hantavirus prevention and exposure risk, visit the CDC at cdc.gov.

Vet Candy covers infectious disease, One Health, and clinical news for 50,000 plus veterinary professionals. myvetcandy.com