YOSEMITE NATIONAL PARK, Calif. — The weapons of choice against the tiny mice that run around here carrying a potentially fatal virus include disinfectant, peanut butter and 20 years’ worth of animal science.
Last year 10 people became ill with hantavirus, an outbreak concentrated among a cluster of canvas-walled tent cabins in Curry Village, a popular destination on the valley floor. Three people died in what is the largest rash of cases related to the Sin Nombre strain of the virus, which was discovered in 1993. The other seven victims, including an unidentified 42-year-old Orange County, Calif., man, survived.
DNC Parks & Resorts of Yosemite, which operates the lodgings and other businesses in Yosemite National Park, has gone to great lengths to prevent another outbreak. Ninety-one Signature Tent cabins, the relatively new structures that turned out to be well-suited for harboring infected mice, were torn down earlier this year.
Park employees now take more time to clean structures — at least 15 minutes for the Curry Village tent cabins — and watch for mouse droppings. Park authorities also have redoubled efforts to educate visitors about the importance of stowing food so mice, as well as bears, deer and other animals, can’t get to it. All over the park, there are fliers urging guests to take precautions.
The preventive surge has brought a comeback for one of the nation’s greatest natural treasures: Visits to the park dropped a bit early last fall, as word of the outbreak spread (thanks in part to a massive outreach campaign by DNC). For the year, visitors numbered 3.996 million, down 2.5 percent from 2011. But through May of this year, 1.023 million people entered the park, up 1.58 percent compared with the same period in 2012. Eighty percent of park visitors go to Yosemite Valley.
From hard-core climbers who pull themselves up the sheer granite face of El Capitan, to hikers who explore the wilderness’ 750 miles of trails, to families who walk in the valley’s meadows and wade in the gentle Merced River, people are flocking here again. And they’re undeterred by last summer’s unprecedented outbreak.
“The transparency of the National Park Service reinforces my confidence that they’re eradicating the problem and keeping everyone safe,” said Kevin Kearn, 45, a retired Army lieutenant colonel from Huntington Beach. After he and his wife, Tammy, hiked 45 miles in four days, they relaxed at Curry Village’s crowded Pizza Deck, enjoying beers and slices.
“It’s a Herculean task to preserve the environment, maintain the park, and still keep it accessible,” he said.
No one, not even the biologists who poured into the valley after the first hantavirus cases were reported, can explain exactly why so many people developed infections in such a short time. And despite the intense preventive efforts, there’s no guarantee there won’t be more cases.
“When you have that many humans, chances are there’s going to be a food source,” said Danielle Buttke, a veterinary epidemiologist at the National Park Service in Fort Collins, Colo.
The “reservoir” for the outbreak was traced to the deer mouse, which is common all over the West. About 14 percent of the deer mice in Yosemite have the Sin Nombre strain of the hantavirus, which was first recorded during a small outbreak in the Four Corners area 20 years ago.
The virus doesn’t spread to any other animals, and the virus doesn’t exist in common house mice. But it’s in the deer mouse feces, urine and saliva, and if any of those substances gets stirred up — say, by sweeping along a floor — it can get aerosolized. If enough of it is inhaled, infection can occur.
In 2009, the park built 91 Signature Tents, which featured canvas on the outside, then a layer of insulation, then a layer of sheet rock. The goal was to have a warmer-cabin option in wintertime for guests who wanted it. Mice were able to get into the insulation and hide, and the virus came from the walls.
“That particular kind of structure, we hadn’t anticipated a rodent infestation would be there, but that was the smoking gun in this case,” said Barbara Knust, an investigator with the Centers for Disease Control and Prevention’s infectious-disease branch in Atlanta.
On Aug. 16, 2012, the California Department of Public Health announced two confirmed cases of hantavirus. On Aug. 27, two more cases were reported; by mid-September, the cases had grown to eight, and three deaths had been confirmed. A total of 10 cases — nine of them in the Signature Tents of Curry Village, another, milder case in a High Sierra camp — were reported.
Hantavirus can’t be spread from one person to another, but one by one the number of cases piled up. The park established a call center at its headquarters in the valley, and more than 260,000 people who had stayed overnight in the park between June 1 and Aug. 28 were contacted, sometimes just by calling people who lived near the guests.
“We were investigating, sleuthing, trying to locate people just to make sure everybody knew of the exposure,” said Tom Medema, chief ranger for interpretation and education at Yosemite.
On the advice of the CDC, National Parks Service and state health officials, the Signature Tents were torn down, and the old-style single-wall tents were built in their place. They have only the canvas outer shell, then the frame of green 2-by-4s. Now, there are fewer places for mice to hide. As part of the “exclusion” process, steps to the entrance were elevated, and more nails were inserted to cut down on the gaps on the canvas. Any mouse droppings found will be sprayed with disinfectant and scooped up.
Guests are asked to store food in metal “bear boxes” outside the tent cabins, which are locked at night, when bears and mice are more active.
The state health department traps some mice and tests them for the virus, and the park also traps mice — baited with peanut butter — to control the population.
DNC Parks & Resorts spokeswoman Lisa Cesaro said in an email: “First and foremost, our thoughts go out to those who have been affected by hantavirus. Once we were notified, we worked hand in hand with the National Park Service and public health officials on addressing this issue. As you know, the Signature Tent Cabins have been removed from Curry Village, and we have taken and continue to take steps recommended by the National Park Service, which were developed in consultation with the California Department of Public Health and the Centers for Disease Control and Prevention.”
Marsha Blount, a San Diego physician staying in Curry Village for the first time with her husband and two children, came prepared: When they arrived at their tent cabin on a recent visit, she immediately sprayed all surfaces with Lysol. “When I looked into it, it seemed like they had done the right things, and I was trying to hedge our bets by cleaning everything,” she said.
A hike up the spectacular rock staircase along the Mist Trail, leading to the top of Vernal Fall, shows why visitors can’t stay away from Yosemite. It’s like a Mardi Gras parade, with hundreds of people cramming the trail to catch a glimpse of a rainbow arching near the thundering falls. Another hiker was spotted literally skipping up a hill.
Yet Yosemite still hasn’t put the outbreak in the past. A Chino Hills, Calif., woman, Cathy Carrillo, 50, has filed a multimillion-dollar lawsuit against Delaware North Cos. She was among the survivors of the outbreak and was hospitalized for two weeks after getting infected last summer, her lawyer said. Although the effects of hantavirus aren’t supposed to be long-lasting, she’s had persistent fatigue and weakness ever since, attorney Mark Algorri said.
“She had pneumonia, kidney failure, acute respiratory failure. It was pretty dastardly. She really was on the brink of death there,” Algorri said.