Hantavirus Appears in Grand Canyon

Although it's hardly cause to pack mousetraps in your drybag, you might want to give paramiscus eremicus a little wider berth next time you paddle the Grand Canyon.

Last July, a commercial river guide developed the canyon's first reported case of Hantavirus Pulmonary Syndrome (HPS)--a respiratory illness transmitted by rodents--after sleeping beneath an overturned raft at Mile 19 Camp. "In the middle of the night I was awoken by a mouse running around on top of me," says the guide, who chose to remain anonymous. "Then I noticed a little moisture on my face--my reaction caused it to urinate."

Although he didn't think twice about it at the time, a week after the trip he became extremely ill, confusing doctors with a high fever, aches, pains and swollen lymph glands. "It fools a lot of doctors," he says. "The symptoms are a lot like the flu--they tested me for everything under the sun." Eventually, a blood test confirmed Hantavirus and the victim remained in intensive care for four days before recovering. He was lucky. According to Dr. Tom Myers of the Grand Canyon Clinic in Grand Canyon National Park, the survival rate for victims of HPS is only 44 percent. And the disease has been responsible for six deaths in Arizona since it was first discovered in 1993.

After the case was confirmed, two studies were conducted to determine the virus' prevalence in the canyon. Larry Stevens of the Grand Canyon Monitoring and Research Center set 100 traps at mile 19.5--where the victim was camped--and brought 43 live mice back for testing. Four tested positive. Last September, the Arizona State Vector Control, a division of the state health department, conducted another study, this time gathering 98 mice from seven different sites along the canyon. After sending specimens to the Infectious Disease Lab, two tested positive. "I don't know if we'll ever be able to determine how long it (the virus) has been there," says Dr. Myers. "It's probably been there a long time...the circumstances were just right this time for it to finally surface."

Helping it surface was last year's abnormally high mouse population, which many attribute to increased moisture from El Niño. "But I don't think people need to lose sleep over it," adds Dr. Myers. "What happened was such a fluke it would be rare to see it happen again. But it does pay to be cautious."

Those wishing to be extra cautious can do so by planning their itinerary; while it might not mean anything, says Myra Leslie of Arizona's State Vector Control, all of the positive tests came from river left. --edb