February 24, 2015

Scientists believe that gerbils may have spread the deadly plague known as the Black Death. (Credit: © Bildagentur Zoonar GmbH/Shutterstock)

Scientists from the University of Oslo report that a series of outbreaks of plague, called the Black Death, which swept across Europe and Asia beginning in the mid-1300′s, may have been spread by gerbils rather than rats. The great gerbil (Rhombomys opimus) is native to Central Asia, where the Black Death originated.

Scientists have long believed that plague was spread to Europe by the black rat (Rattus rattus). The theory was that black rats traveled on ships from Central Asia. The rats carried the bacteria responsible for plague, Yersinia pestis, and once the rats arrived in Europe or Asia, fleas carried the bacteria from rats to people. Scientists theorized that rats living in Europe then acted as a reservoir for the Yersinia bacteria, resulting in later outbreaks of plague.

However, in a paper published this week in the Proceedings of the National Academy of Sciences, scholars claim that when they studied the dates, there was no correlation (relationship) between times in which weather conditions were good for rats and outbreaks of Black Death in Europe. When the scientists studied weather and conditions in Central Asia, however, there was a correlation between times that would have been good for great gerbils (warm and wet weather) and outbreaks of plague in Europe several years later. The scientists believe that when gerbils had a population boom in Central Asia, they may have carried the disease by the land and sea routes to Europe, and the fleas they carried passed the disease on to humans. Thus, the outbreaks in Europe were due to individual instances of transmission by gerbils, rather than a persistence of the disease in the rats.

Genetic studies of the plague bacteria should be able to determine whether the disease came to Europe once, and lay dormant in rats, or whether it was transmitted many times, and by gerbils. Until then, it seems as if we may have unfairly blamed the rat.

Other World Book articles:

• Apocalypse Then: A History of Plague (a Special report)
• Plague (a Research Guide)

The bacterium that caused the Black Death, one of the worst epidemics in human history, was virtually the same as the bacterium causing plague outbreaks in some parts of the world today, according to the surprising conclusion of a study of medieval skeletons. Scientists have long thought that the Black Death bacterium, called Yersina pestis, was so lethal because of certain genetic characteristics that had disappeared over the centuries. Instead, the researchers argued, other factors accounted for the Black Death’s toll. Scholars estimate that the plague killed about 30 million people, about half the population of Europe, from 1347 to 1351.

The researchers’ findings are based on a nearly complete reconstruction of the Y. pestis genome (total amount of genetic information) from DNA extracted from the teeth of three victims of the Black Death. The victims had been among 2,500 people buried in a mass grave in East Smithfield Cemetery in London. When the researchers compared the ancient Y. pestis genome to modern versions, they found that two were nearly identical.

The researchers suggested that the Black Death claimed so many victims, in part, because the immune systems of the Europeans of that time had never before been exposed to Y. pestis. “The Black Death was the first plague pandemic [widely spread epidemic] in human history,” said lead researcher Johannes Krause of the University of Tuebingen in Germany. Plague bacteria may not be as deadly to modern people because genetic mutations (changes) that allowed medieval people to survive the disease were passed down to later generations. The researchers said that the Black Death also spread so rapidly and killed so many because of miserable living conditions that included poor nutrition caused by climate change and crop failures, lack of sanitation, and war.

Additional World Book articles:

• Human Genome Project
• Middle Ages