Climate Change and the Black Plague

I was in Denver recently, where a zoo primate recently died from bubonic plague (Yersinia pestis infection). The animal probably ate an infected squirrel. Bubonic plague is endemic in the American west, but it doesn't get much attention. Human cases are uncommon and usually treatable.

When I read the story I wondered about the intersection of climate and disease. Why does Yersinia pestis infection become epidemic? Why was it a recurrent catastrophe throughout medieval european history?

In the old days I'd wonder if this hypothesis had been studied much, then forget about it. Nowadays, of course, I simply presume someone's studied it and I ask Google. Here's an excerpt from a coincidentally recent review for the layperson (emphases mine):

Geotimes - May 2007 - The Plague: Could It Happen Again?

...The 14th through 19th centuries were challenging times in Europe. Winters were harsh, filled with heavy snowfalls that lasted late into spring and ice that perpetually covered mountaintops and pushed into settled valleys. Springs and summers were so cold and wet that crops would not grow, or became moldy before they could be consumed. People and livestock starved. Wars were fought over scant resources as people traveled farther than before, searching for food and better conditions and colliding with anyone who got in their way.

These desolate conditions forced people to leave their homes and rotting fields in the countryside and head for cities, where crowding and poor sanitation were the rule. Meanwhile, international trade greatly expanded, as ships and caravans brought goods from Asia into European cities. Trade brought more than just goods, however: It also brought diseases.

During these 500 years of cold, extreme and unpredictable weather in Europe, temperatures rose slightly for brief periods of time. But rather than providing a respite from the cold, the warmer temperatures actually promoted the proliferation of infectious diseases. Chief among them was plague. Estimates suggest that up to half of Europe’s already weakened population was wiped out by devastating epidemics, including the infamous Black Death that began in 1347 and the Great Plague of London in 1665, when people died so quickly that bodies piled up on the sidewalks...

... Bubonic plague is “a disease of nature,” Engelthaler says, meaning that climate and landscape play a vital role in the survival and spread of the bacterium that causes the disease. Rodent and flea population dynamics are driven by many factors, Gage adds, including food availability, disease and climate variables, namely precipitation and temperature. In studies published over the last five years, models and observations have shown that precipitation and temperature strongly influence the spread of plague.

The most important factor in the disease, besides the bacterium, Engelthaler says, is the flea that carries and transmits the disease. Not all species of flea will transfer or maintain the bacterium, and some transmit it better than others. The type of flea that lives on cats, for example, is not a good vector, he says. But the fleas that live on black rats and ground squirrels are great vectors. Furthermore, the fleas that carry Yersinia pestis can only survive for long periods in “optimal” conditions, including warm but not hot temperatures and wet environments. And they can only transmit the bacterium under even more specific conditions, he says. If temperatures get too hot, the biology of the bacterium stops it from spreading, by breaking down the bacterial blockages that have built up in the flea vector’s gut and are considered essential for efficient transmission.

In addition to needing the right type of flea, the right type of host needs to be present to keep the cycle of transfer from flea to host and back to flea going, Engelthaler says. Black rats and prairie dogs die within days of being infected, so they might not be the best hosts, he says. Although ground squirrels also often die from plague, they can carry the bacterium around for months, allowing fleas to transfer the plague bacterium from their dying host to another unsuspecting host.

To get widespread epidemics of the disease, the density of host rodents must first reach a threshold level in a region, Gage says. Then the weather has to cooperate to keep it going and to increase the number of human cases, he says.

In the American Southwest, where plague is prevalent in wild rodents and an average of five to 15 people contract the disease each year, increasing rainfall in late winter and early spring leads to a sizable increase in plague 15 months later, Gage says, as seen in models and observations over the past 50 years. It works in a sort of “trophic cascade,” he says: “Heavy precipitation in early spring leads to more plant growth and more insects, which means more food for the rodents, which leads to more hosts for the [plague-bearing] fleas, and thus more plague.” The other important factor, he says, is lower summer temperatures.

The story is similar in Central Asia, says Nils Chr. Stenseth of the University of Oslo. Infection rates and climate data from 1949 to 1995 in Kazakhstan showed that with just a 1 degree Celsius increase in spring temperatures, plague prevalence in gerbils more than doubled a year or two later, Stenseth says. Wetter summers also led to an increase in plague prevalence the following fall, he says.

Ongoing research in China and other parts of the world is finding a similar trend, Stenseth says, though the exact mechanisms may be slightly different, such as whether spring or summer precipitation or temperature is the driving factor. And models are agreeing with the data. “The general message we’re seeing all over the world is that climate is important,” he says. “Furthermore, climate is changing in a way that will affect human plague cases.”

We don't expect to see plague recur, but it now appears likely that it was an interconnected combination of socioeconomic and climate change that led to the plagues that killed up to half of Europe. Now would be a good time to invest in this research domain.