Maybe Rats Aren't to Blame for the Black Death. National Geographic 15 January 2018
Black Death plague spread by dirty humans not rats, study suggests. Daily Telegraph 16 December 2018
The classic explanation for the Black Death plague is wrong, scientists say. Washington Post 16 January 2018
These are typical of the headlines appearing around the world as a result of a paper in PNAS*, and this is how the authors describe it under the subtitle, Significance.
Plague is infamous as the cause of the Black Death (1347–1353) and later Second Pandemic (14th to 19th centuries CE), when devastating epidemics occurred throughout Europe, the Middle East, and North Africa. Despite the historical significance of the disease, the mechanisms underlying the spread of plague in Europe are poorly understood. While it is commonly assumed that rats and their fleas spread plague during the Second Pandemic, there is little historical and archaeological support for such a claim. Here, we show that human ectoparasites, like body lice and human fleas, might be more likely than rats to have caused the rapidly developing epidemics in pre-Industrial Europe. Such an alternative transmission route explains many of the notable epidemiological differences between historical and modern plague epidemics.
Sadly, while the mathematical modelling by these authors supports the view that Black Rats (Rattus rattus) were not important in transmitting plague via fleas during its rapid spread through Europe the media reports do not point out that this is just more work on this theme. Equally sadly, neither the authors of the paper nor the journalists parroting the press release from the journal, mention the fact that the Black Rat theory for transmission in Europe was pretty well shot down by Graham Twigg, formerly of Royal Holloway College, University of London (and student of Eric Thomas Brazil Francis in Sheffield) in his book, The Black Death, A Biological Reappraisal (Batsford), published in Britain in 1984.
Graham Twigg based his claim that Black Rats could not have been involved in the transmission of plague in England (or northern Europe) on knowledge of the biology of the species, then the only species of rat in Britain. Black Rats, nor the Brown Rats (Rattus norvegicus) that came later, are native. The Black Rat, also known as the Ship Rat, arrived from the east, coming ashore from ships carrying them and infesting the buildings of ports. They need a warm environment and rarely leave the shelter of buildings or reach the countryside. They were not, therefore, sufficiently widely distributed across the country for their fleas to carry the infection.
Black Rats and other rodents, though, through the fleas they carry, have been implicated in the spread of plague in other parts of the world, including Hong Kong, where the bacterium Yersinia pestis was first isolated from plague victims in 1894.
Twigg also threw doubt on the infective agent that caused the Black Death but that is a different question I will not address here.
After the 1980s when Twigg’s book made the first headlines, the story emerges at irregular intervals in the popular press. For example, here is the headline from The Independent of 8 October 1994: Maybe the black rat didn't do it: the Black Death.
If present-day journalists had even looked at the Wikipedia article on the Black Death they may have been better informed on the research reported in PNAS. Indeed there are whole websites covering The Black Death and the theories on its cause and rapid spread. The same journalists may then have realised that PNAS paper is just a minor addition to the literature adding apparently quantitative verisimilitude to what has been qualitatively obvious. Worse still, the new ‘discovery’ has already been added to a Wikipedia article on Yersinia pestis as a novel idea!
So, we have even more evidence that the Public Understanding of Science has turned into the Public Misunderstanding of Science, with scientific journals and their publicists putting out hyped stories for gullible journalists to fill media space.
*Dean KR, Krauer F, Walløe L, Lingjærde OC, Bramanti B, Stenseth NC, Schmid BV. 2018. Human ectoparasites and the spread of plague in Europe during the Second Pandemic. Proceedings of the National Academy of Sciences of the USA. Published ahead of print January 16, 2018, doi:10.1073/pnas.1715640115