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updated 9/12/2012 8:21:02 AM ET 2012-09-12T12:21:02

After a third Yosemite visitor died of hantavirus pulmonary syndrome last week, questions about the rodent-borne disease are intensifying.

Besides the fatalities, five more Yosemite National Park visitors have been diagnosed with the illness and are recovering, the park reports. The confirmed cases include six individuals from California, one from Pennsylvania and one from West Virginia.

The disease is spread through contact with rodent urine, feces or saliva; in the U.S., there are no known cases of person-to-person transmission. Symptoms include fever, chills, muscle aches and gastrointestinal symptoms, and appear from one to six weeks following exposure.

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Although the disease is rare, the mortality rate is high. Of 602 known U.S. cases since 1993, 216 people have died. That's 36 percent. Last year, it was 50 percent, says Mark Escott, deputy health authority of the Montgomery County Hospital District Public Health Department in Texas.

The high death rate means no one is taking it lightly. The World Health Organization has issued an alert; the National Park Service (NPS) is updating its park alerts and alerting more than 22,000 people who visited the park; and the NPS and Centers for Disease Control and Prevention (CDC) are investigating possible causes of the unusual cluster of cases.


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"Early medical diagnosis can greatly improve medical outcomes," said Danielle Buttke, a veterinary epidemiologist with the park service. "We are working very closely at the CDC to look at risk factors, and considering long-term studies to understand the disease better." "This is an unprecedented situation to have a cluster situation like this," she added.

The disease was discovered in 1993 when a young Navajo man and his fiancee died of an unexplained pulmonary illness, sparking an investigation, as detailed by the CDC. Five other healthy people who had died after acute respiratory failure in the same southwestern region were quickly identified. A few months later, the virus was isolated.

By examining earlier samples of tissue, previous cases have also been discovered, dating back to 1959.

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The virus usually occurs in warm climates. In the United States, it's most often seen in Southwestern states. Researchers think the area of Arizona, New Mexico, Colorado and Utah hit by the virus in 1993 may have been prompted by a higher-than-average number of deer mice. Heavy snow and rain gave rise to a deer mouse population that was 10 times bigger in 1993 than 1992.

Yosemite had also experienced a similar weather pattern recently that gave rise to a possible theory of a higher-than-usual mouse population this year.

"We're working with the California Department of Public Health on rodent tracking, but the cautionary message is that the mouse population is extremely dynamic," Buttke said.

"So we know what it was the second to last week in August, but that doesn't tell us what it was like in June [when people likely contracted the virus]," she added. "The prevalence in August was about the same as what we would expect."

The park service and the CDC are looking into other hypotheses as well, Buttke said.

Yosemite's popular Curry Village, where many of the cases are believed to have been contracted, seems well-suited to a healthy mouse population, with its lack of predators (foxes stay away due to the human presence), unnatural food sources (mice can get into trash), and habitat (walls of cabins make warm homes for mice). That's led the park to start trapping rodents to control the population, and to new protocols to thoroughly inspect every accommodation in the park.

"Our human impacts on the environment can dramatically influence wildlife populations and the risk for disease," Buttke said. "Biodiversity is very important in protecting human health, and this is an illustration of why we should think not just about human health but environmental and wildlife health as well."

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Meanwhile, officials briefly quarantined a house near Houston, after a woman who had been preparing the home for the TLC television show " Hoarding: Buried Alive " developed a respiratory illness that appeared to be HPS.

On Monday, the woman tested negative, but the incident shows the heightened awareness of the disease and the importance of early detection.

While there is no cure for hantavirus pulmonary syndrome, patients are often hospitalized, Escott said, so they have access to oxygen and medication to help the heart pump. Patients are also usually put on broad spectrum antibiotics, he said, even before a definitive diagnosis is made.

If you live in an area where deer mice infect homes, the best prevention is to contact rodent control professionals and also have your home professionally cleaned, Escott said. When mice urine or feces is present and the area is touched, the virus can be inhaled through dust.

© 2012 Discovery Channel

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