Plague (disease) - History - Second Pandemic: From 14th Century (Black Death) To 19th Century - Nature of The Black Death

Nature of The Black Death

In the early 20th century, following the identification by Yersin and Kitasato of the plague bacterium that caused the late 19th and early 20th century Asian bubonic plague (the Third Pandemic), most scientists and historians came to believe that the Black Death was an incidence of this plague, with a strong presence of the more contagious pneumonic and septicemic varieties increasing the pace of infection, spreading the disease deep into inland areas of the continents. It was claimed that the disease was spread mainly by black rats in Asia and that therefore there must have been black rats in north-west Europe at the time of the Black Death to spread it, although black rats are currently rare except near the Mediterranean. This led to the development of a theory that brown rats had invaded Europe, largely wiping out black rats, bringing the plagues to an end, although there is no evidence for the theory in historical records. Some historians suggest that marmots, rather than rats, were the primary carriers of the disease.

The view that the Black Death was caused by Yersinia pestis has been incorporated into medical textbooks throughout the 20th century and has become part of popular culture, as illustrated by recent books, such as John Kelly's The Great Mortality. Many modern researchers have argued that the disease was more likely to have been viral (that is, not bubonic plague), pointing to the absence of rats from some parts of Europe that were badly affected and to the conviction of people at the time that the disease was spread by direct human contact. According to the accounts of the time the Black Death was extremely virulent, unlike the 19th and early 20th century bubonic plague. Samuel K. Cohn has made a comprehensive attempt to rebut the bubonic plague theory. In the Encyclopedia of Population, he points to five major weaknesses in this theory:

  • very different transmission speeds — the Black Death was reported to have spread 385 km in 91 days (4.23 km/day) in 664, compared to 12–15 km a year for the modern bubonic plague, with the assistance of trains and cars
  • difficulties with the attempt to explain the rapid spread of the Black Death by arguing that it was spread by the rare pneumonic form of the disease — in fact this form killed less than 0.3% of the infected population in its worst outbreak (Manchuria in 1911)
  • different seasonality — the modern plague can only be sustained at temperatures between 10 and 26°C and requires high humidity, while the Black Death occurred even in Norway in the middle of the winter and in the Mediterranean in the middle of hot dry summers
  • very different death rates — in several places (including Florence in 1348) over 75% of the population appears to have died; in contrast the highest mortality for the modern bubonic plague was 3% in Bombay in 1903
  • the cycles and trends of infection were very different between the diseases — humans did not develop resistance to the modern disease, but resistance to the Black Death rose sharply, so that eventually it became mainly a childhood disease

Cohn also points out that while the identification of the disease as having buboes relies on accounts of Boccaccio and others, they described buboes, abscesses, rashes and carbuncles occurring all over the body, the neck or behind the ears. In contrast, the modern disease rarely has more than one bubo, most commonly in the groin, and is not characterised by abscesses, rashes and carbuncles.

Researchers have offered a mathematical model based on the changing demography of Europe from 1000 to 1800 AD demonstrating how plague epidemics, 1347 to 1670, could have provided the selection pressure that raised the frequency of a mutation to the level seen today that prevent HIV from entering macrophages and CD4+ T cells that carry the mutation (the average frequency of this allele is 10% in European populations). It is suggested that the original single mutation appeared over 2,500 years ago and that persistent epidemics of a haemorrhagic fever struck at the early classical civilizations.

However recent research published in the open-access scientific journal PloS Pathogens in October 2010 presented conclusive evidence that two previously unknown clades (variant strains) of Y. pestis were responsible for the Black Death. A multinational team conducted new surveys that used both ancient DNA analyses and protein-specific detection to find DNA and protein signatures specific for Y. pestis in human skeletons from widely distributed mass graves in northern, central and southern Europe that were associated archaeologically with the Black Death and subsequent resurgences. The authors concluded that this research, together with prior analyses from the south of France and Germany,

"... ends the debate about the etiology of the Black Death, and unambiguously demonstrates that Y. pestis was the causative agent of the epidemic plague that devastated Europe during the Middle Ages."

The study also identified two previously unknown but related strains of Y. pestis that were associated with distinct medieval mass graves. These were found to be ancestral to modern isolates of the present-day Y. pestis strains 'Orientalis' and 'Medievalis', suggesting that these variant strains (which are now presumed to be extinct) may have entered Europe in two waves. Surveys of plague pit remains in France and England indicate that the first variant entered Europe through the port of Marseille around November 1347 and spread through France over the next two years, eventually reaching England in the spring of 1349, where it spread through the country in three successive epidemics.

However, surveys of plague pit remains from the Netherlands town of Bergen op Zoom showed evidence of a second Y. pestis genotype which differed from that found in Britain and France and this second strain is now thought to have been responsible for the pandemic that spread through the Low Countries from 1350. This discovery implies that Bergen op Zoom (and possibly other parts of the southern Netherlands) was not directly infected from England or France c. AD 1349, and the researchers have suggested that a second wave of plague infection, distinct from that which occurred in Britain and France, may have been carried to the Low Countries from Norway, the Hanseatic cities, or another site.

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