The Plagues of Global Warming

By Elizabeth Kolbert, The New Yorker

27 February 15

he black rat—also known as the ship rat, the roof rat, and the house rat—is actually gray. It has large ears and a tail that’s longer than its body. The black rat (Rattus rattus) probably evolved in tropical Asia, and then was spread around the world by humans—first by the Romans and later by European colonists. According to Juliet Clutton-Brock, the author of “A Natural History of Domesticated Mammals,” it has been blamed for causing “a greater number of deaths in the human species than any natural catastrophe or war.” But perhaps the rat has gotten a bad rap?

A paper published the other day in the Proceedings of the National Academy of Sciences, which quickly made headlines all around the world, argues that the prevailing theory of how the Black Death spread is unfair to rats. Really, the authors of the study contend, the animal responsible was a Central Asian species like the great gerbil. (Great gerbils are only distantly related to the fuzzy rodents that American kids keep as pets, though they may look a lot alike to parents.)

The authors of the study were trying to address one of the mysteries about the Black Death. Why, after killing something like twenty-five million people in Europe in the mid-fourteenth century, did outbreaks of plague keep flaring up and then dying down again? (The Great Plague of London, in the mid-seventeenth century, killed roughly a fifth of the city’s population.) The prevailing theory is—or was—that bacteria responsible for the plague, Yersinia pestis, lived on Europe’s black-rat population. The rats transmitted the bacteria to fleas, which, episodically, transmitted them to humans. But the scientists who conducted the PNAS study concluded that there were no “permanent plague reservoirs in medieval Europe.”

Instead, they posit, the plague bacterium kept being reintroduced to Europe from Asia, where it lived on the native rodent populations. They came to this conclusion after comparing tree-ring records from Europe and Asia with records of plague outbreaks. What they found was that plague seemed to show up at port cities in Europe several years after climate conditions favored a burst of population growth among rodents in Central Asia. (This theory does not completely exonerate black rats, as they would still have helped their Asian rodent brethren spread the disease once it reached Europe.)

“We show that, wherever there were good conditions for gerbils and fleas in Central Asia, some years later the bacteria shows up in harbor cities in Europe and then spreads across the continent,” one of the authors of the study, Nils Christian Stenseth, a biologist at the University of Oslo, told the BBC.

Plague is no longer a worry in Europe, although there are still occasional outbreaks in other parts of the globe. What’s perhaps the most important insight from the study has little to do with Yersinia pestis or giant gerbils. It’s that climate and human health are, in significant though often roundabout ways, related. As the climate changes, this has important—and, at the same time, hard to predict—implications.

The list of diseases (and disease vectors) that could potentially be affected by climate change is a long and various one. It includes tick-borne diseases, such as Lyme disease, and mosquito-borne diseases—dengue fever, West Nile virus, malaria. It also includes waterborne diseases, such as cholera, and fungal diseases, such as valley fever. An upcoming issue of Philosophical Transactions B, a journal of Britain’s Royal Society, is wholly devoted to the subject of “climate change and vector-borne disease.”

Rising temperatures may already be contributing to the spread of some diseases, like chikungunya, a mosquito-borne virus that, not long ago, was confined to Africa and Asia. (The name of the virus, from Kimakonde, a language spoken in Tanzania and Mozambique, means “to become contorted,” which is what happens to the virus’s victims, who experience severe joint pain.) In recent years, cases have shown up in Italy and the Caribbean, and, just last year, in Florida. While the recent spread of chikungunya probably has more to do with global trade and travel than with climate change, the mosquito that transmits the virus seems to be able to survive in more and more places as the globe warms.

“All the blocks are falling into place,” Walter Tabachnick, the director of the Florida Medical Entomology Laboratory, at the University of Florida, told NPR a few weeks ago. “You’ve got to be worried about this.”

Which brings us back to giant gerbils. 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. 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.