Your name symbolizes all diseases that spread death and terror - the plague itself seems to have been defeated today. But it still lurks in natural springs, perhaps waiting for its return: climate change could help.
"We were off Madagascar and had the plague on board," is a line from the eponymous popular hitman of the seaman, who describes the inadequate sanitary and medical conditions on a leaky and stranded ship far from home. However, neither the sailors on site nor the copywriter at home probably suspected that, in addition to inadequate hygiene and the disastrous supply situation, there was perhaps another point that allowed Yersinia pestis to play easily with the crew: the tropical climate.
Because, according to the new findings of the researchers led by Nils Stenseth from the University of Oslo, increased temperatures and precipitation obviously improve the spread conditions for the deadly germ - at least in its natural host and reservoir, the large gerbils (Rhombomys opimus) of Central Asia. These quite cute rodents live in the arid and semi-arid ecosystems of the region, feed on vegetarian food there and are subject to strong cyclical fluctuations in quantity, in which their populations first increase significantly and then collapse all the more drastically.
The animals are not only running out of food due to the mass reproduction, they are also being plagued in the truest sense of the word: almost two years after the population has exceeded its maximum, the bubonic plague strikes - in the colonies of the sociable rodents settlement density is then still sufficiently high to ensure barrier-free transfer of the pathogen by fleas. At the same time, the lack of food has probably already weakened the animals to such an extent that the disease now has an easy time.
However, as is almost always the case in nature, the situation is not just reduced to the equation "High numbers of gerbils plus parasites plus lack of food equals plague". Because the flea carriers of the Black Death are also dependent on the weather: In the years between 1949 and 1995 examined by Stenseth's team, the risk of devastating epidemics grew to the same extent as temperatures and precipitation rose - with every degree that the spring got warmer failed, the spread of Yersinia pestis in rodent colonies increased by almost 60 percent, according to the scientists.
Favourable weather conditions also allowed the fleas to sprout in addition to the preferred food of the gerbils: the fewer late frosts there were and the more frequently the temperatures were above ten degrees Celsius, the more active the small insects were and the more the nuisances and their offspring survived. Milder springs also increased the number of biting attacks, bugs migrated more quickly to the entrances to the underground gerbil burrows, and the flea population cycle accelerated: eggs that had already been laid matured faster, while the adults produced new clutches more quickly.
If a favorable spring was followed by a warm and at the same time damp summer, the danger of a later plague epidemic among gerbils increased, since different flea generations could then overlap and consequently the sensitive bacterium was more likely to be passed on to one another. Even a tenth more rain than usual increased the influence of the germ by a further seven percent. At the same time, the gerbils took advantage of the favorable weather, produced more offspring and spread - and with them the ominous fleas, which then said two years after the population maximum finished off the majority of the remaining population. On the other hand, intermittent icy springs, hot and dry or humid and at the same time hypothermic summers hindered the development of fleas and thus spared the rodents at least from dying from the plague.
This association between weather conditions and outbreaks of Yersinia pestis also sheds new light on the mass extinctions caused by the plague in medieval Europe, as well as during its last major plague in Asia between 1855 and 1870: both periods thus offered optimal conditions for spread of gerbils and with them the bad bacterium – especially in Central Asia, where the springs were generally warmer and the summers wetter. It was then just a flea hop along caravan routes or near human settlements before the disease spread to humans and from there to the Old World, eventually claiming millions of victims.
In the wake of climate change, conditions for Rhombomys opimus and its deadly stowaway are improving today as temperatures and rainfall rise again in Central Asia, creating a thriving environment for mice, flea and bacteria. And since at the same time the hygienic and medical conditions as well as the epidemic monitoring of rodents in the post-Soviet states of Central Asia have not necessarily improved, Stenseth and his colleagues warn against a more frequent return of the Black Death among humans - at least in its core distribution area. He doesn't necessarily have to make it to Madagascar right away.