About 10,000 people who stayed in tent cabins at Yosemite National Park this summer may be at risk from the deadly rodent-borne hantavirus, the US Centres for Disease Control and Prevention (CDC) has said.
The CDC on Friday urged lab testing of patients who exhibit symptoms consistent with the lung disease, hantavirus pulmonary syndrome, after a stay at the California park between June and August.
It also recommended that doctors notify state health departments when cases are found.
Two men died from hantavirus linked to the Yosemite outbreak and four others were sickened but survived, while the CDC said additional suspected cases were being investigated from "multiple health jurisdictions".
Most of the victims were believed to have been infected while staying in one of 91 "Signature" tent-style cabins in Yosemite's popular Curry Village camping area.
"An estimated 10,000 persons stayed in the 'Signature Tent Cabins' from June 10 through August 24, 2012," the CDC said.
"People who stayed in the tents between June 10 and August 24 may be at risk of developing HPS in the next six weeks."
Yosemite officials earlier this week shut down all 91 of the insulated tent cabins after finding deer mice, which carry the disease and can burrow through holes the size of pencil erasers, nesting between the double walls.
Park authorities said on Friday that they had contacted approximately 3,000 parties of visitors who had stayed in the tent cabins since mid-June, advising them to seek immediate medical attention if they have symptoms of hantavirus.
Nearly four million people visit Yosemite, one of the nation's most popular national parks, each year, attracted to its dramatic scenery and hiking trails.
Roughly 70 per cent of those visitors congregate in Yosemite Valley, where Curry Village is located.
The virus starts out causing flu-like symptoms, including headache, fever, muscle ache, shortness of breath and coughing, and can lead to severe breathing difficulties and death.
The incubation period for the virus is typically two to four weeks after exposure, the CDC said, with a range between a few days and six weeks. Just over a third of cases are fatal.
"Providers are reminded to consider the diagnosis of HPS in all persons presenting with clinically compatible illness and to ask about potential rodent exposure or if they had recently visited Yosemite National Park," the CDC said.
Although there is no cure for hantavirus, which has never been known to be transmitted between humans, treatment after early detection through blood tests can save lives.
"Early medical attention and diagnosis of hantavirus are critical," Yosemite superintendent Don Neubacher said in a statement.
"We urge anyone who may have been exposed to the infection to see their doctor at the first sign of symptoms and to advise them of the potential of hantavirus."
No mass cancellations
Yosemite spokeswoman Kari Cobb said rangers have answered about 1,500 phone calls from park visitors and others concerned about the disease. But she said the outbreak had not triggered a wave of cancellations.
"Right now it's normal numbers for Friday," she said.
"There have been cancellations, but it would be grossly overstated to say they're cancelling en masse.
"There's quite a bit of people out there still. It's still summer and a holiday weekend. It's still the summer crowds."
A national park service officials has said that public health officials warned the park twice before about hantavirus after it struck visitors.
But it was not until this week that the hiding place for the deer mice carrying the virus was found.
Hantavirus is carried in rodent faeces, urine and saliva, which dries out and mixes with dust that can be inhaled by humans, especially in small, confined spaces with poor ventilation.
People can also be infected by eating contaminated food, touching contaminated surfaces or being bitten by infected rodents.