Black rats accused of spreading plague colonized Europe twice

(ORDO NEWS) — According to scientists, this is the first genetic study of Rattus rattus – a black, or ship rat.

A joint study of biologists, zoologists, archaeologists, geneticists from the University of York, Oxford, Liverpool (UK), the Max Planck Institute for Evolutionary Anthropology in Leipzig, Philipp University of Marburg (Germany), Peking University (China), the University of California (USA) and many other universities around the world helped to understand how the black rat Rattus rattus settled in Europe.

Rattus rattus , along with the gray rat ( Rattus norvegicus ) and house mouse ( Mus musculus ), spread through close commensal relationships with humans.

These rodents have long since become real pests: not only do they damage food stocks worth billions of euros every year, but they also act as vectors and reservoirs for numerous diseases. The most famous and especially dangerous of them is bubonic plague , which was carried by fleas that parasitize rats.

The Black Death is the second and most devastating bubonic plague pandemic in world history, caused by the bacterium Yersinia pestis , which is believed to have persisted for centuries in wild rodent colonies in Asia and mutated into a form more dangerous to humans around the early 1300s.

This was preceded by the Plague of Justinian , which struck the world in 541 AD, during the reign of the Byzantine emperor Justinian.

Traditionally, historians have argued that plague epidemics are associated with the transmission of infected fleas from wild rodents to ship rats that live in cities.

However, there are studies that try to put the blame on rodents, and some evidence suggests that the disease must have been transmitted first by direct human contact with rats or mice, and then spread through human fleas and head lice.

Black rats accused of spreading plague colonized Europe twice 2
Funeral of plague victims in Tournai. Fragment of a miniature from the chronicle of Gill Mayzet, XIV century

The ability of rats to colonize entire regions and become dependent on humans makes them ideal for tracking historical processes: archaeological specimens of rodents have often been used to track human migrations, trade, and settlement types. But despite all this, our knowledge of the evolution and taxonomy of rats remains limited.

“The earliest large concentrations of alleged remains of commensal rats come from settlements of the late third or early second millennium BC in both the Indus Valley and Mesopotamia. Commensal black rats may also have reached the Levant and the Eastern Mediterranean region by the beginning of the first millennium BC.

Based on archaeological evidence from Corsica, the Balearic Islands, Italy and Morocco, black rats probably first appeared in the Western Mediterranean basin towards the end of the same millennium.

They note that the colonization of Europe by black rats was associated with the development of cities and trade networks.

The remains of Rattus rattus are found throughout the Roman Empire in the 4th century AD, but rarely beyond its northern borders: it can be assumed that these rodents depended on the Roman economic system, characterized by a network of dense settlements connected by many roads, as well as river and sea routes.

With the fall of the empire, starting from the 5th century, traces of the presence of Rattus rattus become less and less – both in the northern provinces, including Britain, and in the “Italian core” of the Western Empire. Moreover, in the Balkans and Anatolia, black rats continued to exist at least until the 6th century.

Then they reappear in northern European settlements in the 9th and 10th centuries AD, including far beyond their former Roman range: for example, in Hedeby in the very north of Germany and Birka, the largest trading center of the Swedish Vikings in 800-975.

“The subsequent spread of urbanism and large-scale wholesale trade in medieval Europe seems to have favored rats, as it did in the Roman period. By the 13th century AD, they lived in most of Europe and reached the south of Finland by the end of the 14th century.

Rattus rattus were ubiquitous and widely present in Europe until at least the 18th century, before their population declined – most likely due to competition with the recently arrived gray rats.(This species dominates in European countries with a temperate climate today. – Approx. ed.) , ”the researchers continue.

In their work, scientists tried to answer questions that remained poorly understood: was Rattus rattus really exterminated in post-Roman Northern and Western Europe; did the medieval populations of rats in Europe originate from a remnant population in southern Europe, or from rodents introduced from beyond the Mediterranean?

The scientists also analyzed the model of the spread of the Justinian plague and the first pandemic – it was during the period of the Black Death that there was a gap in the archaeological data on rats in northwestern Europe.

To study the demographic history of Rattus rattus in relation to other species, the researchers assembled its genome de novo . They then sequenced 67 ancient and three modern mitochondrial genomes, as well as 36 ancient and three modern nuclear genomes from archaeological sites spanning the 1st-17th century AD in Europe and North Africa.

The results of paleogenomic analysis showed that the black rat was originally brought to the Eastern Mediterranean by land through Southwest Asia, although scientists have not ruled out a route through the Indian Ocean and the Red Sea.

“We identify two waves of rat introductions to temperate Europe. The first probably accompanied the Roman expansion to the north in the first centuries BC, and the second took place in the medieval period (starting from the 8th-10th centuries AD).

The rodents of the second wave probably descended from the same ancestral population as the rats of the first wave, ”the authors of the study concluded.

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Lower jaw of a black rat / © Evan Chipping, University of York

The fact that the populations and range of Rattus rattus declined during the Early Middle Ages is confirmed by the scarcity of archaeological remains of these animals in the 6th-8th centuries AD, especially in the north and west of Europe.

It is likely that this is indeed due to the collapse of the Roman Empire, and with it the networks of interconnected settlements that helped keep black rats alive and spreading (mainly through the transportation of grain). In addition, the first plague pandemic and a cold snap in the middle of the 6th century could negatively affect European Rattus rattus.

But to work out such scenarios more precisely, further zooarchaeological and genomic studies of ancient rats are needed that would cover these centuries over a wider geographic range, the scientists added.

As for the second wave of rat settlement in Northern Europe, this is confirmed by their presence in Germany at the beginning of the 10th century (at the latest): archaeologists have found bones of Rattus rattus of that period all over the continent.

Re-colonization, according to researchers, is due to the presence of rats in the northern emporia around the North and Baltic Seas, the development of trade routes in the empire of Charlemagne (controlled most of Western and Central Europe, as well as Northern Italy in the 9th century) and, consequently, the restoration of large-scale trade links between the Mediterranean and northern Europe. Thus, trade with Russia, which was mentioned earlier, did not play a role here.

“Black rats seem to have been a constant presence in Europe from this point until the post-medieval period, from the black death of the 14th century to the 17th century,” the scientists concluded.

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