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MONO COUNTY, Calif. (MARCH 26, 2025) – Mono County Public Health has confirmed a second death related to Hantavirus in the Town of Mammoth Lakes. Hantavirus is a serious and often fatal illness spread primarily through contact with infected deer mouse droppings, urine, or saliva. Deer mice are widespread in the Eastern Sierra region.
“Hantavirus Pulmonary Syndrome is uncommon but extremely dangerous,” said Dr. Tom Boo, Mono County Health Officer. “We urge residents and visitors to pay attention to the presence of mice indoors and other enclosed spaces and take precautions to reduce the risk of infection, especially when cleaning up areas where rodent activity is present.”
Ongoing Investigation
Each Hantavirus case is investigated by local and state public health officials. The exposure for one of the two confirmed cases this year may have occurred while vacuuming rodent waste, while the other individual was likely exposed at home during a known infestation. A third possible case is currently under investigation. Prior to these cases, the last confirmed Hantavirus infections in Mono County occurred in 2019.
Since the first local case in 1993, Mono County has recorded 26 confirmed Hantavirus cases tied to exposures in the county—the most in the State of California. Twenty of these infections were in county residents, and six in people who visited Mono County and became infected. Two cases within a three-week span during late winter are unusual, as most cases are typically reported in spring and summer.
Rodent trapping in places where the first case may have been exposed found that 27% of captured deer mice tested positive for Hantavirus—consistent with the county’s typical baseline of around 25%.
Health Risks and Symptoms
Hantavirus causes a severe illness that progresses rapidly once the lungs are involved. Early symptoms usually include fever, fatigue, headache and muscle aches, sometimes with gastrointestinal symptoms, followed by coughing and shortness of breath. The disease is often fatal, with a mortality rate of approximately 30%. Prompt diagnosis and treatment improve outcomes, but testing is only available in specialized laboratories, making early recognition challenging.
Prevention is Key
Public Health urges community members and visitors to remain vigilant for signs of rodent activity, particularly in homes, cabins, sheds, garages, and other enclosed spaces. Infested vehicles may be risky too. Follow these guidelines to reduce your risk:
- Seal up all gaps in your home larger than a pencil’s width to prevent mice from entering.
- Store food in rodent-proof containers.
- Trap mice using snap traps (not glue or live traps).
- Avoid vacuuming or sweeping rodent droppings, nests, or urine.
- Air out enclosed spaces for at least 30 minutes before activities.
- Spray contaminated areas with a disinfectant or a freshly made 10% bleach solution and let sit for five minutes before wiping clean.
- Wear gloves and an N-95 (or higher) mask when cleaning. Contact Public Health for free N-95 masks.
Do not eat food that may have been contaminated by rodents, and always wash your hands thoroughly after any potential exposure.
Mono County Public Health can be reached at (760) 924-1830. The California Department of Public Health has more hantavirus information here.
