Why the Black Death is still a puzzle.

The Black Death, a bubonic plague, devastated Europe and North Africa from about 1346 for over a decade. The plague was spread by fleas on rats, and on humans. It was quickly transported across Europe and into the British Isles through trade routes. It is estimated that between 1348 and 1350 The Black Death killed 25 million people in Europe .

The East Smithfield plague pits, which were used for mass burials in 1348 and 1349. Image Credit: Museum of London Archaeology (MOLA)

The plague is responsible for the two largest and most deadly pandemics in human history. Its origins continue to be a puzzle which scientists are still exploring because if we can understand how the plague was able to be transmitted then it gives us a better understanding of how to deal with future pandemics, including the present one, Covid-19.

Researchers at McMaster University, University of Sydney and the University of Melbourne, have completed a painstaking granular examination of hundreds of modern and ancient genome sequences, creating the largest analysis of its kind in order to track bubonic plague.

Their research includes an analysis of more than 600 genome sequences from around the globe, spanning the plague’s first emergence in humans 5,000 years ago, the plague of Justinian, the medieval Black Death and the current (or third) Pandemic, which began in the early 20th century.

Evolutionary geneticist Hendrik Poinar, director of McMaster’s Ancient DNA Centre explained:

“The plague was the largest pandemic and biggest mortality event in human history. When it emerged and from what host may shed light on where it came from, why it continually erupted over hundreds of years and died out in some locales but persisted in others.   And ultimately, why it killed so many people.” 

The plague of Florence, 1348; a scene from Boccaccio’s Decameron. Etching by L. Sabatelli the elder after G. Boccaccio. Wellcome Images

The team studied genomes from strains with a worldwide distribution and of different ages and determined that Y. pestis has an unstable molecular clock. This makes it particularly difficult to measure the rate at which mutations accumulate in its genome over time, which are then used to calculate dates of emergence.

Because Y. pestis evolves at a very slow pace, it is almost impossible to determine exactly where it originated.

The Covid19 virus has mutated many times. This means, and with modern methods, scientists are able to track where each mutation is coming from and how it is being transmitted to other countries.

Today plague is treatable through antibiotics and preventative measures. Plague epidemics have occurred in Africa, Asia, and South America; but since the 1990s, most human cases have occurred in Africa. The three most endemic countries are the Democratic Republic of Congo, Madagascar, and Peru. In Madagascar cases of bubonic plague are reported nearly every year, during the epidemic season (between September and April).

Key facts

  • Plague is caused by the bacteria Yersinia pestis, a zoonotic bacteria usually found in small mammals and their fleas.
  • People infected with Y. pestis often develop symptoms after an incubation period of one to seven days.
  • There are two main clinical forms of plague infection: bubonic and pneumonic. Bubonic plague is the most common form and is characterized by painful swollen lymph nodes or ‘buboes’.
  • Plague is transmitted between animals and humans by the bite of infected fleas, direct contact with infected tissues, and inhalation of infected respiratory droplets.
  • Plague can be a very severe disease in people, with a case-fatality ratio of 30% to 60% for the bubonic type, and is always fatal for the pneumonic kind when left untreated.
  • Antibiotic treatment is effective against plague bacteria, so early diagnosis and early treatment can save lives.
  • Currently, the three most endemic countries are the Democratic Republic of the Congo, Madagascar, and Peru.
World Health Organisation

In the Middle Ages bubonic plague spread faster than its genome evolved. Genomic sequences found in Russia, Spain, England, Italy and Turkey, despite being separated by years are all identical, for example, creating enormous challenges to determining the route of transmission.    

The researchers developed a new method for distinguishing specific populations of Y. pestis, enabling them to identify and date five populations throughout history, including the most famous ancient pandemic lineages which they now estimate had emerged decades or even centuries before the pandemic was historically documented in Europe.

“You can’t think of the plague as just a single bacterium,” explains Poinar. “Context is hugely important, which is shown by our data and analysis.”

To properly reconstruct pandemics of our past, present, and future, historical, ecological, environmental, social and cultural contexts are equally significant.

He explains that genetic evidence alone is not enough to reconstruct the timing and spread of short-term plague pandemics, which has implications for future research related to past pandemics and the progression of ongoing outbreaks such as COVID-19. 

Click on this link to access, Plagued by a cryptic clock: insight and issues from the global phylogeny of Yersinia pestis, published in Communications Biology

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