* National park to serve as a giant laboratory for tests
* Researchers to study park's employees, rodent population
* Insights sought for transmission, variability of virus
By Ronnie Cohen
SAN FRANCISCO, Sept 26 (Reuters) - California researchers and public health officials have launched what they describe as a groundbreaking series of studies of a rare mouse-borne virus that has infected at least nine Yosemite National Park visitors, killing three of them, since June.
By using the 1,200-square-mile (3,100-square-km) park and its rodent and human populations as a giant natural laboratory, scientists hope to gain new insights into how hantavirus is transmitted, how varied it might be and why certain people seem more susceptible than others.
The effort will include the first whole-genome sequencing for the hantavirus strain that struck Yosemite over the summer in the biggest cluster of cases since the disease was first identified in the United States in 1993.
Public health officials are also developing an unprecedented voluntary medical screening of the scenic park's 2,500-plus employees.
Dr. Charles Chiu, an infectious disease specialist from the University of California, San Francisco, said researchers would take blood samples from workers in an effort to find clues to how the virus infects people and how it might be prevented.
Chiu already has begun performing genome sequencing of the virus using tissue samples taken from patients infected in this summer's outbreak, as well as with tissue taken from rodents carrying the virus in Yosemite and throughout California.
Danielle Buttke, a National Park Service veterinary epidemiologist, said: "We want to take this opportunity to learn as much about the disease as we possibly can ... I think there's a lot to be learned here."
Chiu said his study will examine whether the virus that infected Yosemite visitors last summer is the same as the strain first identified in 1993, when a then-mysterious disease now known as hantavirus pulmonary syndrome struck 18 people in the U.S. Southwest.
"Is this a different strain, potentially a mutated virus? Has the virus evolved?" Chiu asked. "I actually don't think so, but the only way to confirm that is to do the study."
Deer mice carry the airborne virus in their droppings, urine and saliva. People can inhale it when it mixes with dust, especially in confined, poorly ventilated spaces. Humans have never been known to transmit it.
All but one of the Yosemite visitors who contracted the disease over the summer are believed to have been exposed in Yosemite's Curry Village area while staying in double-walled tent cabins later found to have been infested by deer mice.
There is no cure for hantavirus pulmonary syndrome, which kills more than a third of those infected. But early detection through blood tests and supportive hospital care increase the chances of survival.
The disease can incubate for as long as six weeks after infection. Initial flu-like symptoms can lead quickly to severe breathing difficulties and death.
Investigators are trapping and examining rodents in Yosemite to gauge the size of the population of deer mice and the percentage carrying the microbe.
In one area of the national park, researchers found that the deer mice population had tripled since 2008, probably a partial explanation for the outbreak.
Chiu is also considering launching a study to look for hantavirus antibodies in patients and their families.
In addition, he is asking northern California blood banks to screen donors for hantavirus antibodies and inquire about whether they visited Yosemite and, if so, where they stayed, to try to determine the prevalence of hantavirus exposure.
Weather conditions since the Curry Village tent cabins were erected in 2009 may have created the perfect environment for deer mice to multiply, Buttke, the veterinary epidemiologist, said.
"We don't know if it was a specific factor or a confluence of factors. So we're looking at people, animals and the environment to understand the situation. When the health of the environment is poor, that's often when you see infectious disease occur," she said.
When hantavirus pulmonary syndrome struck healthy young adults in the Four Corners region of Utah, Colorado, Arizona and New Mexico in 1993, Navajo elders linked the outbreak to an abundance of pine nuts and an explosion of mice coming out for the feast.
Buttke said hantavirus infection rates tend to be lower in areas where rodents' natural predators, such as foxes, thrive.
Both the employee survey and the blood-bank survey are expected to shed light on whether everyone who comes in contact with the virus gets sick or only some do. (Editing by Steve Gorman and Claudia Parsons)