If there is something that pervades the last few millennia more than the plague, it must certainly be sensationalist stories about it. The only difference now is that instead of God smiting the world, it’s gerbils.
Plague stories erupt like swollen buboes in the media at least every few months. Mostly it’s stories of recent outbreaks in Madagascar or cases of people becoming infected in not-Madagascar. But occasionally it’s something different, as has been the case in the last few weeks when we’ve learned that there isn’t plague on the New York subway (nor platypus) or that church graffiti commemorating Tudor plague victims has been discovered.
On Monday a new study was published in the Proceedings of the National Academy of Science (PNAS), ‘Climate-driven introduction of the Black Death and successive plague reintroductions into Europe’. It’s a very interesting paper, but more on that in a minute.
Because the release of the… press release, appears to have sparked what is now becoming an annual event where we are told that ‘rats are exonerated’ from their involvement in the Black Death (no they’re not); or ‘the plague could happen again’ (but it’s pretty unlikely, and besides it never really left); and ‘everything we knew about the Black Death was wrong’ (not it’s not).
Now, I read the study before a lot of places had picked the story up – it’s four pages that include lots of pretty pictures, one page with further study details, and a final page of references, so I was able to have a first go-over with my morning cuppa. I decided pretty rapidly that I would write a blogpost about it. As I said, it’s a really interesting paper, but I do have some thoughts (and questions) about the research I figured might be worth sharing. However, before I can get to the discussing the results of the study, I think we need to discuss how they have been (mis)represented in the media.* (If you’re interested in the why, I discuss that in a separate post.)
But in order to have this discussion you will first need to know what the results of the study were, if you don’t already. So, here is a summary of the paper:
[Below is a full summary, which is 1550 words. If that doesn’t work for you, I’ve written a snappier summary here, which is 550 words, although it skips a lot of the detail. TD;LR for them both: Schmid et al suggest the plague bacteria may have been reintroduced into Europe multiple times during the Second Wave plague epidemic (1346-1859 ish) from reservoir rodent populations in Asia – instead of only being introduced once at the Black Death and then establishing reservoirs in the local black rat populations. If you want to skip the full summary (either because you read the snappy one, already know the details of the study, or don’t get the point of this post) click here to go to the rest of the discussion.]
Schmid et al set out to test the assumption that once the plague was introduced into Europe in 1347, it remained in the region by establishing a reservoir in the local populations of rodents, namely the black rat (Rattus rattus). And that it was then from these local reservoirs that the human population was repeatedly reinfected, with outbreaks of the disease occurring in the region until the early 1800s. This epidemic of plague is commonly referred to as the Second Wave.
Schmid et al thought that if they looked at the climate fluctuations during this period, they may be able to locate the local reservoirs. This is because modern research into rodent species that are still reservoirs for the plague bacteria, such as great gerbils (Rhombomys opimus), show populations increase during warm and wet weather (because that generally means there are more delicious plants to eat), which also leads to an increase in the fleas that live on them. This is key, because fleas are a vector for the disease, which means they can spread it from one gerbil to another (or something else, that isn’t a gerbil). When the climate changes from warm and wet (such as to cool and/or dry), the gerbil population can collapse. This means there are less gerbils for the fleas to live on, so they may seek out other hosts, which spreads the disease.
In order to look at the climate, Schmid et al used tree ring based data from fifteen samples (nine in Europe and six in Central Asia). To see if there was a pattern between climate fluctuations seen in the tree rings and outbreaks of the plague in Europe, they had to compare them. They started with 7711 references to plague outbreaks from 1346 to 1859. By removing outbreaks further than 500km from any tree-ring sample, this was narrowed to 4119. If they thought any outbreaks could have been caused by something other than direct infection from a local reservoir (which meant for each outbreak, if there was another recorded outbreak within the two years before it and within 1000km of a tree-ring sample) they were out. This left Schmid et al with 24 outbreaks in 19 towns. Of these 24 outbreaks, eight could be traced back using historical documents to a maritime source from another city. And then there were 16. Still with me? In the years ahead of these 16 outbreaks, the tree-ring samples from Europe showed no climate fluctuation patterns that Schmid et al associated with the boom and bust a reservoir rodent population.
