Yosemite Hantavirus Surrounded In Mystery, Deer Mice Population Increase May Be Key
September 13, 2012

Yosemite Hantavirus Surrounded In Mystery, Deer Mice Population Increase May Be Key

Lawrence LeBlond for redOrbit.com - Your Universe Online

Doctors and health officials working to find why a hantavirus outbreak at Yosemite National Park this summer has been so severe, have struggled to come up with a “good explanation,” according to an epidemiologist with the US Centers for Disease Control and Prevention (CDC).

“For Yosemite, why this year of all years is there an increased number of cases?” pondered CDC´s Barbara Knust. Nearly 20 years after first being identified in the US, the virus is better understood but no less troublesome.

Doctors now know it causes hantavirus pulmonary syndrome (HPS), a severe respiratory disease. It is transmitted through the droppings of deer mice, and not transmissible between humans. Early treatment offers a better chance of survival, but there is no cure and a third of all patients die. Doctors are calling this year´s outbreak unprecedented--as more than one hantavirus case from the same location in the same year is extremely rare.

However, recent trapping related to the investigation of the outbreak has indicated that the deer mouse population has swelled and is larger this year than in most other years, said Dr. Vicki Kramer, head of the California Department of Public Health´s vector-borne disease section.

Agency officials laid traps out on two different occasions (August 21 and 23) at Curry Village, where seven of the eight cases of hantavirus had stemmed from. About 50 percent of the traps caught mice, and 13.7 percent of the rodents tested positive for antibodies of sin nombre virus, indicating they have, or have had, deadly hantavirus, said Kramer.

Trapping resumed last week, after additional hantavirus cases cropped up and were linked to Yosemite. Traps were also laid in Tuolumne Meadows, an area of the park close to the High Sierra Loop where one other hantavirus case hailed from. Traps there yielded a 45-percent success rate in capture.

The higher success rate in capturing mice could indicate a larger mouse population, as two previous years of trapping only succeeded in catching 17 percent and 25 percent in 2007 and 2008 respectively. Experts seem to agree that a larger deer mouse population would lead to more cases of hantavirus. The 1993 outbreak in the Four Corners region of the Southwest was attributed to an abundant population of deer mice that year.

For Yosemite, “that could be a contributing factor,” Kramer told Los Angeles Times℠ Kate Mather. “This seems to be supporting that hypothesis.”

The mice collected in the trapping events were euthanized and stored in freezers in case experts needed blood and tissue samples in the future, said Kramer.

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

Officials are looking at all possible factors for this year´s outbreak. One factor is an increase in the development of Curry Village, noted Kramer. More people are visiting the campground now, and they bring in more food, which attract mice. Also, increased construction of tent cabins offer mice more protection as well, as natural predators of the mice are more likely to stay away from the human-populated campground.

Yosemite may still provide more clues. Health officials and epidemiologists are continuing to compile information about the cases, hoping to determine the exact combination of factors that led to the outbreak.