How Yosemite campers were exposed to lethal hantavirus

Sloppy campers may have inadvertently triggered a viral outbreak that has killed two people and sickened four more who stayed at Yosemite National Park in California in June.

Sin Nombre virus is a hantavirus carried by deer mice. It can spread to humans if they inhale dust containing dried mouse urine or faeces, but it does not spread from person to person.

The virus affects the lungs, and kills one-third of those who become ill. Infection is very rare, though, with only 556 cases of Sin Nombre reported in the US between its discovery in 1993 and the end of 2011.

A population of deer mice was found nesting inside the double walls of tent cabins in Yosemite occupied by those who have succumbed to the virus. Some 10,000 people slept in these insulated tents this summer, and have been warned to watch for flu-like symptoms, which can take up to six weeks to show.

Human cases are more likely when mouse-contaminated dust accumulates. "If you do not clean a cabin with holes through which mice can enter, you are asking for trouble," says Charles Calisher of Colorado State University at Fort Collins, who helped discover Sin Nombre in 1993.

Cleaning eliminates two hazards: dust and food scraps, which attract the mice and boost their numbers. "We've been telling people since 1993: keep it clean," says Callisher.

Yet campers in the same tent village this summer have reported dust and discarded nuts in the tents.

"If the tents were cleaned of foodstuffs each day, they would be less attractive to rodents," says Tony Schountz of the University of Northern Colorado in Greeley.

How the virus got its name

Sin Nombre virus was discovered after an outbreak among Navajo Indians in northwest New Mexico in 1993. Navajo traditional healers told investigators at the time that they avoid deer mice, as they were believed to cause illness, especially after rains led to bumper crops of the seeds eaten by the mice. The 1993 outbreak followed such rains.

In 1993, following virologists' tradition, the virus was named Muerto Canyon virus, after the place where the Navajo outbreak started.

"But in Navajo tradition, you don't mention death or dead," says Calisher. Muerto means dead: the canyon was the site of a massacre of Native Americans by US troops. This made the name useless for health officials who needed to talk with the Navajo about the virus.

The next choice was Four Corners virus, named after the region where four US states meet. Local authorities trying to promote tourism nixed that, says Calisher. He wasn't involved in the final choice, and did not know where the name came from.

Now the story can be revealed. "We decided to name it after anything we could find on a topographical map," says CJ Peters of the University of Texas Medical Branch in Galveston, to whom the job fell. A name was required urgently: outbreak investigators needed patients who might be infected to come forward.

"We found this little arroyo [creek] called Sin Nombre," he says. "Sin nombre" means nameless in Spanish. "No one could complain about that."

The arroyo was within 50 kilometres of the first outbreak. He speculates that local Spanish-speaking sources told the original English-speaking cartographers that the place didn't have a name ? and the cartographers dutifully wrote that down.

Nameless or not, the risk of Sin Nombre remains real, but low, says Schountz. "This won't dissuade me from going to Yosemite again. It's a superb park."

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