Researchers studying rodents in the Pacific Northwest have uncovered surprising evidence that a potentially deadly virus may be quietly circulating much more widely than experts once thought.
The virus, known as the Sin Nombre virus, can cause hantavirus pulmonary syndrome, a rare but very dangerous lung disease in humans.
The study was carried out by scientists from Washington State University’s College of Veterinary Medicine and published in the medical journal Emerging Infectious Diseases.
The findings suggest that many rodents living near farms and rural communities in Washington and Idaho have either been infected in the past or are currently carrying the virus.
Hantaviruses are a group of viruses mostly spread by rodents. Different types of hantaviruses exist around the world. Some can cause severe lung disease, while others mainly affect the kidneys. In North America, the Sin Nombre virus is the main hantavirus linked to hantavirus pulmonary syndrome.
Although human infections are uncommon, the disease can become deadly very quickly. Early symptoms often look similar to the flu and may include fever, muscle aches, headaches, chills, nausea, and tiredness. But within days, some patients develop severe breathing problems as fluid fills the lungs.
Doctors first identified the Sin Nombre virus during a mysterious outbreak in 1993 in the Four Corners area of the southwestern United States. Healthy young adults suddenly became critically ill, and many died from sudden respiratory failure. Scientists eventually traced the disease back to infected deer mice.
Since that outbreak, 864 cases of hantavirus pulmonary syndrome have been reported across the United States between 1993 and 2022. More than one-third of patients died from the disease.
The Pacific Northwest has reported far fewer cases than some southwestern states, but researchers believed there was not enough information about how widespread the virus truly was in local wildlife.
To investigate, scientists carried out field work during the summer of 2023 across parts of Washington and Idaho. Rodents were trapped in both natural habitats and farming areas in Whitman County, Washington, and Latah and Benewah counties in Idaho.
The research team collected samples from 189 rodents, including deer mice, voles, and chipmunks. Deer mice are known to be the main carriers of the Sin Nombre virus and are commonly found around barns, garages, homes, sheds, and agricultural buildings.
When scientists tested the animals, they found unexpectedly high infection rates. Nearly 30% of the rodents showed signs of previous exposure to the virus. About 10% were actively infected and potentially capable of shedding the virus into the environment.
These findings suggest that people living or working in rural areas may encounter the virus more often than previously recognized.
Humans usually become infected by breathing in contaminated dust particles from rodent droppings, urine, or nesting materials. Cleaning old storage buildings, cabins, sheds, or barns can increase exposure risk, especially if rodent waste is disturbed.
Experts warn against dry sweeping or using leaf blowers in enclosed spaces because these activities can push virus particles into the air. Instead, health officials recommend opening windows for ventilation and using wet-cleaning methods to safely remove contaminated material.
Lead researcher Stephanie Seifert said the team was surprised by both the high local infection rates and the lack of previous data from the region.
The researchers also discovered something else important. Evidence of infection appeared not only in deer mice but also in voles. This may mean the virus can move between rodent species more easily than scientists once believed.
In the laboratory, researchers performed full genome sequencing of the virus strains found in the Pacific Northwest. This marked the first time complete Sin Nombre virus genomes from this region had been mapped.
The genetic analysis revealed large amounts of viral diversity and signs of reassortment, a process where viruses exchange genetic material. This kind of viral mixing may influence how the virus evolves over time.
Understanding these genetic changes may help public health experts improve future disease surveillance, identify infection sources more accurately, and develop better testing systems.
One interesting mystery remains. Even though the virus appears common in rodents, relatively few human cases have been officially reported in the region. Scientists believe some infections may be mild or may go undiagnosed because symptoms can resemble other common illnesses.
Co-author Pilar Fernandez explained that severe cases are far more likely to receive hantavirus testing. Milder cases may never be recognized.
The researchers hope to continue studying the virus in the future if additional funding becomes available. They want to examine how often humans are exposed, which activities carry the greatest risk, and why some people become severely ill while others may not.
The study has also gained attention because hantaviruses have recently appeared in international news due to outbreaks involving the Andes virus in South America. Unlike the Sin Nombre virus, the Andes virus can spread from person to person, making it especially concerning.
Reviewing the findings, this study appears highly valuable because it reveals that Sin Nombre virus circulation in wildlife may be far more widespread than previously understood in the Pacific Northwest.
The discovery of active infections in multiple rodent species and the identification of genetically diverse viral strains suggest the virus ecosystem is more complicated than earlier studies indicated.
However, the research also raises important unanswered questions about why human infections remain relatively rare despite the high rodent infection rates. More studies will be needed to understand how exposure leads to disease and whether mild or unnoticed infections are occurring more often than currently recognized.
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Source: Washington State University.
