When the climate gets wetter, plagues can get worse, according to a new study that reveals why the plague was much worse in China's north than in the south.
The results also suggest that climate change could mean more virulent plagues in northern China and North America, as parts of the globe get wetter.
A bacterium called Yersinia pestis, which is carried by rodents, is responsible for three types of plague: bubonic (also called Black Death ), septicemic and pneumonic plague. Together, these illnesses have been responsible for the deaths of millions of people the world over, including an estimated third of Europe's population during the Middle Ages. While modern antibiotics can effectively treat plague, thousands of cases are still reported each year to the World Health Organization, and the bacterium has been identified as a possible biological warfare agent.
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Chinese and Norwegian researchers examined the association between climate and the severity of human plague in China during the most recent outbreaks between 1850 and 1964, when 1.6 million people became ill. They analyzed the plague data along with an index of precipitation over a 500-year period for 120 locations across China.
"We have found (a) very clear relationship between the amount of precipitation and the occurrence of human plague: the more precipitation, the more plague in the north of China whereas the less in the south,” study author Nils Stenseth, of the Centre for Ecological and Evolutionary Synthesis at the University of Oslo in Norway, told LiveScience. The study results were published last week in the Proceedings of the National Academy of Sciences.
More precipitation is expected in certain parts of the globe with a warming climate, according to the researchers, which might mean more cases of plague in the future.
They found that in the northern regions of China, which generally has a dry climate, increased rainfall was linked to more cases of plague; the researchers suspect the wetter conditions gave rise to more vegetation, so flea-bearing rodents had more food. More fleas that can carry the Y. pestis pathogen would mean more plague cases.
However, where the climate was more humid in China, heightened precipitation generally decreased plague severity, probably due to the fact that rats, not acclimated to rainy days, died in floods, cutting off the pathogen’s path to human hosts. There are still many unknowns to be filled in on exactly how this works, the authors say.
What about other places in the world? Stenseth said that North America has a similar relationship between rainfall and plague to what was found in northern China, where plague increased with more rainfall. "However, in North America one expects less precipitation," he said, meaning that the future increase in rainfall would likely be less in North America than in northern China.
Zhi-Bin Zhang, another of the study’s authors, raises another concern: while more rainfall in arid continents like Africa could mean an increase in plague, the reverse may also be true; humid continents could also see a rise in disease prevalence if they experience a drought. The researchers think that in humid places, less flooding would make it easier for rodents to move into human spaces.
"Climate-linked immigration of rodents between fields and houses may increase the risk of plague occurrence," Zhang said, because people would have more contact with disease-carrying rodents.
Stenseth says more rodent-borne disease can be expected in a wetter future, but that is not a reason to panic. "I think there is no reason to fear a big epidemic, because antibiotic treatments are much more developed today than in the past," he said, adding that we do need to be vigilant since pests can evolve resistance to such antibiotics. He suggests experts and officials should prepare for different epidemics in the future by learning more about the evolution of drug resistance.
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