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Black Death 'spread by humans not rats' (bbc.com)
172 points by curtis on Jan 16, 2018 | hide | past | favorite | 50 comments

Last author here. The paper itself comes out this week on PNAS, but the press embargo is lifted already. When it comes out, you can find it here: http://www.pnas.org/cgi/doi/10.1073/pnas.1715640115

What we have done is compare the rise and fall of daily or weekly mortality levels during plague outbreaks against three models of plague transmission - two that are generally accepted (rat-borne plague and pneumonic plague), and one that has been speculated about for a long time (human ectoparasites like body lice and fleas). We allowed the models to achieve the best fit they could within biological parameter constraints, and see how well each of these models could mimic the observed mortality curve.

There is a bit more detail in this interview: https://news.nationalgeographic.com/2018/01/rats-plague-blac...

The code/models we used are available online for one of the outbreaks (Barcelona 1490) https://zenodo.org/record/1043924

A pre-review version of the paper is available as a poster here: http://www.mn.uio.no/cees/english/people/phd/katharrd/kd_yer... Note that we changed the lice model a bit since then, on recommendation of one of the reviewers.

Could it be a combination of several transmission methods? Like rats + lice?

That indeed could be. We didn't test mixed models of transmission, but we took the first step here by testing all three models independently.

Xavier Didelot did some work on testing mixed models for two cities, 17th century Eyam and 19th century Cairo. He did have to further simplify the models though - there are some restrictions on how many floating parameters you can have while the models are trying to converge to the parameter set that results in the best match with the observations.

Epidemiological analysis of the Eyam plague outbreak of 1665–1666: http://rspb.royalsocietypublishing.org/content/283/1830/2016...

Model-based analysis of an outbreak of bubonic plague in Cairo in 1801: http://rsif.royalsocietypublishing.org/content/14/131/201701...

Great paper. Certainly gives credence to the idea that All Conclusions Are Provisional. (Also will make me panic the next time the kids get head lice)

Thanks! You might be able to postpone the panicking. Although I haven't looked into head lice (we specifically looked at body lice), this redditor had some links in that head lice apparently are less likely to carry diseases.


The lice would also need to be coming from a human with the plague, right?

And we can treat the plague today.

So the original theory of the black death was that the disease originated in Asia, and was borne to Europe via rats on ships. Is the new theory arguing that it was the sailors transmitting it instead, or is the origin of the black death in Europe now unknown? Also, is the paper arguing that rats are not vectors for the disease?

Paper is online now :-).

I also put a popular science summary of the paper online here: https://medium.com/@boris.schmid/human-ectoparasites-and-the...

Attention-grabbing headlines and bad research has conditioned me to not update my beliefs based on anything that I only briefly stumble upon like this. There might have been a time when reading an article like this would blow my mind, and make me excitedly share it with others who had a similar belief, but now my first impulse is to just feel sceptical.

Not a bad first impulse :-). It often pays in science to wait a few years/follow-up studies before fully accepting something. What is thought to be true in plague is especially fluid now with ancient DNA studies on human remains coming out regularly, each with its own narrative on how to explain the phylogenetic tree.

That said, there are some long-standing questions in plague research, one of which is why the first and second plague pandemic were that much more lethal than the third plague pandemic. Prior to the aDNA work, people speculated that medieval plague was a different pathogen altogether, but that hypothesis has been put to rest now. An alternative theory has been that plague could spread through human ectoparasites, and we found a novel way to test that theory. That resulted in this paper.

Could it be that by the third plague more people developed immunity and therefore less were killed.

> more people developed immunity

Better yet, those who survived prior plagues were more likely to already be immune to the disease, and evolutionary pressure led to a more resilient population against this type of disease.

Potentially, but I am a bit sceptical. Plague reached a lot of new territory during the third pandemic (for example, australia and the americas), while still having a low mortality. One could check whether native populations to those continents were especially hard hit compared to people from European descent. I haven't heard anything like that.

If you want to get into the mindset of what it was like to deal with the black death, check out Plague Doctors: https://www.youtube.com/watch?v=oDz0OrRZOZ0

On the upside, Black Death survivorship may explain Europeans' relative resilience to HIV (edit: smallpox) [1].

[1] https://www.nature.com/news/2005/050307/full/news050307-15.h...

If the contention of this page's article is true (BD spread by humans), then the link you posted is more in favor of smallpox being the cause of relatively greater resistance to HIV in Europe.

Cute video. Too bad the costume had a few weak spots, the idea of sealing/isolation is deeply valuable. If it hadn't failed there, it would have helped many other diseases.

I think this was already known in the 90s. At least a plaque in the Antwerp Zoo has already been stating this since the beginning of this millenium.

You are right that there have been people speculating about it for a long time, going back more than a hundred years. Some of the earliest reports are from the time of the Indian Plague Commission, and there have been some snippets of evidence since then.

One of the groups that is doing most of the research on body lice as vectors of plague is the group of Didier Raoult. Michelle Ziegler made a nice summary of that work here:


Research clearly funded by rats.

This may mean good things for the outbreaks in remote parts of Madagascar and anywhere else one may occur, because the solution could be as simple as sanitation and delicing.

The 2017 madagascar one might actually have been a combination of rats and pneumonic plague. But yeah, it is good to pay attention to all possible routes of transmission and see where you can intervene.

