Editor's note: This updates with news that a ninth park visitor contracted Hantavirus.
Though the calendar is running down on the incubation period for Hantavirus infecting visitors who stayed in Yosemite National Park this summer, a ninth case has been confirmed, park officials said Thursday. Three of those individuals died from the rare, rodent-borne disease, while the others are recovering.
In a brief release park officials said they have "received confirmations from national and state public health agencies of hantavirus infection in nine individuals who stayed one night or more in Yosemite since June of this year. Eight of the confirmed cases developed hantavirus pulmonary syndrome (HPS). Three cases of HPS have resulted in fatality; the six other individuals have recovered. The confirmed cases of hantavirus infection were in seven individuals from California, one from Pennsylvania, and one from West Virginia.
Meanwhile, epidemiologists tracking the deadly outbreak might never definitively know what spurred it.
In the North America, the disease is carried by deer mice and can be found throughout most of the country. In California, about 14 percent of the deer mice in the High Sierra carry the disease, according to National Park Service staff.
So what spurred this summer's outbreak of the disease? It could be weather-related, due to a lack of predators in the valley, or possibly connected with the structural design of the "Signature Tent Cabins" in one area of Curry Village.
"Hantavirus doesn’t survive in the environment for that long. It is readily killed by sunlight," says Dr. Danielle Buttke, an epidemiologist for the National Park Service who has been investigating the outbreak. "It’s a disease that doesn’t last long in the environment in general. ... The most common pathway is to have a mouse urinate in an area, and if it is an infected mouse the virus is spread in the urine and then that urine can be stirred up in dust, and a person would inhale that virus in the dust that’s stirred up. That’s the most common route of infection."
Yosemite officials have known of the dust connection; after a 2010 visitor contracted the disease brooms were removed from the Curry Village cabins, Dr. Buttke said during a telephone interview from her Fort Collins, Colo., office.
The outbreak this summer led to the discovery of deer mice nesting within the double-walled Signature Tent Cabins. It would, in theory, be possible for urine-contaminated dust from those nests to migrate into the cabin's interior, she said.
However, Dr. Buttke went on, "we don’t know if there's anything specific about the ventilation or design of the Signature Tent Cabins that may have influenced someone stirring up or breathing in the virus, but we are investigating that as well."
Investigators were following up with those sickened Yosemite visitors who are getting over the disease to see if they can help identify any behavior or risk factors that caused them to contract the disease, she said.
Incubation Period Approaching End
Health officials for the Park Service and the California Department of Public Health have placed the incubation period of Hantavirus in infected humans at one-six weeks, although the Centers for Disease Control's outer limit is five weeks. With the Signature Tent Cabins in the Yosemite Valley having been closed since August 28, the six-week wait will run its course by Oct. 1.
Of course, one of this summer's victims contracted the disease while staying in the park's High Sierra tent camps, which like the Curry Village accommodations are operated by DNC Parks & Resorts at Yosemite.
"That individiual stayed at several High Sierra Camps, and (a tent cabin at) Tuolumne Meadows. We know that Hantavirus occurs throughout the High Sierras, all over California," Dr. Buttke said. "And the two cases that have occurred in California outside of the park both occurred in the High Sierras. So that is a known risk factor, and likely that (Yosemite) case is a typical sporadic Hantavirus case.
“Unfortunately, we may never know where that individual was exposed, because they stayed at many different locations.”
While the Signature Tent Cabins at Curry Village have been closed and undergone a thorough cleaning, neither Yosemite nor DNC officials have returned phone calls to say what precautions were being taken with the five High Sierra tent camps as well as with the tent cabins at Tuolumne Meadows and White Wolf.
Park and concession officials have reached out to tens of thousands of visitors who stayed in the park from early June through late August, including some who came from overseas, to alert them to the outbreak and to urge them to seek medical attention if they come down with any of the flu-like symptoms the disease spawns.
"The National Park Service Office of Public Health is working with the Centers for Disease Control and Prevention (CDC) and state public health offices to heighten public health awareness and detection of the disease," a release on Yosemite's website said.
Weather, Lack Of Predators Could Be Behind Outbreak
In the end, investigators might determine that the unusual outbreak was a fluky combination of weather, too many mice, and too few predators.
"The deer mouse population is extremely dynamic. They reproduce very quickly, and can reproduce at six weeks of age," pointed out Dr. Buttke. "So at any point in the year the population will vary significantly. Along with that, the percent of mice infected can vary significantly. At this point in time, the California state average is approximately 14 percent. Historically it has been 20 percent, but it varies quite a bit from year to year and from location to location.”
As to what was behind this summer's outbreak, it could have been weather-related.
"We know that when Hantavirus was first identified in 1993, it occurred after two very, very wet summers, and a very mild winter," said the Park Service epidemiologist. "So the mice had an abundance of food, followed by a mild winter that allowed more of them to survive the winter. And that’s when they had the outbreak in 1993. And there’s a lot of Native American literature and folklore that also tells us of a similar disease to Hantavirus that occurs during years of lots of mice.
“So, that’s typically why we see an upswing, or a spike of cases of Hantavirus in an area," Dr. Buttke went on.
And while the Yosemite Valley's deer mice population might have spiked this summer, there's a lack of natural predators to tamp it down, she said.
"We know that predators are very important in the control and spread of zoonotic diseases, and so when you have an area like Curray Village where there aren’t predators because they’re afraid of humans, they don’t come into that area, you have a very adequate rodent habitat," the doctor said. "They are protected from predators, and also have a large food source if people are bringing food into their cabins."
For the moment, though, "'We don’t know' is the short answer," Dr. Buttke replied when asked to identify the cause. "We’re still looking into what environmental factors, what epidemiologic factors, and what behaviorial factors might have influenced this cluster of cases.”
Once all those factors have been reviewed, the Park Service will look for ways to prevent a similar outbreak in Yosemite or any other unit of the National Park System where mice are prevalent.
"This is an unprecedented situation to have a cluster of cases like this in an area. We do know that there are certain risk factors, including having an abundance of mice in an area with close contact to humans," she said. "The park has always had excellent protocols in place to mitigate those risk factors, and so we are looking at ways to bolster those mitigation factors and try to identify any other situations that might have been important here.