The Black Death is an historical example of infectious disease with a rapid onset, local persistence for more than 4 centuries and eventual extinction due to causes that are not currently understood. Few days ago Nature published an interesting paper that sheds light on the journey through Europe of the bacterium responsible for the plague during this black period of history.
The second plague pandemic, better known as the Black Death, devastated Europe and the nearby regions, between the 14th and 18th centuries. Historical documents seem to indicate that the Black Death pandemic began in 1346 in the regions of the lower Volga and the Black Sea. That year the Mongol armies besieged the Kaffa port (on the Crimean peninsula), controlled by the Republic of Genoa. The plague affected the Mongolian troops, who toke advantage of their little fortune. Launching the corpses of their dead in the city, they began the epidemic of plague at Kaffa. The Genoese, fleeing of the siege of Kaffa, brought the infection to the heart of Europe with their ships, which docked at the port of Messina (Sicily) with most of their crew dead or infected. The siege of Kaffa is one of the most important cases in the history of bioterrorism, but above all it is the arrival of the Black Death pandemic in Europe.
Besides the great mortality associated with the great outbreak of the 14th century, the most devastating effects were those associated with the small recurrent outbreaks that took place until the end of the 18th century. The Black Death appeared constantly from its beginning in the fourteenth century, punishing Europe for more than 400 years. This climate of terror, related with the beginning of the Renaissance, suppused a change in the world of art. Death is much more present, it adopts a phantasmagoric or skeleton form, and triumphs over life.
In 1546, Girolamo Fracastoro published the first theory on the infectious nature of the plague. Fracastoro proposed that the disease was caused by infectious agents of tiny size called “seminaria contagionis”. However, it was not until 1894 that Alexandre Yersin described the bacterium, during the third pestilence pandemic, in Hong Kong.
- Bubonic plague: Most frequent form, in which the bacteria enters the body by the sting of an infected flea. Y. pestis settles in the lymph nodes of the affected person, which inflate and can become open and suppurative wounds (buboes).
- Pneumonic or pulmonary plague: It is the most virulent form. Without treatment it is mortal and it is transmitted easily between people through the aerosols generated when breathing.
This month Nature journal presents a study that aims to decipher the genetic history of Y. pestis during the second plague pandemic. For this study, human remains have been analyzed from ten European archaeological sites dating from the period of the pandemic. One of the results of this study is the low genomic diversity of the bacteria during the initial wave of Black Death, which occurred during the seven years after the siege of Kaffa.
Previous studies proposed an entry of Y. pestis to Europe by several points, and that these varieties of the bacteria would be related to the different outbreaks that occurred in the years after the beginning of the pandemic. In the study of Spyrou et al. determine that all the genomes associated with the outbreaks after the Black Death derive from a single ancestral strain, present at the beginning of the second pandemic in the South, Center, North and West of Europe.
These results seem to indicate a single entry of Y. pestis in Europe. A unique genome that already within Europe was divided into two lineages. The first would be associated with strains that are currently found all over the world, especially related to the third plague pandemic. But the second lineage would be linked to a set of strains only present in the Europe post-Black Death and without modern descendants.
The second pandemic of the plague finished in the 18th century, it seems that due to the local elimination of reservoirs of the disease. Maybe this is why this second lineage has not endured until our time. The study presented by Nature magazine detects the loss of a genomic region, which includes genes related to virulence in strains of this second line, probably extinct. Deletion has also been identified in genomes related to the first plague pandemic (541-750 CE), suggesting a comparable evolutionary trajectory of Y. pestis during both events. However, more studies will be needed to relate this loss of genes related to virulence and the end of the two pandemics.
The game Plague Inc. can give us a clue in the loss of these genes. Plague Inc. is a strategy game developed by Ndemic Creations and it is based on creating and evolving a pathogen to end humanity. The strategy is to make the pathogen spread throughout the world. If it is too lethal it will kill the infected people so quickly that it can not be transmitted. Therefore, it is about finding the balance between transmission and lethality. Is this what happened with the lineage of Yersinia pestis responsible for the Black Death and subsequent outbreaks?