So, next Schmid et al decided to look for outbreaks of plague in harbours. While Schmid et al noted, “The black rat (Rattus rattus) likely played a role in maintaining plague outbreaks on ships, as well as importing plague into harbours” they also state, “its role as a potential plague reservoir in Europe is rather questionable”. By searching for harbours that may have been at the beginning of a series of outbreaks (like how the Black Death is thought to have first arrived in Europe), they were trying to find out if the plague could have been repeatedly reintroduced from reservoirs outside of Europe.
For a harbour outbreak to be considered there could be no other outbreak within the previous two years either in the city associated with the harbour, in other cities with 500km, or in other harbours within 1000km. This provided Schmid et al with 61 possible outbreaks that began from an outside source. These 61 outbreaks occurred in 60 different years in 17 different harbours (of around 46 main harbours in Europe during this time).
The majority of these harbours were along the Mediterranean and Black Sea coasts (although not all) and were a part of the network that connected Europe to Asia, which began first over land and then continued largely by sea. Many of the routes over land go through areas that are plague foci, or regions with reservoir rodent populations. So Schmid et al looked into whether reservoir populations in the medieval period in this region could have been a source of plague that was then repeatedly reintroduced to Europe via ships.
To narrow down which of the 60 different years of outbreaks were more likely to originate this way, Schmid et al only looked at those harbour outbreaks that took place within five years of new outbreaks in southern Russian or the Caucasus (again, those not proceeded by any outbreak in the region in the previous two years). They narrowed it down this way to reflect the timeframe of the Black Death’s initial outbreak to its eventual arrival in Europe. This left them with 16 outbreaks, which occurred in the years: 1346, 1408, 1409, 1689, 1693, 1719, 1730, 1737, 1757, 1760, 1762, 1780, 1783, 1828, 1830, and 1837. Schmid et al note that this list is probably not exhaustive, as it is difficult to identify outbreaks that are the result of reintroductions, while there are other outbreaks still occurring for other reasons. They then compared these outbreak dates to the tree-ring samples from Central Asia
These tree-ring samples come from areas along the trading routes and border the Chinese plague foci (in addition to being within the Central Asian plague foci). Most come from mountainous regions, where the rodent species that are reservoir populations for plague are mostly ground squirrels (Spermophilis undulatus) and the Altai marmots (Marmota baibacina). They looked for evidence of climate fluctuations that would lead to that boom and bust scenario in a reservoir population; above average conditions, followed by a decline in conditions. If this was identified they labelled the year of the transition between the two as a ‘climate event’. They then looked to see whether there was a pattern between these climate events and outbreaks of the plague in Europe.
Schmid et al found that for 11 of the 16 years when harbour outbreaks with possible origins outside of Europe occurred (those listed above), there was a climate event 15±1 years before. This was seen in one of the tree-ring samples from the Karakorum mountains in northern Pakistan – specifically from juniper trees. [This sample has 28 total climate events between 1250 and 1850 (although less so between 1400 and 1500).] Because of the ±1 year it was actually narrowed to nine distinct climate events (1408/1409 and 1828/1830 harbour outbreak years each share an event).
Based on these results Schmid et al suggest a scenario where climate events that affected the above sample from the Karakorum mountain region would have also impacted a wider geographic area and the reservoir rodent populations living there. These climate events that lead to the boom and bust scenario previously described in reservoir populations would see fleas seeking new hosts. This would result in plague outbreaks in Europe after a delay of 15±1 years.
The reasons for this delay are proposed to be three-fold: 1) fleas find alternative hosts and come into contact with humans after 1-2 years; 2) the disease is transported approx. 4000km from the mountains of Central Asia to the coast of the Black Sea, taking 10-12 years; 3) this new plague from Asia is introduced to Europe via the harbours and spread to the mainland, in less than 3 years. Schmid et al suggest that the transmission from stage one to stage three may have been through caravans, with camels first becoming infected from the reservoir population and then the disease establishing within the caravan animals, humans, and fleas. After which, one caravan could spread the infection to further caravans along the trading routes.
Schmid et al state they have found no evidence for a reservoir rodent population in Europe (at least not one that reacts to climate fluctuations as they have investigated). This could explain why there is no plague foci in Europe after the 19th century (because there never was one), whereas they remain in Asia, Africa, and the Americas. If this were the case, it would leave us having to explain how the disease was able to persist in Europe in between reintroductions from Asia. Possible explanations might include: plague circulating between cities and villages re-infecting susceptible populations following previous outbreaks (suggested in 1984 by Ell) or black rats acting as urban reservoirs in harbour cities.
