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  • Brendan Shaw

Exonerated!: Rats not to blame for the 14th century Black Death

Updated: Feb 3, 2023

Brendan Shaw

“ .. the Porters who opened the infected Bales of Goods in the Lazaretto's [sic] of Marseilles, died upon the first Appearance of Infection, as it were by a sudden stroke … death insued sometimes in a few hours”

- Richard Mead, A discourse on the plague, 9th ed., London, 1744, pp. 48.

“Doctors and chroniclers marvelled at the Black Death's lightning transmission, reporting that mere speech was enough to pass it directly and immediately from one person to the next.”

- Samuel Cohn, “Epidemiology of the Black Death and Successive Waves of Plague”, 2008

Spare a thought for the humble rat.

Long blamed for the ‘Black Death’, one of the worst bubonic plagues in human history, it turns out that Rattus rattus may have been framed, so to speak.

In one of the great detective stories of the 21st century, more research is coming out to indicate that rats were not to blame for the ‘Black Death’ plague pandemic that killed millions of people in Asia and Europe in the mid-14th century.

What was the ‘Black Death’?

The ‘Black Death’ in 14th century Europe – originally known at the time as the ‘Great Pestilence’ – was a bacterial pandemic brought about by the Yersinia pestis bacteria. The Black Death was part of the second plague pandemic that swept through Europe. The first started in the early 6th century – the Justinian Plague – and lasted until the 8th century. The second plague pandemic, including the Black Death, started in the 14th century and continued for five centuries, while the third plague pandemic began in 1894 and continues to this day in a few places including California and Madagascar.

Source: Wikipedia.

The 14th century Black Death is particularly noteworthy due to its high mortality and speed of its transmission. Historical records of the time talk of arriving ships instantly infecting Europe’s trading ports, thousands of sudden deaths across cities in a very short space of time, and whole villages being wiped out in a matter of weeks.

For example, tax records show that Florence in Italy lost at least 75% of its population during the summer months of 1348 and chroniclers of the time record that some towns in Italy were just abandoned after 1348. Similarly, manorial records show that villages in Cambridgeshire and the bishopric of Worcester in Britain, and the region around St-Flour (Auvergne) in France lost up to 80% of their population.

In Europe, up to 60% of the population died from the Black Death during a relatively short period from 1347 to 1353. Other estimates suggest that while 25 million people died in Europe, an equal number died in Asia and Africa at the time. These pandemics largely involved the bubonic form of plague – an infection of the lymph nodes – as opposed to pneumonic plague which infects the lungs.

Spread of the Black Death plague, 1346 - 1353

Source: Wikipedia.

Plagues of the second pandemic like the Black Death differed radically from other plagues in part because its mortality rate was so high, reaching 50%. Other plague pandemics only reached 1% mortality in some parts of the world, with Europe having even lower mortality rates. The second big difference was how fast the second pandemic moved. Even chroniclers at the time noted its speed of transmission, spreading more widely and much faster than any other disease of the period, or even any other disease in history until the 19th century cholera outbreaks or the ‘Spanish Flu’ influenza pandemic of the early 20th century.

Were rats and their fleas to blame?

One of the often-repeated statements about the Black Death was that it was spread by rats and the fleas that live on rats. (Indeed, I confess to have recounted it myself not too long ago in a blog discussing advice in 17th century London on how to combat plague). The theory that rats were responsible for transmitting the Black Death has existed through history from the time that it hit Europe up to research in the 20th century. Indeed, rats and their fleas can provide transmission for modern plague, so the assumption in studies for decades was that the same applied to the Black Death.

However, research has emerged through a variety of fields in more recent times that exonerates the humble Rattus rattus from the trail of destruction left by the Black Death.

The most recent research released at the end of last year provides further evidence that rats are unlikely to have been primarily responsible for the Black Death. This multinational study compared soil chemistry and rodent diversity in Europe with that found in China and the western United States. It found that differing environmental conditions and lack of host rodent species in Europe suggest that wild rodents like rats were not the cause.

Critically, the study shows that:

“… the rapid spread of the Black Death and subsequent outbreaks of the next few centuries also suggest slow-moving rats may not have played the critical role in transmitting the disease that is often portrayed.”

