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What did climate have to do with the human plague?

Chinese researchers found that the infectious disease, which killed about 200 million people during three historic pandemics, was linked to variations in rainfall (and rats with fleas, of course).
Written by Janet Fang, Contributor on

Today’s history lesson will be on The Human Plague, a disease that killed about 200 million people around the globe during 3 recorded pandemics.

The first was the Justinianic Plague, which occurred in AD 541 in the Egyptian port of Pelusium, on the eastern edge of the Nile Delta. It spread quickly to the Middle East, and then to Mediterranean Europe, where it remained virulent for more than 2 centuries.

The second pandemic – The Black Death – was responsible for about 25 million deaths in Europe from 1347 to 1665. Some say it killed as much as 60% of Europe’s population.

The third pandemic originated in the Yunnan Province of southwest China in 1772. First it spread out to the coastline, and from there it went out into the world by ship.

It wasn’t until Hong Kong in 1894 when Alexandre Yersin first isolated the pathogen that caused the plague, the bacterium Yersinia pestis, which is usually vectored by fleas.

Infected people start out with flu-like symptoms, followed by fever, chills, and vomiting – if treated with antibiotics, the disease shouldn’t be the same scourge it was before. The 3 plague forms are:

  1. Bubonic plague results from a bite from a Y. pestis-positive flea. The plague enters the body and travels to the nearest lymph node, making it swell into a bubo.
  2. Septicemic plague occurs when infection spreads directly through the bloodstream.
  3. Pneumonic plague ensues after inhaling Y. pestis-laden droplets. This is transmitted from human to human.

As of the turn of the millennium, the plague had been reported in 38 countries. A new study now shows how climate may have controlled the disease’s intensity.

To explore that link, an international team of scientists, including researchers from the Chinese Academy of Sciences and the Chinese Centre for Disease Control and Prevention, analyzed plague data from 1850 to 1964 in China, where the third recent pandemic originated.

This included 1.6 million cases of infection, along with 500 years worth of precipitation data that spanned across China.

Looking at the ‘human plague intensity’ (plague cases per year) and the yearly wetness and dryness cycles, they found:

  • In the northern part of the country – which has an arid climate – increased rainfall elevated plague intensity. The wet conditions prompted seed production and vegetation growth, which led to more food for rodents, who then harbored more fleas who carried the pathogen.
  • On the other hand, in southern China – which has a more humid climate – increased rainfall lowered plague severity. Turns out that there, flooding and such probably killed off lots of rodents, inhibiting the flea population.

Now it’s up to future research to untangle the separate effects of climate on these regional differences in precipitation, human migration, bacteria survival in droplets, flea survival, and in particular, rodent behavior.

Although, in September of last year, 2 cases of human plague were reported in Oregon, the only cases in the US that year. The patients were 17 and 42 years old, and they were likely exposed by their dog.

The study was published in the Proceedings of the National Academy of Sciences last week.

Image: Dance of Death by Hans Holbein the Younger via Wikimedia

This post was originally published on Smartplanet.com

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