They note the importance of black rats in the epidemic is disputed though, as outbreaks are known to have occurred in regions without any rat populations, during periods of climate that do not favour either the black rat or their common plague vectors of rat fleas (Xenopsylla cheopis and Nosopsyllus fasciatus). They conclude that it is more likely the plague spread through Europe in many different ways, such as was seen in India during the last major pandemic (often referred to as the Third Wave), documented transmissions occurred human to human, via infected goods, and from rats (although they again state that in the medieval plague rats may only have been involved on ships or in harbours). Schmid et al intend this paper to provide an alternative perspective on the introduction of plague into Europe from a single introduction to multiple reintroductions, driven by the impact of climate fluctuations on reservoir rodent populations in Asia. Schmid et al then conclude that the correctness of this hypothesis will be determined after ancient DNA (aDNA) from plague victims during this time period is analysed.
Right, so now that we know what Schmid et al said in their study paper, I can point out that they definitely didn’t say*** that gerbils were to blame, nor that they arrived in Europe from Asia (either on ships or via the Silk Road) laden with plague. Poor, poor, poor gerbils. Why not blame ground squirrels and Altai marmots who were also mentioned? WHY NOT THE FLEA-INFESTED CAMELS?
Beside, if it was gerbils (which no one was saying it was) then it was great gerbils, not ‘scary as fuck’ giant gerbils. Great gerbils are very different from your everyday gerbil, but we’re not talking Twilight Beast sized things here, well, unless you count the Giant Maltese Dormouse, which is only like 10cm bigger...
And if apportioning ‘blame’ to the Black Death and subsequent outbreaks of plague is something that we’re going to do, why is no one blaming the bacteria?
Since they didn’t say gerbils travelled to Europe, they definitely never said that happened during warm spells or that gerbils thrived in Europe at this time because of the warmer temperatures. I’m pretty certain they also never said that Europe always experienced plague outbreaks after central Asia had a wet spring followed by a warm summer.
Schmid et al never said that that rats are exonerated. Nope, they did not say it. May have suggested they played less of a role than people think, but still not exonerated. Oh and don’t even get me started on the whole, but gerbils = rats? Even if you consider that gerbils were called rats in the medieval period they still wouldn’t be off the hook BECAUSE THAT’S NOT WHY PEOPLE USED TO SAY RATS WERE INVOLVED – THEY DID NOT MISTAKE RATS FOR GERBILS! *cries*
The links included above aren’t exhaustive of everybody that has screwed up in reporting these study results. It probably includes the majority of the worst offenders though, based upon my reading of every popular news article that came up when searching Google within 24 hours of the story breaking, which is absolutely how I recommend you conduct any literature review…
I took the liberty of capturing some of the awesome headlines (this includes good and bad and there are more, so many more). Look, even though misleading, incorrect, sensationalist headlines bother me, I get it. I do. But when the actual content of the article is incorrect, well that’s just unacceptable – journalists, especially science journalists, just… do better. Okay? It’s an open access article and it’s four pages long. Even if you don’t understand the nitty gritty, you can at least read it through once, which would have told you enough to know that teh gerbuls dint doit.
As soon as I became aware that most headlines had people picturing medieval gerbils in Europe left right and centre, I made this.
If this super dry blogpost has you still searching for other articles to read on this study, I can recommend the following. They all do a good job writing up the results with lots of lovely plague context. For non-exonerated rats, check out the Smithsonian. Want input from a dendochronologist (tree ring researcher)? I suggest CBS. Looking for a plague researcher to comment on the results? In that case, have a read of Science News. For other decent write-ups there is also the Guardian (well, this version at least), BBC, Pop Sci, New Scientist, and quite a few others. There are even more articles that are ‘meh’, but oh so many (including some of the aforementioned) take the ‘killer gerbils’ route, which is a shame.
Oh and then there is this article, which I suppose is pretty good at representing the study’s results. But, seeing as how it was written by some of the study authors, I guess that should be expected.
Finally, you could do worse than the Onion.
For a follow-up on how it went so wrong, please see this post.
As I said previously, I have thoughts (and questions) about the actual results of the study, but those are to come in Part II (you’re welcome).