Amongst other things, the authors demonstrate that the speed with which the Black Death swept across Asia, the Middle East, Africa and Europe was far faster than what you’d expect if rats were transmitting the plague bacteria. Some researchers have calculated that the Black Death travelled through Europe between 1.5 km and 6 km per day, much faster than any plague transmission in the 20th century.

No other human epidemic has shown such a propensity to move so fast, perhaps until modern times. In the Middle Ages, rats would have had a hard time sustaining constant travel at 6 km per day.

Other studies in the last few decades have used a variety of historical research, archaeology, genomics, mathematical modelling and science to demonstrate that rats are unlikely to have been responsible for the Black Death, including:

  • Historical zoological records and archaeological digs from medieval Norway and other Nordic locations finding little evidence of black rats in communities despite the many plague epidemics that occurred there

  • Mathematical modelling using Bayesian inference techniques to model different explanations against mortality data from nine outbreaks in Europe, finding that the rodent transmission theory does not explain the observed patterns

  • An examination of the vast swathe of historical records across Europe from the period 1347 to 1352 provides no evidence of a link between late medieval or early modern European plague from that time and the description or even suspicion of an epizootic of rodents that preceded or accompanied human plague, and

  • Studying tree ring data in Asia from 1250 to 1850, particularly the Karakorum Mountains in northern Pakistan, to link climate fluctuations and resultant wild rodent populations there with plague outbreaks in European port cities. The study suggested a 15-year time lag between climate changes in Asia and European outbreaks.

Thus, a range of evidence is emerging that possibly the worst pandemic in human history was not caused by the long-assumed offender.

So, who’s the culprit?

While there’s no definitive answer, most of the studies instead point the finger at human lice for the Black Death. So, while for the last 600 years we’ve tried to pin the blame on the rat and its fleas, it turns out it may be our own parasites and our propensity to share them in close quarters that were major reasons for the bacteria’s rapid spread to the Black Death in the 14th century.

The speed with which the second plague pandemic and the Black Death swept through Europe suggests that it almost had to be transmitted through human-to-human contact, rather than waiting for fleas on rats to migrate from Asia to Europe.

Source: Wikimedia.

Recent studies using DNA sampling and genomic sequencing from the bodies of people buried after a 14th century outbreak in the Chu-Valley in Kyrgyzstan suggest that the strain of Yersinia pestis that led to the Black Death may have originated from a stopover on the Silk Road trade route in the 14th century in Central Asia.

Another study combining genomic sequencing of DNA samples of Black Death-era plague with economic history and legal historical records suggests the second plague pandemic was brought to Europe through the fur trade that transited from Central Asia along the Silk Road trade routes in the 13th and 14th centuries.

However, the reasons for the startling speed and mortality of the Black Death might still be more complicated.

There’s evidence that the Black Death plague of the 14th century was spread by human lice, but also by coughing and touch between humans. Doctors and chroniclers at the time were amazed at the speed of transmission from person to person, either through coughing or touch. There are stories at the time of sailors leaving their trade ships and infecting the local population instantly, with people becoming infected just by opening bales of cargo and European port cities rapidly succumbing to the disease after trading ships arrived.

Implications for today

So, what does all this mean for us here today who are living through a global COVID-19 pandemic in the 21st century?

First, it can take a long time to work out how pandemics start. The Black Death occurred 600 years ago, and we still don’t know exactly how it started and how it was transmitted, despite it being one of the worst pandemics in human history.

Second, conclusions made by people at the time and subsequent researchers in modern times that rats and their fleas were the major cause of transmission of the second plague pandemic look like they were wrong. This was in part due to the biases, assumptions and the knowledge and technologies available at the time. Yes, the rat was ‘framed’ by the frames of reference used by commentators and researchers. There’s nothing necessarily wrong with this, but it’s something to be expected.

Finally, the scientific and technological tools we have today provide an enormous opportunity to study past pandemics and help us prepare for future ones.

As well being a fascinating story of history, evidence, science and research, this story provides some useful lessons for us today ... and exonerates the humble rat from primary responsibility for one of the worst pandemics in human history.

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