Update: The California Department of Public Health has announced that a second visitor to Yosemite National Park was infected with “presumptive positive” case of plague. The hiker, from the state of Georgia, is hospitalized and being treated with antibiotics.
There are going to be a few unhappy campers in Yosemite National Park this week.
Park officials shuttered the Tuolumne Meadows campground in Yosemite on Monday after finding the carcasses of two squirrels that died of plague. Last month, a child who had stayed at Crane Flat Campground in Yosemite was infected, the first case in the park since 1959 and the first in California since 2006.
So far, eight people have contracted plague across the United States this year, including two people who have died in Colorado this summer, according to the U.S. Centers for Disease Control. The cases involving plague, which typically is spread by fleas that have bitten infected rodents or by the infected animals themselves, are a reminder of the persistent presence of a disease that has caused pandemics throughout history, and that wiped out an estimated 50 million people in Asia, Africa, and Europe in the 14th century.
Experts are hesitant to say there’s been a spike in plague activity this year. In recent decades, the U.S. has seen an average of seven plague cases annually, but the numbers vary significantly year to year. In 2010 there were only two cases of plague nationwide, while in 2014 there were 10.
This year’s count of eight cases so far “is not extraordinary by any means,” says Kenneth Gage, Chief of Entomology and Ecology Activity with the CDC. “But some of the circumstances have been somewhat unusual,” he says, including the number of fatalities in Colorado and the human case in Yosemite.
Between Monday and Friday, the National Park Service will dust the Yosemite campground with a powdered insecticide to kill the fleas that transmit the plague-causing bacteria, Yersinia pestis. “Although this is a rare disease, and the current risk to humans is low, eliminating the fleas is the best way to protect the public,” said Karen Smith, California Department of Public Health, in a statement. The park remains open, and visitors are being told to avoid contact with wild rodents.
Typically the disease moves back and forth between fleas and their rodent hosts, but the plague isn’t picky. It can jump to people when they—or their pets—come in close contact with rodents such as the California ground squirrels and prairie dogs that can carry the disease in the West. Often, this happens when rodents die, leaving their infectious fleas to find a new warm-blooded host to feed on.
“It can infect a huge range of mammals,” says Dan Salkeld, a research scientist at Colorado State University in Fort Collins. “And people, obviously,” he says. He called the plague bacterium a “generalist.”
One culprit in this year's number of plague cases might be the weather. After an El Niño in 1982, plague cases spiked across the U.S., with 40 cases in 1983 and 31 cases in 1984.
Gage explains that wet winters mean more food for the rodents, leading to more rodents for fleas to feast on, which in turn leads to greater disease transmission. Plus, fleas seem to like wet weather. When the sickened rodents die, all these disease-transmitting fleas can turn to humans and their pets for blood meals.
The Denver Post reports that the area has had a relatively warm and wet winter this year. But an increase in water leading to larger rodent populations doesn’t explain the cases in California, which has been experiencing a record-setting drought for the last four years.
Gage says that this might actually be a paradoxical example of the same phenomenon: Drought conditions drive rodents in larger numbers to the areas that actually have food and water—like campgrounds.
“It’s definitely a good theory, but I don’t think we’ve had enough research in that area to be able to say definitively,” says Danielle Buttke, an epidemiologist with the National Park Service. “But it is plausible. It’s definitely plausible.”
“We're seeing early activity now, even while we're in the middle of a drought, and then predicting to have a wet winter where we might have an increase in rodent populations,” Buttke says. “I think it's something we really need to keep an eye on, because climate change is predicted to have a lot more climate variability—which can have some pretty significant implications for disease.”