YOSEMITE NATIONAL PARK (CBS / AP) — Yosemite National Park will offer testing to all employees at the site to determine if they have ever been infected with a deadly mouse-borne virus, officials said Thursday.

The testing to be done by state health officials will be voluntary and available to all workers for the National Park Service and its concessionaire, DNC Parks and Resort, park spokesman John Quinley said. He declined to say when the testing would start.

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There have been no confirmed or suspected hantavirus cases among park employees so far, Quinley said. The park did not offer the testing earlier because public health officials did not recommend it, he said.

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Nine people who visited the park this summer have been infected, the majority after staying overnight at the “Signature” cabins in Curry Village. Three of those people later died.

Between 2,500 and 3,000 people work in the park every year, depending on the season. A little less than half are National Park employees, and the rest work for the concessionaire. Many workers also live in the park.

The California Department of Public Health conducted a pilot testing program on Wednesday, taking blood samples and questionnaires from 96 employees who live or work in the El Portal area of the park.

Hantavirus is carried in the feces, urine and saliva of deer mice and other rodents, and carried on airborne particles and dust.

People can be infected by inhaling the virus or by handling infected rodents. They usually develop flu-like symptoms at first, including fever, shortness of breath, chills and muscle and body aches.

The illness can take six weeks to incubate before rapid acute respiratory and organ failure.

The tests for employees will cover all past infections, said Danielle Buttke, veterinary epidemiologist with the National Park Service. People who have been infected at any time in their lives and developed antibodies will test positive, but the test does not pinpoint time of the infection, or where the person was infected, she said.

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The goal of the testing, which was proposed by public health officials, is to further the understanding of the rare virus. Park and public health officials hope to learn more about why no park employees have thus far been struck with the disease, even though many could have been exposed to it, Buttke said.

Officials also hope to learn whether there are any people who had the infection but never developed symptoms of the disease. And they plan to evaluate training or knowledge gaps employees might have about the disease.

It’s possible that no employees have contracted hantavirus because they have received training about the illness and take more precautions because they are more aware of the risks than park visitors, Buttke said. Employees also don’t stay in visitor facilities overnight, she said.

During the outbreak this summer, park officials say they sent all employee emails about the virus while increasing training and holding meetings about the disease.

Another reason for lack of worker infections could be the rarity of the disease. Previous studies of people who directly handle mice found that only a few had hantavirus antibodies, meaning few were infected by the disease during their careers, said Barbara Knust, an epidemiologist with the Centers for Disease Control and Prevention.

In rare cases, people who did develop antibodies never had any symptoms.

“We think it’s actually quite rare that people get infected,” Knust said. “And in most cases, people who do get infected with hantavirus show symptoms.”

Employees tested in Wednesday’s pilot program included maintenance and facilities workers, who open park buildings in the spring.

The effort included questions about employees’ work activities and living environment; if and when they were exposed to mice; and what they remembered about hantavirus training.

While individual test results will remain confidential, overall results from the pilot testing could be available sometime next month.

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