The deadly outbreak of hantavirus in Yosemite National Park has sent disease experts and rodent researchers scrambling for answers as people across the country second-guess their plans to visit California’s most famous landscape.
The California Department of Public Health and the U.S. Centers for Disease Control and Prevention are trying to figure out why the mouse-borne virus struck suddenly, spread quickly and infected so many people this summer, but not in any other years.
One prominent theory is that the mysterious cluster of hantavirus cases was caused by a skyrocketing mouse population in Yosemite Valley.
“I suspect that the underlying factors likely are simple – more deer mice,” said Douglas Kelt, a professor of wildlife ecology at UC Davis. “Why more deer mice? Presumably more food and a less rigorous winter.”
It’s a pattern that has occurred in the past with similar results, most notably in 1993 when the disease was first identified after a deadly outbreak on the Navajo reservation in New Mexico.
Also suspect are the tent cabins, built in 2009, where the people who were infected had stayed.
How disease spreads
Since June, nine Yosemite visitors have gotten sick, and three people have died, from hantavirus pulmonary syndrome. It is the largest outbreak of the virus in a single geographic area since the Navajo cluster.
The pathogen is carried by a common species of deer mouse known scientifically as Peromyscus maniculatus. The virus doesn’t make mice sick, but humans can catch it by inhaling airborne particles of mouse feces and urine that accumulate in dust. Mouse saliva also contains the pathogen, so people can conceivably get the disease after being bitten.
Flu-like symptoms begin one to six weeks after exposure and quickly progress as the chest cavity fills with fluid, causing breathing difficulties. The pathogen has killed 36 percent of people known to be infected.
Kelt, who wrote a paper outlining field strategies for avoiding hantavirus, said the more mice there are, the more likely it is for people to be exposed to hantavirus. He said modeling studies he conducted in the Plumas National Forest over the past eight years suggest that winter severity and cone crop productivity drive the following year’s deer mouse numbers.
“In a nutshell, we found that severe winters (and lots of snow) tended to reduce deer mouse numbers,” he said, “but strong cone crops from the preceding year provided a solid food source and tended to favor higher deer mouse numbers.”
Cabins a factor
In other words, the severe winters two and three years ago are believed to have kept mouse numbers down, but the moisture helped produce a bumper crop of pine cones with juicy pine nuts this year, when the weather was mild, allowing mice to multiply.
Combine that with the fact that the questionable tent cabins were built in 2009, meaning that up until this year people had stayed in them following harsh winters. This was the first in which people stayed in them after a mild winter.
Jim Patton, curator and professor emeritus of integrative biology for the Museum of Vertebrate Zoology at UC Berkeley, believes the tent cabin conditions at Curry Village had more to do with the spread of the virus than the mouse population or the amount of pathogen circulating among rodents.
Eight of the nine people who contracted the disease in Yosemite slept in the higher-end “signature” tent cabins, on the east side of Curry Village between early June and mid-July. The other victim hiked and camped around the same time in Tuolumne Meadows and the high sierra camps about 15 miles away.
Perfect mouse habitat
“You have to have a threshold population for the virus to build up in the first place, but it is conceivable the mice are surviving better and producing more because of the local environment in the cabins,” Patton said. “The cabins being insulated keeps the warmth in and keeps the air from circulating, and those are the conditions that keep the virus going.”
The 91 cabins were built to replace tents removed from another location after a rockfall. All of the new ones have drywall and canvas siding. Health department officials ordered them closed after discovering that the extra insulation created a gap in the wall that was a perfect habitat for mice to crawl inside and build nests.
In fact, many of the known hantavirus infections over the years have occurred inside mice-infested buildings, usually after workers clean out a mountain cabin that was uninhabited during the winter and filled with stale air.
The first known outbreak of hantavirus occurred in 1993 when a young Navajo woman died suddenly of respiratory failure in the Four Corners region of New Mexico. Her death was followed a few days later by her fiance’s. Those and several other cases caused a near panic throughout the country. Eventually, hantavirus – so named because the first strain was identified near the Hantaan River in Korea – was pinpointed as the cause of the deaths.
A Centers for Disease Control analysis of the Four Corners outbreak determined that El Niño conditions in 1991-92, followed by a warm winter and a rainy spring in 1993, led to explosive growth of vegetation, which in turn provided enough food and cover to allow for a tenfold increase in the number of deer mice. In all, 48 people throughout the United States died of hantavirus that year, more than 56 percent of the identified cases.
Present in tribal lore
Navajo elders reported similar outbreaks in 1918, 1933 and 1934. It turned out that Navajo tribal stories had long identified mice in the home as a source of bad luck and sickness.
Cases have been reported since then in 31 states, but the vast majority have been in the western United States. Hantavirus deaths have been traced back to 1959, but experts believe the virus has co-evolved with rodents for millions of years.
Since 1993, 59 hantavirus infections have been reported in California, including cases in Yosemite’s Tuolumne Meadows in 2000 and 2010. The 2010 case involved a 54-year-old San Mateo woman who recovered after three days in the hospital. She told investigators that she saw mice in the cabin where she was staying and swept out the room.