*I mean, not that it bothers me or anything when research is poorly reported.
**Okay, look, I know that was a LONG summary but now I can use it for both this blogpost and for my next one about the actual results of the study. And it is like 2500 words less than the paper and in non-academic-speak…
***Following the press release the study authors have clearly spoken to various journalists, so some articles may include statements not found in the study paper, but I’m willing to bet they didn’t say any of this. Please feel free to prove me wrong and I’ll update this.
BONUS NOTE: I got wondering about the whole ‘analysing the aDNA from plague victims’ things that has been reported in a few places and also mentioned at the end of the paper. There is a little more information in this article from a few weeks ago.
The rats get a bad rap. It was the FLEAS on the rats that spread the disease. So, you could put it down to the unhygienic nature of the people in that area.
The whole idea that plague entered Europe only once at the start of the Black Death seems to me to be a straw man argument. Who ever claimed that it was only imported once? Showing that it was imported more than once (which it doesn’t exactly do) does not mean that there were no reservoirs in Europe. There are some huge gaps between some of their dates, especially between 1409 and 1689.
Thank you for commenting Michelle – and for letting me know I’m not the only one! I’ll mention in my post about the actual study results, but yes, straw man was my very first thought. The references for that claim are three: two from before 1986 and one from 2000. Not exactly comprehensive! They could have still written a paper if they’d framed it differently, the data is still the same.
I too can’t see how it could persist for so long in Europe without a reservoir (their figures are interesting here), so to say ‘We may need to rewrite that part of history’ or whatever it was, is a bit much…
We also don’t know what the lifespan of the average plague reservoir is. Sure, some in Asia could be over a thousand years old (thought I expect them to shift around some even there), but the average reservoir may only last a couple hundred years (or less). We have to remember how young all the 3rd pandemic reservoirs are relative to the start of the other two pandemics. Australia is an example of plague reservoirs that completely died out fairly quickly.
I agree that they have plenty of interesting data to write the paper without making extraordinary claims.
This is definitely something that I’d be interested to learn more about. How long is long enough to be considered a reservoir?
On a related that note, is there such a thing as a ‘temporary reservoir’ or would that be considered an ‘unsuccessful reservoir’ (even if it is successful at temporarily spreading infection)?
Michelle – I find straw man a bit too strong :-). There are not that many references out there that explicitly talk about whether or not plague was repeatedly reintroduced during the second pandemic (that I could find) – it seemed to have been something of a silent assumption, and with the assumption the implicit existence of European reservoirs. So one part of the paper is to make the assumption explicit and get it tested.
Part of the problem is that most of the attention for the second plague pandemic goes to the Black Death, rendering the question of reimportations irrelevant.
There are gaps between the climate events (but not as big as 280 years – those 280 years are a consequence of us not being able to tell an incoming wave of plague if there is too much plague already circulating in Europe), meaning that we have to explain how plague could persist in Europe for almost a 100 years. There are several possible explanations there.
* One is that plague was fed into Europe from several reservoirs in Central Asia, so there were more reimportation events
* Another is that our climate event description describes what we think were massive plague outbreaks in Central Asia, but even small plague outbreaks can be picked up and transported to Europe. The odds are just smaller. This would also result in more reimportation events
* A third way, suggested already by Ell, 1984, is that metapopulation dynamics could help plague to persist – it could bounce around between cities in Europe, and between human population centers in Europe, northern Africa and the near Middle East, similar to the way obligatory human pathogens like measles do today.
* Urban rodents (rats or otherwise) in those cities that had such populations them might still have formed a temporary reservoir. We just couldn’t find them looking at climate data.
As for the lifespan of the most permanent versions of plague reservoirs, as we know them from Asia. That is tough to answer until we start sequencing zooarcheological rodent teeth or something like that. Would be great to know.
Excellent post – thanks Alison – I have recommended it to all our first year students (hot off the heels of my lecture on the archaeology of the Black Death, which coincidentally took place on the day the story broke). Far more interesting is the impact of the Black Death on human society…but that doesn’t involve malevolent rodents of any kind….
Thanks! I hope the students enjoy it. Yes, I’m always astonished by what makes for ‘good’ news stories and what doesn’t when it comes to science/archaeology/history!
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Great post. My friend is fascinated by stuff like this. I’ve referred him. Thanks for sharing:-)