Despite its reputation as a scourge of antiquity, Yersinia pestis–the bacterium that causes bubonic plague–still causes thousands of human illnesses every year. In modern times, most of these occur in Africa, and to a lesser extent in Asia, though we have a handful of cases each year in the U.S as well.

When Y. pestis was first confirmed as the cause of bubonic plague during an 1894 outbreak in Hong Kong, most people assumed that we also now knew the cause of the 14th-century Black Death, and the later plague outbreaks that resurfaced periodically. However, there has been lingering resistance to the idea that Y. pestis actually caused the Black Death. I covered the reasoning behind this resistance in a series of posts back in 2008, so I’ll just give the Cliff notes version here. Basically, many of those advocating “not Y. pestis” pointed to differences in the epidemiology of the Black Death compared to modern outbreaks of Y. pestis. Today, people are much less likely to die of plague; the outbreaks aren’t nearly as big; and the pneumonic form (which infects the lungs and is therefore able to spread directly person-to-person) seems too rare to account for the number of cases that occurred during the Black Death. Also, they argue that transmission across Europe was much too fast, given that rodents (typically rats) are the disease vector. Instead of Yersinia, some authors have suggested that the Black Death was instead caused by a hemorrhagic fever virus, or perhaps by an unknown microbe that went extinct sometime in the last 600 years.

More recently, we’ve been able to test these claims, using paleomicrobiology to look for molecular evidence of Y. pestis in skeletons that presumably died of plague. Many of these come from mass graves that have been dated to the time of the Black Death–some also have parish or other town records to attest to the timing of the grave. In most cases, investigators found Y. pestis DNA. In a few cases, they didn’t, which led to controversy and charges of contamination in the positive samples.

However, the tide has turned. In 2010 and 2011, three papers came out which, um, put the nail in the coffin for the Y. pestis naysayers. At the time, the papers got press not necessarily because of what they explained, but because the ancient Y. pestis strains looked fairly ordinary–there was nothing obvious to suggest why, from the bacterial point of view, the Black Death was so deadly. However, I hadn’t had a chance to read these closely until now, and one of the punches never made it into the mainstream media. From the discussion section of this paper, the authors note:

Two of the authors (SW and JM) have previously argued that the epidemiology, virulence, and population dynamics of the Black Death were too different from those factors of modern yersinial plague to have been caused by Y. pestis (13). Given the growing body of evidence implicating this bacterium as responsible for the pandemic, we believe scientific debates should now shift to addressing the genetic basis of the epidemic’s unique characteristics.

The reference cited within is this paper, where the authors cast doubt on another group’s finding of Y. pestis DNA in ancient corpses. So it took them 10 years and probably a dozen or more papers, but two “Black Death doubters” have now come around. Score one for the weight of scientific evidence changing minds.

Works cited

Schuenemann VJ, Bos K, DeWitte S, Schmedes S, Jamieson J, Mittnik A, Forrest S, Coombes BK, Wood JW, Earn DJ, White W, Krause J, & Poinar HN (2011). Targeted enrichment of ancient pathogens yielding the pPCP1 plasmid of Yersinia pestis from victims of the Black Death. Proceedings of the National Academy of Sciences of the United States of America, 108 (38) PMID: 21876176

Bos KI et al. A draft genome of Yersinia pestis from victims of the Black Death. Nature, 2011.

Haensch, S et al. Distinct Clones of Yersinia pestis Caused the Black Death. PLoS Pathogens, 2010.

Previous posts in the series

Part 1

Part 2

Part 3

Part 4

Comments

  1. #1 Pierce R. Butler
    August 25, 2012

    … they argue that transmission across Europe was much too fast, given that rodents (typically rats) are the disease vector.

    I kinda hope we have better rat control now than they did then…

  2. #2 onkelbob
    August 26, 2012

    If the plague of Justinian was the bubonic plague, then there must have been some reservoirs present when 1346 rolled around. Perhaps the introduction of new bacteria reinvigorated a previously dormant reservoir. I also suspect that the increasing fur trade along with the opening of trade routes from central Asia via the North and Baltic Seas through the Hanseatic League was a contributing factor. If you look at art, furs, especially Russian and Central Asian furs (silver fox,ermine, mink) suddenly become popular items and indicators of prestige and wealth. Taken together, this could offer some explanation how the plague spread so quickly.

  3. #3 Confounding
    August 26, 2012

    There’s a mathematical modeling study in there somewhere. Build a model of the Black Plauge using modern parameters, and then start seeing what needs to be different to get the progression that happened historically.

