Trapping of deer mice indicates a larger population, a possible factor in the outbreak of infections. Three Yosemite visitors have died from the disease.

Recent trapping related to the investigation indicates that the park’s deer mouse population is larger this year, said Dr. Vicki Kramer, head of the California Department of Public Health’s vector-borne disease section. Deer mice are the primary carriers of hantavirus in the U.S.

Rodent Trappings Provide Evidence

Agency officials have twice laid peanut butter-laced traps for the rodents at the park, Kramer said. The first traps, set out between Aug. 21 and Aug. 23, were centered on Curry Village, where seven of the eight hantavirus cases have been traced to tent cabins.

About 50% of the Curry Village traps caught mice, and 13.7% of the rodents tested positive for antibodies of sin nombre virus, indicating that they either have, or have had, hantavirus. The statewide average is about 14%, Kramer said.

Trapping resumed last week, after additional cases of hantavirus were linked to Yosemite — including one traced to the High Sierra Loop that links Yosemite Valley with Tuolumne Meadows and other areas. Traps were also laid in Tuolumne Meadows, where about 45% were successful, Kramer said.

That could indicate a larger mouse population, she said. In 2007, only 17% of traps in the area caught mice; in 2008, 25%. Antibody results for the second set of traps were not yet available.

Some experts have wondered if a population boom of deer mice contributed to the Yosemite outbreak. Scientists have attributed the 1993 outbreak in the Four Corners region of the Southwest to an abundant deer mouse population that year.

“That could be a contributing factor,” Kramer said of the Yosemite cases. “This seems to be supporting that hypothesis.”

Officials have called the Yosemite outbreak unprecedented — more than one hantavirus infection from the same location in the same year is rare. The disease is typically transmitted to humans as they inhale dust or dirt containing the droppings or urine of infected mice.

The mice collected from Yosemite were euthanized and stored in freezers in case experts need their blood or tissue for additional research, said Kramer, who added that fewer than 100 mice had been trapped.

“Our objective is not rodent reduction but risk assessment, by trying to get a general idea of mice abundance,” Kramer said.

Park Visitors May Increase Rodent Populations

It could take months to complete the investigation into the Yosemite outbreak, which is being conducted by state and federal agencies, said Danielle Buttke, a veterinary epidemiologist with the National Park Service.

Officials are looking at other factors, among them the density of development in Curry Village, she said. The popular campground offers more food for mice, as well as protection — natural predators are more likely to be scared off by such a large human presence.

kate.mather@latimes.com