Gerbils replace rats as main cause of Black Death

The Black Death, also known as plague or Yersinia pestis began in Europe as the second plague pandemic in 1347 CE and by 1353 had killed an estimated 75 to 200 million people. Unlike most pandemics that sweep through a population and then decline, plague re-emerged by fits and starts in various places until the early 18th century.

It is commonly thought that rats were responsible for spreading the disease and that when rat populations increased the disease would re-emerge and spread again.

However, new research published by the Proceedings of the National Academy of Sciences, casts doubt on these assumptions. The researchers blame populations of gerbils, rather than rats, for the outbreaks. Additionally they suggest that plague was regularly re-imported from Asia rather than going dormant.

The researchers studied more than 7,500 plague outbreaks as well as climate and weather patterns. According to the researchers, a booming rat population requires warm summers with a moderate amount of rainfall, neither too wet or too dry.

On the other hand, when the team looked at Asia they found conditions that would have been ideal for population explosions in the giant gerbil, another plague carrying rodent. In total the team identified 16 outbreaks in Europe between 1346 and 1837. They found that in every case, weather conditions in Pakistan would have been right for an increased rodent population within about 15 years prior to the European outbreak.

“We show that wherever there were good conditions for gerbils and fleas in central Asia, some years later the bacteria shows up in harbour cities in Europe and then spreads across the continent,” Prof Nils Christian Stenseth, from the University of Oslo, told the BBC.

The team has now taken a variety of DNA samples from the skeletons of ancient plague victims for analysis. If the material shows substantial variation between plague bacteria, it would lend support to the team’s theories.

A large amount of genetic variation between one wave of plague and the next, for example, would imply that it was re-imported from Asia instead of re-emerging from a local reservoir of the disease in a rat population.

One of the reasons that plague is so dangerous is the versatility of the pathogen. The specific symptoms of the disease depend on the area of the infection. Bubonic plague begins in the lymph nodes while septicemic plague starts in blood vessels and pneumonic plague in the lungs.

While most would like to think of the new research as purely a historical exercise, plague hasn’t actually gone away. According to the World Health Organization (WHO) there were 800 cases of plague resulting in 126 deaths in 2013.

A paper published in the American Journal of Tropical Medicine and Hygiene, suggests that agricultural expansion in East Africa has caused conditions ripe for a new plague outbreak. After the experience of the Ebola outbreak in 2014, a better understanding of how the plague emerges and spreads prior to a human outbreak could become very important.