Vincent Jansen is Professor of Mathematical Biology at Royal Holloway, University of London. In his research he uses mathematics and models to understand how things work in biology, evolution and epidemiology.

This story is about the research of Dr Graham Twigg, who was made an honorary fellow to Royal Holloway in March 2019. This is an edited version of a contribution to The Scientists’ Scribe, the biology student led magazine of the Department of Biological Sciences, Royal Holloway University of London.

The artwork on this page was done by Yasmin Azizbayli and Ulviya Ahmadova. Yasmin was editor-in-chief of the Scientists’ Scribe from 2019-2020.

What zoology has taught us about the Black Death

It started with a small flotilla of ships, pulling into a Sicilian port. The ships were filled with refugees fleeing from a war, biological warfare even, and carried a dreadful disease.

As port authorities spotted the symptoms, such as swellings and blackened bodies; they quarantined the ships. But it was too late, the disease has started spreading through the town and over the island of Sicily. And it didn’t stop there: the Black Death spread with a terrifying speed over the whole of Europe. After landing in 1347 in Messina, Sicily, the disease reached England in 1348 and Norway in 1350, travelling at a speed of 1-5 miles a day. It would change things forever.

The carnage was of a magnitude never seen before. In the pandemic’s wake, villages were left deserted, fields left fallow. It changed society: peasants demanded better wages and gained economic freedom. Life would never be the same again: the Black Death heralded the end of medieval society.

What caused the disease? Medieval people realised the disease was contagious, without fully understanding the biology of the disease. It probably didn’t matter, the disease could not be controlled or contained.

Centuries later, Alexandre Yersin investigated an outbreak of bubonic plague in Hong Kong. He established that bubonic plague is caused by a bacterium, transmitted from rats to humans via rat fleas. The symptoms of bubonic plague are buboes (swellings) and gangrene (leading to blackened body parts): the same symptoms as the Black Death.

The conclusion was obvious. Black Death had to be the same as bubonic plague and consequently, it would have been transmitted by rats and their fleas. It was generally thought that hygiene in medieval times left a lot to be desired for, so that would all fit.

This became, and still largely is, the accepted vision of what caused the Black Death. Just look at the BBC bitesize website: it has a diagram with arrows pointing from Italy to England for rats to follow, and a video where rats are seen moving through sewers. But is this how the Black Death actually moved through Europe?

Graham Twigg didn’t know too much about the Black Death, but he did know about rats – a lot. Before he became a lecturer at Royal Holloway he had worked for the Colonial Office as their “Colonel Rodent Liaison Officer” in Guyana and had studied rats in coalmines for his PhD. And one thing he was very certain about: when the Black Death arrived in Europe there were rats, and they weren’t at all abundant. The rats were black rats (Rattus rattus), which are very different from brown rats (Rattus norvegicus), the type that thrive in sewers, and which did not appear on the scene in these parts until the eighteenth century.

The black rat is essentially a tropical animal. It had arrived in Europe by ship. Its distribution is very much limited to ports where it lives in warehouses and grain stores, and it is hardly found further inland. It didn’t add up: the black rat and its fleas were unlikely to be the transmitter of the disease that caused the Black Death. In 1984, he published his findings in great detail in a book in which he asserted that it would not have been possible for a rat-borne disease to spread at that speed.

That leaves the question how the Black Death did spread so fast. Could it be that, perhaps, it wasn’t bubonic plague after all, but something different like a nasty virus? In recent years, DNA analysis has shown that the plague bacterium was present in medieval skeletons, so the disease was the bubonic plague.

The debate has been ongoing, and the latest is that there have been very detailed analysis of the spread of historical Black Death using clever mathematical models (see paper here). The conclusion from the modelling exercise is that it is indeed unlikely that it was rats and rat fleas that were responsible. More likely is that human fleas were in on the act. Although not as efficient in transmission, human fleas can transmit the plague, and if there are enough of them (and in medieval times they were very common) they could spread the disease

So 35 years after Dr Twigg published his book, the question of how the disease spread is still unanswered. But the explanation we all know – rats and their fleas – is unlikely to be the whole story.

By using his knowledge of zoology, and the power of deduction, he had discovered that the common explanation for the cause of the Black Death was likely to be a myth. Without being an expert on disease or medieval society, he had worked out something very fundamental from zoological facts. Even if the full answer is still elusive, it is a remarkable piece of science.

Vincent Jansen, September 2019

 A big thank you to Bethany Nichols for her proofreading and improving the text.

Further reading:                     

Graham Twigg: The Black Death: A Biological Reappraisal. London, Batsford. 1984

<<<< Read the previous blog ------------------- Read the next blog >>>>