Rats were not the main vectors of the Black Death, according to scientists

Rats were not the main vectors of the Black Death, according to scientists

The Black Death is one of the world’s best-known pandemics, killing millions and afflicting Europe especially between the years 1347 and 1353, but continuing to cause outbreaks well into the 19th century. It is popular wisdom that the pathology would be spread by rats, which carried the responsible bacteria (Yersinia pestis) in their fleas, especially wild rodents, called “reservoirs” of the disease. There is a possibility, however, that we have overestimated the role of animals.

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It is true that the disease starts in rodents, escaping to humans only occasionally. The European continent was also often home to animal reservoirs that caused some of the plague outbreaks — but science is now trying to understand the role of repeated reintroductions of the disease across Asia, to and from where many travelers went and returned, bringing the bacteria again. Which scenario was more prevalent, or whether they both had a shared responsibility, we’re still trying to figure out.

How was the plague transmitted?

In new research, scientists from the Scottish universities of Glasgow and Stirling found that the environmental conditions of medieval Europe would preclude long-lived animal reservoirs. Based on this finding, we have to consider other possibilities for the persistence of the plague.

One is the repeated reintroduction from Asian reservoirs, and the other is the existence of short- and medium-term reservoirs on the European continent. The speed of transmission, however, and the widespread outbreaks would not be possible with the slow movement of the mice. It is more likely, therefore, that human-to-human transmission played a greater role than previously thought.

To determine the ideal environmental conditions for rodents, factors such as soil characteristics, climate, terrain types and rodent varieties were taken into account. The presence of copper, manganese, iron and high pH, ​​in addition to lower temperatures, higher altitudes and less rainfall are some of the factors that lead to more persistent reservoirs, for example — even if we don’t know why.

With comparative Reviews, the researchers determined that centuries-old plague reservoirs in rodents were less likely from the Black Death of 1348 to the 19th century than they are today: they couldn’t even exist in Europe, and the ideal locations would be some regions of modern China and the western United States, where we can see rodent reservoirs of the plague today.

In central Asia, reservoirs like these may have existed for millennia, especially in rodents such as the steppe marmot (Marmota bobak). Ancient DNA and textual evidence show us that once the plague traveled all the way to Europe, it would have initiated short- and medium-term reservoirs, with the most likely location being Central Europe. With the impossibility of staying there for long periods, the plague would have to be reintroduced to continue being transmitted, at least sometimes.

In defense of rodents

The role of rats — or lack thereof — is evident when looking at the different outbreaks. The first occurrence of the plague, at the beginning of the 6th century, lasted until the 8th century, while the second, which includes the Black Death, began in the 1330s and lasted for 5 centuries. The last one started in 1894 and continues today in places like Madagascar and California. For the most part, the occurrences were of the bubonic plague, when the bacteria invades the lymphatic system, part of the body’s immune defense. In pneumonic plague, those affected are the lungs.

The second plague pandemic was very different from the current one, from transmission to the characteristics of the disease. Mortality was much higher, reaching 50% in the past, while in the third pandemic, this figure rarely exceeded 1%. In Europe, these numbers are even lower for the latter.

The frequency and speed of transportation of goods, animals and people in the Middle Ages was much lower than today or even in the 19th century, but still the transmission was incredibly fast. On land, she ran almost as fast in a day as she did in a year in current outbreaks. Records of chroniclers, physicians and other medieval scribes report that the disease spread faster and to more places than any other pathology of the time, surpassed only by cholera in 1830 and the influenza epidemic of 1918-20.

European rodents, wild or not, move much slower than that. Outbreaks of the third pandemic, except for the rare cases of pneumonic plague, match the fertility cycle of rodent fleas very well, but in the second pandemic it was months on end of bubonic plague, even during the winter, even in extreme cold. of the Baltic, as seen from 1709 to 1713. This was also true of the Mediterranean heat from 1348 until the next century, with contagion peaking in June and July, the region’s hottest months.

In addition to differing greatly from recent outbreaks, these data do not match the ideal temperature for the plague to jump from rodents to humans. This suggests that transmission would be more efficient from person to person, also thanks to ectoparasites such as lice and fleas, or through the respiratory system and touch.

There is still much to discover about the human and rodent role in these pandemics, redeeming the wronged rats, but one thing is certain — when the work of historians joins that of biologists, we get much further in discoveries.

Source: PNAS