Third Hantavirus-Related Death Confirmed in Mono County

MONO COUNTY, Calif. (April 3, 2025) – Mono County Public Health has confirmed a third death due to Hantavirus in the Town of Mammoth Lakes.  Hantavirus is a serious and often fatal illness which people can get through contact with infected deer mouse droppings, urine, or saliva. Deer mice are widespread in the Eastern Sierra region.

“A third case of Hantavirus Pulmonary Syndrome (HPS), each of which has been fatal, is tragic and alarming,” said Dr. Tom Boo, Mono County Public Health Officer. “We don’t have a clear sense of where this young adult may have contracted the virus. The home had no evidence of mouse activity. We observed some mice in the workplace, which is not unusual for indoor spaces this time of year in Mammoth Lakes. We haven't identified any other activities in the weeks before illness that would have increased this person's exposure to mice or their droppings.

We’ve been aware of this suspected case for some weeks, but it has taken time to obtain testing. The occurrence of three cases in a short period has me worried, especially this early in the year. Historically, we tend to see Hantavirus cases later in the spring and in the summer. We’ve now gone about a month without any additional suspect cases, but remain concerned about the increase in activity.

We believe that deer mouse numbers are high this year in Mammoth (and probably elsewhere in the Eastern Sierra). An increase in indoor mice elevates the risk of Hantavirus exposure. Therefore, it is crucial to take precautions and follow the prevention steps outlined below.

I want to emphasize that as far as we know, none of these deceased individuals engaged in activities typically associated with exposure, such as cleaning out poorly ventilated indoor areas or outbuildings with a lot of mouse waste. Instead, these folks may have been exposed during normal daily activities, either in the home or the workplace. Many of us encounter deer mice in our daily lives and there is some risk. We should pay attention to the presence of mice and be careful around their waste.”

Ongoing Investigation

Each Hantavirus case is investigated by local and state public health officials.  Each of our recent cases lived and worked in Mammoth Lakes and experienced illness beginning in February. When a person has died, we can only talk to people who knew them about the places they were and things that they did in the weeks before they got sick. We know that one person had numerous mice in their home. No evidence of mice was found in the other two homes. In all three cases there was some evidence of mice in places they had worked, but no major infestations were found. Investigators did note that one person did some vacuuming in one or more areas where investigators later found mouse droppings. Vacuuming can aerosolize the virus from mouse waste spreading the virus through the air and lead to infection.

Mono County has now recorded 27 cases since it was first reported here in 1993, the most in the State of California. Twenty-one of these infections affected County residents, and six occurred in visitors who were infected in Mono.   Hantavirus more commonly occurs in the late spring or summer, so three cases this early in the year is strikingly unusual.

Health Risks and Symptoms

People get infected with Hantavirus through contact with the feces, urine, or saliva of infected deer mice. Most often this happens by inhaling contaminated particles in the air, although getting waste in your mouth from contaminated hands can cause infection too.

The incubation period after infection is usually two or three weeks but can range from one to more than seven weeks, after which illness typically begins as a non-specific flu-like illness with fever, headache and body aches, also often with gastrointestinal symptoms such as nausea, vomiting, diarrhea and occasionally abdominal pain. These early symptoms are usually not mild. Muscle aches, for example, are often severe.

Typically, there is no cough in this early phase, and the presence of cough, runny nose or sore throat in the first day or two points more to common respiratory viruses such as flu or COVID.

Hantavirus often, though not always, progresses to involve the lungs after a few days, at which time a cough develops. Shortness of breath is a worrisome sign of worsening disease. This is Hantavirus Pulmonary Syndrome (HPS) and it frequently worsens rapidly, with potential death within a day or two.

About one-third of people with Hantavirus infections die. Prompt diagnosis and treatment improve outcomes, but testing is only available in specialized laboratories, making early recognition challenging.

The only source of Hantavirus in our area is from deer mice. It does not spread between people and other rodents do not carry it.

Prevention is Key

Be vigilant for signs of indoor rodent activity. Risk is highest in poorly ventilated spaces and with cleaning, which may stir up the virus. 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.

For more information please visit the Mono County Public Health Division and the California Department of Public Health. 

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