Yosemite's war on hantavirus

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 Yosemite's 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 survived.

DNC Parks and 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, thanks in part to a massive outreach campaign. And the visitors are 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.

"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.

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.