Black rats have borne the blame for spreading the bubonic plague and killing 75 million in Europe over approximately 800 years. But, new research seems to have exonerated that rodent. Instead of “smelling a rat,” it appears people should be looking at a new culprit. Scientists now theorize that gerbils are guilty of spreading the Black Death, and rats should no longer be “plagued” by the blame.
Black rats, hosting the disease-carrying fleas, were previously believed to be responsible for the outbreaks. The fleas jumped on and bit infected rodents; they then bit humans and passed on the disease. Scientists now believe, based on tree-ring evidence, that gerbils carried the plague host insects.
The repeat epidemics of the plague or Black Death in Europe starting in the mid-14th Century have been traced back to gerbils that came from Asia. “If we’re right, we’ll have to rewrite that part of history,” noted Prof Nils Christian Stenseth, who is from the University of Oslo.
As the scientists outlined in the Proceedings of the National Academy of Sciences, the plague originated in Asia. It arrived in Europe in 1347 and, over the next 400 years, killed by some estimates 30 to 60 percent of the people in Europe in repeat pandemics.
The researchers analyzed tree-ring records from Europe and compared them with historical plague outbreaks. They were looking to determine what the weather conditions were during each epidemic since a rat-driven outbreak would have required warm, somewhat dry summers.
After reviewing 15 tree-ring records that show yearly weather conditions, they found that the weather that correlated with the plague outbreaks was not in Europe; it was in Asia. Europe experienced its plague outbreaks in years after central Asia had a wet spring then a warm summer.
That was their big clue. Those weather conditions were not conducive to breeding black rats, but ideal for the Asian gerbils to multiply. They now believe those rodents and their fleas thrived during those years then traveled to Europe along with Silk Road trade to wreak epidemiological havoc.
The findings also explain why the disease popped up so intermittently throughout the centuries. The rats remained around and did not carry the plague in the intervening years.
The team of researchers now plans to test their theory by analyzing plague bacteria DNA that was found on ancient skeletons buried across Europe during the epidemics. They hope to find large amounts of variation in the genetic material, which would suggest their theory is accurate. If the plague came from Asia at various times in history, the genetic waves of the plague would show more differences than a strain that emerged from a rat reservoir.
While finding out the sources of the plague in Europe may seem historical and learned, there continue to be plague outbreaks in parts of the world today. So, finding more information about the sources would be helpful. In fact, the World Health Organization reported nearly 800 plague cases worldwide in 2013, resulting in126 deaths. They hope, knowing for sure that the rats were not to blame for the plague Europe, they can do something about the guilty gerbils in areas that still are periodically grappling with the deadly disease.
By Dyanne Weiss