  4. #4 Tara C. Smith
    August 26, 2012

    Alison Galvani at Yale has actually done some of that. She was also one who contributed to the research showing that CCR5 delta 32 probably didn’t increase due to plague (as had been suggested earlier; see posts 1-2 in the series).

  5. #5 Victor Suntsov
    Moscow
    September 30, 2012

    It is easy to understand history of plague if you understand origin of plague microbe in Sartan in Central Asia.
    Origin of the Plague Microbe Yersinia pestis:
    Structure of the Process of Speciation
    V. V. Suntsov

  6. #6 Joe McPhee
    November 19, 2012

    I’m an author on the Bos paper and one thing to realize is that the genome enrichment depended on using modern strains as the baits for the ancient DNA. Any genome fragments that were exceptionally divergent from modern strains could/would have been missed. Given the amount of recombination that is happening in Y. pestis it’s hard to say what might have been missed. The fragments we saw looked fairly typical by modern standards, but if there was any selection for attenuating gene loss, we would completely miss it. There’s still loads of work to do here. Keeps us in business.

  7. #7 Tara C. Smith
    November 19, 2012

    Thanks for that!

  8. #8 Jeppe
    Denmark
    December 8, 2012

    Hi Tara
    You are wrong. It is true that some scientific investigations of plague graves concluded that there were no signs of Yersinia Pestis, but those are old investigations. Theres been numerous investigations later that did indeed find ancient Yersinia Pestis in the corpses from the plague graves. They even mapped most of the DNA from the ancient speciments using a new technique called Target Enrichment. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2632865/#fn21/

  9. #9 Tara C. Smith
    December 9, 2012

    I’m wrong? The title of the post says “nail in the coffin” and it’s covering papers confirming the findings of Y. pestis. If you go back to the previous series, I outlined why some scientists doubted this connection, but I never took the position that Y. pestis wasn’t the cause–and indeed I pointed out weaknesses in their case. So wrong how exactly?

  10. #10 Elliott Bettman MD
    United States
    September 8, 2013

    I have found this all fascinating. YES it appears that Duncan and Scott were Wrong about Black Death not being Yersinia, but their hearts were in the right place and some of their reasoning sound. Huge lymph nodes that burst in the groin and issue forth foul smelling fluid IS pretty suggestive, if not pathognomonic for the Bubonic form of the ailment. As for the spread, I can only conclude that there Were Black Rats on the ships, and that it spread by fleas and probably pneumonically at close quarters and maybe a short distance inland from the ports “spread by breath those exposed doomed to die..” or whatever. But still, the lack of masses of dead black rats in the hamlets, or stinking up the walls when they died indoors, is a bit of a Paradox. In close quarters it may have spread pneumonically in the towns and then bubonically when the victims died and their body lice or human fleas spread to others. There is also pharyngeal plague which is common in the modern Mideast from consumption of undercooked Camel Meat. Some livestock like goats carry the plague fleas and peasants would have drove them from town to town while fleeing an epidemic. BTW they are criticized after the fact for killing off cats and thus hindering “rat control.” I call BS..cats are NOT good ratters, in general. The aerial Black rats would have been easy prey only when infected..and cats and their fleas spread plague in endemic areas to the present day. As for immunity and mutation to kinder, gentler Yersinia this has not been observed or proven. I think one overlooked reason why it became less severe or pervasive is that the Asia Minor trade routes were closed-or too much “street tax” was extorted by the Ottoman Empire. It is This that led to explorers going all the way around Africa with their spices as an end run..and eventually to Columbus accidentally discovering the New World. A Plague ship that sailed thousands of miles would have been a “Ghost ship” with dead seamen long before it reached European ports, unlike if it sailed from Caffa say to Venice. (but there WERE outbreaks in London and Marseilles for centuries after the initial outbreak in 1347-1351) Brown rats did replace black rats with time, farms around manors became larger which meant city rats were not in proximity to squirrels and marmots. And the Third Pandemic, while it certainly had the ability to spread Quickly by rail and steamship, also would have been hindered by better sanitation (less rats cleaner non louse-infested humans). The latter killed 10 million in places like Calcutta but was less lethal in both Europe and China. There is also Pestis Minor, the low mortality strain. It could have conferred immunity but classically merely presaged Black Death epidemics.

    Then there is the issue of Groups such as Jews not dying as much. Some of this is probably exaggeration by contemporary gentiles who burned them at the stake for being in League with the Devil. But Jews and Moslems believed in hot baths, where as other groups felt they were “unhealthy” and therefore were more lice and flea infested.

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