There were several outbreaks in the centuries 14th-19th. Did the data used by the author cover all the major ones? My impression is that it is easier to recover data for the most recent outbreaks than for the older ones.

There were indeed hundreds of outbreaks during those centuries. The outbreaks we selected were the ones for which we could find daily or weekly mortality records, as that gives you an epidemic curve that you can fit mathematical model to.

We didn't use all of the cities for which we had outbreaks, but selected nine that covered a large part of the time period, and a large geographic region. Here is the list:

Givry, France, 1348. Florence, Italy, 1400. Barcelona, Spain, 1490. London, England, 1563-1564. Eyam, England, 1666. Gdansk, Poland, 1709. Stockholm, Sweden, 1710-1711. Moscow, Russia, 1771. Malta, Malta, 1813.

PSA: "Plague is widespread in much of California, including in the Sierra Nevada mountains and foothills. In a typical year, the Centers for Disease Control and Prevention (CDC) reports about ten cases of plague in humans per year in the western United States. However, during 2015, 16 human plague cases were reported."


Does this shed a new light on the mystery of why the area of modern Poland was spared, remaining largely unaffected by the plague?

Also it makes a certain novel sound a little bit less realistic.

It seems to be an internet myth that Poland was laregly spared. Some guy over at AskHistorians actully dug into the evidence: https://www.reddit.com/r/AskHistorians/comments/79dwua/why_d... - it is a fascinating writeup.

Maybe 'known' is a bit strong. Surmised, perhaps? This study adds some interesting modeling

So I guess, one question this would raise, is, of modern infection, what is the vector found?

Not that they must be the same, but at least they might have an explanation as to why modern infections would be different from historical ones, if they are any different.

There are some studies that looked at the ectoparasites found during plague outbreaks, but just finding an ectoparasite with plague-infected blood in it doesn't tell you much yet on the importance of the transmission route.

Piarroux R, et al. (2013) Plague epidemics and lice, Democratic Republic of the Congo. Emerg Infect Dis 19:505–506.

Laudisoit A, et al. (2007) Plague and the human flea, Tanzania. Emerg Infect Dis 13: 687–693.

Ratovonjato J, Rajerison M, Rahelinirina S, Boyer S (2014) Yersinia pestis in Pulex irritans fleas during plague outbreak, Madagascar. Emerg Infect Dis 20:1414–1415.

Boris, thanks for answering questions here.

One more: has this raised any doubts that the Black Death was a Bubonic Plague?

Depends on how you mean the question. ancient DNA analysis has settled that the first two plague pandemics were Yersinia pestis, so if your question was on what kind of bacterium caused the Black Death - that is Yersinia pestis.

If it is about the manifestation of the disease (the clinical symptoms), those we only know from historical descriptions and that is not something a model can change :-). So yeah, many infected people would still have had buboes. Buboes are also the expected result from plague acquired from fleas (whether they were human fleas or rat fleas).

What might be less known is that there is a pretty high chance (10-20%) that a person suffering from bubonic plague progresses to pneumonic plague. That basically is a death warrant for that person, but it also means that he/she might spread the disease further through air-droplets. If the conditions are right, you might get a pneumonic plague epidemic intermingled with a bubonic plague epidemic. Model-wise, that is one scenario we haven't looked into, but we have seen something like that play out in Madagascar last year.

Rodent Plague Surveillance in California


I never really understood why people hate rats, but love squirrels. I think rats are cool too.

I like rats in general (they make great pets) but I'd imagine people dislike them more than squirrels because they come into the home more readily. I don't know if you've ever had to live in a house with a rodent problem, but in case not, it's miserable.

I like rats but once you had one decide to make your house its home it makes more sense why people don't like them.

I haven't had to clean a bunch of squirrel feces out of my house yet.

The color of urban rates is darkish and ugly. And the fluffy tale. Rat tails are gross.

Most people hate squirrels, at least around here.

In NYC it's pretty common to see people watching them and taking pictures of them.

Those are tourists.

And those who like rats are tourats ? (Sorry)

And what, they don't count as people?

They don't count as people who have to deal with squirrels. Bears are also cuddly.

... Is this a trick question?

Yet another paper discovers the obvious. The "cure" for the plague was 40 days off isolation of affected homes or cities. Rats and fleas don't respect human quarantines.

Another factor is rats. They are killed by plague just like humans. At least one person should have noted large volumes of rats dropping dead everywhere, but that isn't mentioned in writings of the time.

> " Rats and fleas don't respect human quarantines."

rats dont travel as much as you think, even within cities. only when the rats piggy back on human travel, could they have spread the disease.


"But feral cats won’t stray three blocks beyond where they were born, and few mice will venture more than a hundred feet from their burrows in a lifetime."

If rats don't travel, then how do they spread the plague? If they do travel, then quarantine would not work. If they travel with humans, then we would expect to see massively higher infection and death rates among travellers due to prolonged times with infected rats as they dragged them everywhere (where is this evidence?) The plague spread across Europe at an enormously fast rate (some believe that hemorrhagic fever with a separate bubonic outbreak in Italy).

> The "cure" for the plague was 40 days off isolation of affected homes or cities.

In what way is that obvious?

Maybe it spread from the sacramental bread eat in church ...

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