A new study has found that a dangerous virus carried by rodents may be far more common in parts of the Pacific Northwest than scientists once believed.
Researchers studying rodents in Washington and Idaho discovered that many animals were infected with the Sin Nombre virus, a hantavirus that can cause a severe and sometimes deadly lung disease in humans.
The study was led by scientists from Washington State University’s College of Veterinary Medicine and was published in the journal Emerging Infectious Diseases. Researchers say the findings raise important questions about how often people may be exposed to the virus without realizing it.
The Sin Nombre virus, often shortened to SNV, is the main cause of hantavirus pulmonary syndrome in the United States. Hantavirus pulmonary syndrome is a rare but extremely serious illness that attacks the lungs and can quickly become life-threatening.
The disease first gained national attention in 1993 after a deadly outbreak in the Four Corners region of the United States, where Arizona, New Mexico, Colorado, and Utah meet. Doctors at the time were puzzled by healthy young adults suddenly developing severe breathing problems and dying within days. Scientists later identified the Sin Nombre virus as the cause.
Since then, health officials have reported 864 hantavirus pulmonary syndrome cases in the United States between 1993 and 2022. About 36% of infected people died. While the disease remains rare, its high death rate makes it especially concerning.
The new study focused on the Palouse region of Washington and Idaho, an area known for farmland, rolling hills, and rural communities. Researchers trapped rodents in farms, grasslands, and natural environments across Whitman County in Washington and Latah and Benewah counties in Idaho during the summer of 2023.
In total, scientists collected samples from 189 rodents, including deer mice, voles, and chipmunks. Deer mice are considered the main carriers of the Sin Nombre virus and are commonly found near homes, barns, sheds, and farms.
The results surprised the research team. Nearly 30% of the rodents showed evidence of previous infection, meaning they had been exposed to the virus at some point in their lives.
Even more concerning, around 10% of the rodents were actively infected at the time of testing. This means they were likely capable of spreading the virus through saliva, urine, droppings, or nesting materials.
Lead researcher Stephanie Seifert from Washington State University said the team was surprised both by how widespread the virus appeared to be and by how little information existed about hantavirus in the Pacific Northwest.
Scientists believe the virus mainly spreads between rodents through direct contact and saliva. Humans usually become infected by accidentally breathing in tiny virus particles from contaminated rodent droppings, urine, or nesting materials.
This can happen when people clean old sheds, garages, barns, cabins, or storage areas where rodents have been active. Activities such as sweeping or using leaf blowers may stir virus particles into the air, increasing the risk of inhalation.
Health experts recommend ventilating enclosed spaces before cleaning and using wet-cleaning methods instead of dry sweeping. Wearing gloves and masks while handling rodent waste may also reduce risk.
The study also revealed another important discovery. Researchers found evidence that the virus may not be limited to deer mice alone. Signs of infection appeared in both deer mice and voles, suggesting the virus could move between different rodent species.
Scientists additionally created the first complete genome sequences of Sin Nombre virus strains from the Pacific Northwest. Genetic sequencing allows researchers to examine how viruses change and evolve over time.
The team found high levels of genetic diversity and signs of something called viral reassortment. This happens when viruses exchange genetic material, potentially creating new viral combinations. Tracking these changes may help scientists better understand how the virus spreads and evolves in nature.
The findings may also help public health officials improve testing and identify where future outbreaks come from.
Interestingly, although the virus appeared relatively common in rodents, confirmed human cases remain uncommon in the region. Researchers believe this may mean some human infections go undetected, especially milder cases that never receive testing.
Co-author Pilar Fernandez explained that severe cases are more likely to be identified because patients become seriously ill and seek hospital care. Mild infections, however, may be mistaken for flu-like illnesses or may never be diagnosed at all.
The researchers hope to continue studying the virus if future funding becomes available. They want to better understand how often people are exposed, which behaviors increase risk, and why some infections become severe while others do not.
The research has become especially timely because hantaviruses have recently gained attention in global health news. Another hantavirus called Andes virus, found in South America, has been linked to outbreaks and is unusual because it can spread between people. In contrast, the Sin Nombre virus has not been shown to spread from person to person.
Reviewing the study findings, the research appears important because it highlights how little scientists still know about hantavirus circulation in many parts of the United States. The high infection rates found in rodents suggest human exposure may happen more often than previously believed.
However, the relatively low number of reported human cases may indicate that severe disease is uncommon or that mild infections often go unnoticed.
The study also shows the value of monitoring wildlife diseases before they become larger public health problems. Continued surveillance and better public awareness may help reduce future infections and improve early detection.
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Source: Washington State University.
