Mono County California Confirms Third Hantavirus-Related Death

Mono County Public Health has confirmed a third fatality due to Hantavirus in the Town of Mammoth Lakes, a favorite outdoor destination for vacationers in Northeast California.

As of April 3, 2025, Mono County has recorded 27 cases since it was first reported here in 1993, the most in California. Twenty-one of these infections affected county residents, and six occurred among visitors who were infected in Mono. In 

As of 2022, 864 cases of Hantavirus disease were reported in the United States.

According to the U.S. CDC, Hantavirus is a serious and often fatal illness contracted through infected deer mouse droppings, urine, or saliva. It most commonly occurs in the late spring or summer.

“A third case of Hantavirus Pulmonary Syndrome (in 2025), each of which has been fatal, is tragic and alarming,” said Dr. Tom Boo, Mono County Public Health Officer, in a press release.

“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 weeks, but obtaining testing has taken time. The occurrence of three cases in a short period has me worried, especially this early in the year.

We’ve gone about a month without additional suspect cases, but remain concerned about the increased activity.

We believe deer mouse numbers are high this year in Mammoth (Eastern Sierra). An increase in indoor mice elevates the risk of Hantavirus exposure.

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 in the home or the workplace.

Many of us encounter deer mice daily, and there is some risk. We should pay attention to the presence of mice and be careful around their waste,” stated Dr. Boo.

Without a preventive vaccine available in 2025, the county says to avoid eating food that may have been contaminated by rodents and always wash your hands thoroughly after any potential exposure.