The two deaths are both rarities, and neither case seems likely to spread to anyone else.
But they’re also reminders that the world that was once ravaged by the Black Death is not as distant as we might think. In both cases, officials say, animals and environment played a role in infecting the victims — just as they did hundreds of years ago. Our health remains, as it always has been, inextricably linked to the world around us.
Plague comes in three forms — septicemic (focused in the bloodstream), pneumonic (an infection in the lungs) and the most notorious and most common, bubonic (which causes the characteristic boils) — all caused by the bacteria Yersinia pestis. It’s capable of spreading from human to human, but is most often carried by fleas, usually on the backs of rats, gerbils or occasionally cats and dogs.
Even though the devastating plague pandemics of the past are some of the most infamous and influential events in human history, scientists have struggled to understand why they were so repeatedly deadly. Again and again over the course of hundreds of years, the disease would flare up and then die out, taking millions of people each time. What could explain the disappearance and resurgence of the disease?
One explanation is climate: A study published earlier this year in the Proceedings of the National Academy of Sciences found that plague outbreaks in Europe were curiously tied to the weather in Asia. If Central Asia had a wet spring followed by a warm summer, the disease was sure to rear its ugly head in Europe quickly after. Their explanation was that the particular weather combination bolstered the population of great gerbils, whose fleas were something of a reservoir for the pesky Y. pestis. The gerbils then hitched a ride to Europe via the Silk Road and sent their fleas out to wreak deadly havoc. When the weather changed and the gerbil population fell, the epidemic receded. (The gerbils are still responsible for cases of the plague in Central Asia today.)
That study made for a lot of catchy headlines, mostly because it seemed to exonerate the much-maligned black rat. But the deadly combination of climate and critters it describes would likely resonate with Colorado officials, who are coping with a similar problem right now, albeit on a much smaller scale.
Just as rats — and maybe gerbils — bore the plague-carrying fleas that haunted Europe, rodents in the West are currently the country’s biggest reservoir of the disease. Earlier this summer, Colorado health officials confirmed the presence of Y. pestis in prairie dog colonies at a Denver-area park. The bacteria is also common in rats, mice and squirrels, all of which are capable of carrying the disease without dying out at such high rates that the plague itself dies along with its hosts.
Normally, humans stay far enough away from these creatures that we’re safe from their illnesses. Until last year, Colorado had seen only one case of plague since 2007.
But shifts in the weather can cause the carrier populations to boom, bringing their pathogens along for the ride. With more rodents, more fleas and more plague hanging around, it’s a matter of time before other animals — including humans — start to notice.
When a disease spreads beyond the population where it’s endemic (in this case, rodents), it’s called an epizootic. And Colorado is starting to see more of them. Last year, a pit bull was found to be the root cause of a minor plague outbreak that infected four people. All the victims survived, though the the dog did not.
Officials are blaming the weather. For the past two years, some parts of the state (the ones that aren’t parched with drought) have seen a wetter spring than usual. That leads to particularly lush vegetation, which in turn promotes larger rodent populations. The consequence? Colorado had eight cases of plague in 2014 and four this summer, including the two deaths.
“We’ve had a very wet winter, very wet spring and it’s really a cool summer for Pueblo and we are exploding with rodents and rabbits,” Vicky Carlton told KKTV-11, a Colorado City new station. Carlton is a program manager for the health department in Pueblo County, where the second victim fell ill.
“In the area where I live, I see rodents everywhere,” she continued. “And that just means we have twice as many fleas, as well. So, the risk is there for pets and people. This is an unusual summer for us.”
Colorado isn’t the only state to see this trend. A 2012 report from the U.S. Geological Survey found that, between 1969 and 2000, 75 percent of plague epizootics (cases where the disease spread beyond the animals where it is endemic and into other species) happened during an El Niño year, when the winters were warm and wet. That could change with global warming, the report said — as the Southwest gets hotter and drier, the rodents and their fleas might flee, spreading the disease north to Wyoming and Idaho.
Or they might not. As science writer Elizabeth Kolbert pointed out in the New Yorker, the PNAS gerbil study shows how complex the relationship between climate and public health can be.
“If the new PNAS study is correct, then millions in Europe died because the climate conditions were sometimes favorable for these rodents a quarter of the way around the world,” she wrote. “The indirect nature of the connection makes it hard to foresee what warming will mean for human health, which — in case you needed it — is another thing to worry about.”