Did Rats Spread the Black Death?

Everyone knows that black rats harboring fleas infected with Yersinia pestis (the bacterium that causes bubonic plague) were transported on ships throughout medieval Europe, spreading the Black Death and killing maybe half of the population.  Right?  Since Paul-Louis Simond proposed the idea in 1898, there’s mostly been agreement that rat fleas from black rats were the main vector of plague transmission.

Last week, I was lit searching something unrelated when I stumbled across an April 2013 paper entitled, “Rats cannot have been intermediate hosts for Yersinia pestis during medieval plague epidemics in Northern Europe.”  Wow!  In the paper, Hufthammer and Walloe (2013) argue that archaeological evidence suggests that the black rat was not widespread or abundant in Northern Europe at the time of the Black Death, so fleas from rats couldn’t be the main vector of plague.  I’m not an archaeologist, so I can’t really judge the archaeological evidence presented by Hufthammer and Walloe (2013).  But I was fascinated by the literature review that they presented, and I wanted to share some of their ideas on the blog.  Here are some of the arguments against rat-facilitated transmission, as expressed by Hufthammer and Wallow (2013) and other sources:

1. Many rodent species can be infected by the plague, and rodent species vary in their ability to tolerate the plague.  For instance, mice and voles don’t tend to die when infected with the plague, while black rats are killed by the plague.  Apparently black rats don’t just quietly die, either – they get really sick and behave abnormally, so it would be hard not to notice the diseased rats.  If black rats were experiencing a plague epidemic, then massive mortality events should have been observed in black rat populations.  Therefore, if there are written accounts of plague epidemics in humans that do not include notes about how massive number of black rats also recently experienced horrific deaths, then black rats probably weren’t involved in the spread of the plague.  Similarly, there should be archaeological records (i.e., rat bones) of black rat population declines coinciding with human plague epidemics.  But in many cases, there aren’t! I find that fascinating!
2. If the plague kills rats within a few weeks, then rat populations carrying the plague wouldn’t be able to survive any ship voyages that long. (Is your mind blown yet?!)
3. In many places, the plague spread so quickly that it does not seem possible that rats could have been the cause of transmission.  The rats would need to travel to the new town and die of infection before the rat fleas left the rats to bite and infect humans.  Apparently that process takes several weeks, which is just too slow to explain how rapidly the bubonic plague spread among towns.
4.  The human flea (Pulex irritans) can be infected with the plague.  In present-day Tanzania, where the plague occurs in humans, human fleas are the most common fleas in households (Laudisoit et al. 2007).  Furthermore, towns that frequently have plague epidemics also have greater proportions of houses containing human fleas.  So, perhaps human fleas play(ed) a role in plague transmission?
5. The human body louse (Pediculus humanus) can also become infected with and transmit the plague (Ayyadurai et al. 2010).

Does that mean that rats and rat fleas never caused plague epidemics?  Nope.  There is evidence of rat plague epidemics before human plague epidemics in some locations.  Similarly, the plague has proceeded slowly enough in some locations that rat-facilitated transmission is feasible.  But in other places, like Northern Europe, the evidence may be iffy.  Perhaps rat fleas originally brought the plague to the region, but then the main vectors were human ectoparasites?

This is all fascinating, and I hope we see more related research in the future!

References:

Ayyadurai, S., F. Sebbane, D. Raoult, and M. Drancourt. 2010. Body lice, yersinia pestis orientalis, and black death. Emerging infectious diseases 16:892–3.

Hufthammer, A. K., and L. Walløe. 2013. Rats cannot have been intermediate hosts for Yersinia pestis during medieval plague epidemics in Northern Europe. Journal of Archaeological Science 40:1752–1759.

Laudisoit, A., H. Leirs, R. H. Makundi, S. Van Dongen, S. Davis, S. Neerinckx, J. Deckers, and R. Libois. 2007. Plague and the human flea, Tanzania. Emerging infectious diseases 13:687–93.