90-Second Read: Hantavirus Is More Common in Pacific Northwest Rodents Than We Knew

A new WSU study found nearly 30% of rodents in parts of Washington and Idaho showed evidence of Hantavirus infection. 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. Human cases in the three-state Pacific Northwest region over nearly 30 years total 109. 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.

Between 1993 and 2022, 864 cases were reported nationally, with a case fatality rate of 36%. 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. 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. Samples were collected from 189 animals including deer mice, voles, and chipmunks.

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. Nearly 30% of the rodents showed evidence of past infection with Sin Nombre virus, meaning they had antibodies indicating prior exposure. Rodents spread the virus among themselves through saliva and direct contact. 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.

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 Palouse region does not have a particular history as a Hantavirus hotspot in the way the Four Corners region does. Health officials recommend ventilating spaces before working in them and using wet-cleaning methods rather than dry sweeping to reduce the risk of inhalation.