Rats and the Black Death

Another excuse to feature Ghost…(relevant video here)

This is one that I’ve been meaning to catch up with for a while; the historical role of rats in the spread of Plague. Having seen the ‘Shadiversity’ ‘Misconceptions’ YouTube video on the subject I thought it was time to dig in. In this, Shad states that rats were responsible for later outbreaks of plague, but not the 14th century ‘Black Death’ pandemic:

“…the assumption that it was spread by the fleas on the rats… there’s actually no evidence for this…no such correlation exists for the medieval black plague.’ 

The video fails to provide an alternative explanation, other than something about a combined effect of the Bubonic, Pneumonic and Septicaemic forms of plague, which speaks to its severity, not its transmission or spread. Incidentally, whilst correctly pointing out that the plague doctor ‘beak’ mask is not medieval, it just as incorrectly states that it dates ‘from the Renaissance.’ As I covered previously, the ‘beak doctor’ is actually an Enlightenment phenomenon (and not a common sight at that).

So is the video right to say this? Yes, but with a hefty caveat that absence of evidence is not evidence of absence; the rat hypothesis is shaky, but so are all the others. And rodents or one species or another were almost certainly responsible for the spread of this plague… 

The Evidence

As far as I know there is no controversy over the role of fleas in carrying the Yersinia pestis bacterium, and to be fair I don’t think he is trying to claim that the fleas weren’t the main carrier. I have previously seen reports that the role of rats had been overstated, so I was prepared to accept that for this particular pandemic, scholarship had identified another mammal as the carrier of the infected fleas. However, I’m also very wary of the ‘pendulum’ effect in the media narrative of historical events; similar to medical reporting, where every new study is hailed as the new gospel truth, rather than just another datapoint. This is how the tabloids are able to claim that everything either causes or prevents cancer… 

Regardless, fleas have to live on some species of mammal in order to spread disease. I was aware that other mammals had been blamed instead/as well of rats, such as gerbils, but as noted, wasn’t sure what the current consensus on rats was. Although his video and description cite no sources, Shad does provide one in response to a commenter; a popular history website article from ‘historyextra.com’. His video appears to be based very closely on this one article, including the author’s confusing description of rat population changes. The article conflates rises and falls in populations, implying that an epizootic (animal-caused) outbreak must necessarily be preceded by a boom in the local rat population. The real indicator of a rat-borne plague in the various literature is die-off, not population increase. This is the real meaning of the term ‘rat fall’, not some sort of waterfall of rats as Shad seems to think. As Theilmann and Cate point out (more of them later);

“…chroniclers, however, paid so little attention to the plague that their failure to mention rats is hardly a surprise.”

So a lack of reports of a multitude of rats in the medieval period is irrelevant. The bigger problem for the rat fans is that Black Rats don’t seem to have been very common in 14th century Northern Europe. One of the first academics to challenge the rat hypothesis was David E. Davis, who published his article ‘The Scarcity of Rats and the Black Death: An Ecological History’ in The Journal of Interdisciplinary History in 1986 (Vol. 16, No. 3 (Winter, 1986), pp. 455-470). This is free to read at JSTOR with a free account, and I recommend it. However, even this also makes clear that a lack of ‘rat fall’ does not mean that rats were not spreading the disease;

“…mortality of rodents was not noted in some epidemics when it seems likely that Rattus was present.”

There is no mention of large numbers of rats preceding an epidemic here either; nor in the other sources that I checked. This seems to be a misconception borne of popular culture (meaning that Shad was right to query its appearance in a video game; but he should also be querying it in historical reality as well). It’s the die-offs that matter, but even these don’t preclude a given species as a factor. Having said this, Davis is emphatic that the evidence against (or rather, in favour of) rats is overwhelming; the rapidity of spread seen in the 14th century could not be down to rats alone (or even at all, as he argues). He also highlights the problem of sustaining the disease on board ship, when rats, being susceptible to plague, will die off in the course of a voyage and therefore be unable to spring from the gunwales and infect the landlubbers. Their fleas, on the other hand, and any infected humans, would have remained to spread the disease. This seems a little semantic though; in this scenario the rats have still spread the disease, possibly at both ends of the trip; they just weren’t physically there to pass it to their European cousins. Note that Theilmann and Cate challenge this; rats could survive some strains of Y. pestis, and/or survive through hibernation long enough to pass on the infection.

