Why did plagues stop?

In the early middle ages up to half of the population of Europe were wiped out by theBlack Death, caused by the bacteriumYersinia pestis. But, following its arrival, the initial pandemic wave waned and was replaced by a series of localised epidemics around Europe until the 18th Century, when it abruptly disappeared. So how do we account for this behaviour? From the University of Tuebingen, Kirsten Bos explained why to Chris Smith...

Kirsten - We’ve identified the strain of the bacterium responsible for bubonic plague, Yersinia pestis in archaeological material from Marseille, France from 1722. Plague was at one time a very pervasive infectious disease in Europe. We know it most famously from the Black Death. However, the disease persisted in Europe for the next several hundred years as local epidemics. However, for some reason, it’s gone extinct in Europe. We don’t have large plague infections today within the European continent. So the question becomes, why did pestis go extinct? Why did these plague epidemics stop happening. We wanted to address this question by looking at archaeological material first to type the strain that was present in some of these older European pandemics.

Chris - I suppose it boils down to: did something change in the bug, or did something change in the host whether that’s the rodent that carries it and then gives it to us, or even all of these factors – the environment, the host down to the pathogen itself?

柯尔斯顿——当然可以。所以我们所做的is we have looked at the genetic profile of the bacterium that caused the great plague of Marseille from 1722. We have found that this lineage surprisingly is an extinct lineage or so we believed it to be extinct because it’s not been typed in any modern pestis material. So with that, it appears that pestis was probably living somewhere in a population in Europe itself or easily accessible to Europe, perhaps in west Asia. And that might have been contributing some of these pandemics or epidemics that we were observing within Europe from the time of the Black Death until the mid-18th century.

Chris - What was the material from Marseille that you were analysing to get this data?

Kirsten - We used teeth from a large plague pit that was excavated in the mid-90s. Teeth are excellent material to use for this type of analysis because the thick enamel coating on the tooth actually functions like a shell to preserve the DNA that’s in the interior. In the interior of the tooth, you find an area that’s called the pulp chamber. This is highly protected archaeological material and that’s the best part in the skeleton to go if you're trying to look for a bacterium that might have been presented in the bloodstream of an individual at the time of death.

Chris - Other archaeologists, yourself included have looked at teeth from other plague sites including London from periods earlier on in the course and history of plague. How does the sequence profile that you’ve got from Marseille compare with those earlier victims?

Kirsten - Interestingly, we find that the strain we’ve typed here is a direct descendant of the strain that was circulating in London at the time of the Black Death. Even more interestingly, this strain does not show any close genetic similarity to any strains that are circulating today. To the best of our knowledge, this strain is currently extinct. So what this tells us is that this was probably a daughter population of the strains that gave rise to the Black Death. The big question is, where was that daughter population residing during that second wave, so between the 14th century and the mid- 18th century.

Chris - You think there's therefore some kind of reservoir whether that’s a human or an environmental reservoir which keeps receding populations over hundreds of years and eventually just peters out.

Kirsten - Yes, that’s correct. We figured there was a plague population somewhere. A compelling plausibility is that it was somewhere within Europe itself and is just no longer present or it could’ve been somewhere very close to Europe and accessible to it by marine transport so for example in west Asia. We can't identify where that reservoir was, because of course, with genetic data, we’re providing only one important piece of the puzzle. It’s important to know that this lineage appears to be extinct. However, where that plague reservoir was residing is going to require information from historians so we can determine where people were moving and when. We need to get information from zoologists who learned what plague species were present in different areas at different time periods. We need to get information on climatic data because there's a relationship between climate and bubonic plague outbreaks to arrive at some hypothesis about where plague might have been residing in that very essential time period.