(CBS/AP) The California Department of Public Health (CDPH) said four more cases of hantavirus infection have been discovered in Yosemite park visitors who stayed in Curry Village this summer. Six people have been confirmed to have contracted the rodent-borne illness that typically causes death in one-third of those infected.
"CDPH is working closely with the National Park Service and the Centers for Disease Control and Prevention to further investigate the cluster of Hantavirus Pulmonary Syndrome cases in Yosemite and reduce the risk of other visitors becoming ill from this virus," CDPH Director, Dr. Ron Chapman said in the press release. "CDPH is continuing to monitor cases of Hantavirus Pulmonary Syndrome in persons who visited Yosemite National Park."
from contracting the virus at the park, the Centers for Disease Control and Prevention (CDC) reported.
The hantavirus outbreak occurred despite efforts by park officials to step up protection efforts last April. A 2010 report from the California Department of Public Health warned park officials that rodent inspection efforts should be increased after a visitor to the Tuolumne Meadows area of the park fell ill.
The new hantavirus policy, enacted April 25, was designed to provide a safe place, "free from recognized hazards that may cause serious physical harm or death."
It came after the state report revealed that 18 percent of mice trapped for testing at various locations around the park were positive for hantavirus. The report said park officials should take steps to prevent mice from entering areas where people sleep.
In 2009, the park installed the 91 new, higher-end cabins to replace some that had been closed or damaged after parts of Curry Village, which sits below the 3,000-foot Glacier Point promontory, were determined to be in a rock-fall hazard zone.
The new cabins have canvas exteriors and drywall or plywood inside, with insulation in between. Park officials found this week when they tried to shore up some of the cabins that mice had built nests in the walls.
The deer mice most prone to carrying the virus can squeeze through holes just one-quarter-inch in diameter. They are distinguished from solid-colored house mice by their white bellies and gray and brown bodies.
Yosemite's hantavirus plan also calls for awareness training of park employees and prescribes protective measures and equipment to reduce exposure.
The state concurs that officials in Yosemite took steps to deal with potential hantavirus exposure, but there are limitations, given the location.
Meanwhile, the park sent warning emails and letters on Wednesday to another 1,000 people who stayed in tent cabins in Curry Village, after officials found that a computer glitch had stopped the notices from going out with the original 1,700 warnings on Monday. The warning says anyone with flu-like symptoms or respiratory problems should seek immediate medical attention.
In 2011, half of the 24 U.S. hantavirus cases ended in death. But since 1993, when the virus first was identified, the average death rate is 36.39 percent, according to the Centers for Disease Control.
Most of the nearly 600 cases reported since 1993 have been in New Mexico, Colorado, Washington, Arizona and California. Most often they are isolated, so having this cluster of cases from a small area in Yosemite has perplexed public health officials.
The federal government has two epidemiologists working in the park. They are trapping mice and rodents in an effort to determine how much of the population carries the virus and to see whether there are more mice in Yosemite Valley this year than in other years.
Kramer warned people never to sweep or vacuum mouse droppings. Instead spray them with a mixture of bleach and water then wipe it up with paper towels or a mop.