Ultimately, Davis’ article throws much doubt on Rattus rattus as the main carrier, but is not conclusive and nonetheless forms part of a broad consensus that it Y. pestis was in fact spread by as the University of Michigan puts it, “various rodents,” whether water voles, gerbils, some other species, or a combination of two or more of them. Paul D. Beull confirms in his article ‘Qubilai and the Rats’ (Sudhoff’s Archive, Vol. 96, H. 2 (2012), pp. 127-144) that “many other rodents are common vectors.” Yet there is clearly the same dearth of medieval references to mice, voles, or other rodents. The argument that there’s no evidence for rats spreading this plague is seriously undermined by the lack of evidence for any other species having spread it. Perhaps also by the significant part that rats did in fact play in other plague outbreaks. Basically, if rodents weren’t involved (and of these, rats remain a prime suspect), then bubonic plague wasn’t involved either; the Black Death would have to be one of the other forms of Plague or another disease entirely. If it was, then there is very clearly still room for rats to have some role. Even if the main disease of 14th century Europe wasn’t the bubonic form, it could still have originated in bubonic form within rats. If so, rats are ‘to blame’ (not that we should blame animals going about their business) just as much as the (likely) bats that originated the present pandemic. Bats aren’t flying around infecting us, yet we still blame them (rightly or wrongly). All we can say with any surety is that rats were not the only spreaders; the speed of the spread means that other mammals (including humans, as some emphasise) were involved. Still, it’s useful to know that the rat hypothesis is based upon such thin evidence and became received wisdom in the absence of a better explanation. It’s an educated guess more than a scientific hypothesis and as such should continue to be challenged. 

There’s a twist, however. Not only might rats not be involved in the Black Death, we don’t even know for sure that the three forms of plague caused by Y. pestis were actually the problem. If they weren’t, then the involvement of rats is obviously moot. Perhaps the real misconception here is that we know much of anything about the Black Death. It’s all hypothetical and/or speculative. We have no conclusive historical, DNA, or other evidence. All we know is that the pandemic doesn’t fit with the other outbreaks of plague. This is painfully clear from Samuel K. Cohn’s article ‘The Black Death: End of a Paradigm’ (The American Historical Review, Vol. 107, No. 3 (June 2002), pp. 703-738). Cohn offers no answers, and I’ve seen nothing to suggest that we are any further forward in our understanding. In ‘A Plague of Plagues: The Problem of Plague Diagnosis in Medieval England‘, John Theilmann and Frances Cate (The Journal of Interdisciplinary History, Vol. 37, No. 3 (Winter, 2007), pp. 371-393) make the very valid point that;

‘In one sense, the question of whether the Black Death was Yersinia pestis or some other ailment is a moot point, because only laboratory testing can provide conclusive evidence for a clinical diagnosis.’

As other authors do, they also explain that the medieval history is too sparse and too vague to support a conclusive identification. They do fall into the Y. pestis causal camp, as it were, and they still support rats as hosts. This alone challenges Shad’s confidence that there is ‘no evidence’ for rats causing the Black Death. There is as much evidence – more in fact, given the close association of Y. pestis with rats – as there is for any other possible host. The exclusively human hypothesis remains unproven and unlikely. Yet at the same time the article acknowledges that the rate of spread and mortality rate does not support plague alone – the devastation of the Black Death was likely not just a combination of plague variants, but of other diseases as well. The fact that there remains healthy debate about what diseases were even present renders the question of specific species of rodents involved almost irrelevant. We may never know the level of involvement of plague in the Black Death, never mind the extent of transmission or spread via rats. Of course, you might argue that this makes the formerly confident blaming of rats even less justified and even more in need of debunking. As usual, the more you know, the less you know…

2 thoughts on “Rats and the Black Death

  1. Thanks for the great article! With regards to the last paragraph and in the case of plague, rats would not be considered vectors, rather hosts for plague. Fleas or lice are the vectors that transmit Yersinia pestis from vertebrate hosts to humans.

    1. Ah, good catch, thank you. I’ve corrected my usage, but note that Beull uses ‘hosts’ in his article, which I’ve quoted. Is this not correct?

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