Friday, April 05, 2013

Return of the Black Death by Susan Scott and Christopher Duncan

The Return of the Black Death slowly uncovers a scientific mystery.  The two authors, Susan Scott and Christopher Duncan, have studied the Black Death for many years and have found many puzzling features which suggested to them that the commonly accepted idea that the plague was a fast acting bacterial infection reliant on rats and fleas was an incorrect one.

Here, in summary, are some of these features.  The book provides more detail and context.

1.  The Black Death came to Sicily in 1347 and from there  moved very quickly through Western Europe - 2,200 miles in three years. This was too fast for the disease to be spread just by rodents and therefore was spread by humans, which implies a long incubation period given the primitive and slow transport of the time.

2.  It remained active during very cold winters and they provide evidence that in 1402 and 1494  it spread through the scattered communities of Iceland (but Black Rats only thrive in warm conditions, and the flea also needs warm conditions to breed).

3.  It lasted 9 months in a village (they show how this indicates a long incubation period of 32 days (from time of infection to display of symptoms), and an infectious period of 27 days only the last 5 of which are with symptoms).  Bubonic plague has much shorter periods than this (incubation period 2-6 days).

4.  It had a death rate of around 50% which is too high for bubonic plage.

5.  There were no accounts of dead rats which would be expected if it were bubonic plague since Black Rats are susceptible to bubonic plague and they are usually found first before a bubonic plague outbreak.  Also, Europe uniquely has no native rodent population which is resistant to bubonic plage and would act as a reservoir - necessary for  recurring epidemics.

4.  After death the internal organs were showing necrosis (much evidence for this from symptoms - one of the first being the sweet smell of the air due to internal organs beginning to break down).  This is not a feature of bubonic plague but is a feature of haemorrhagic fever viruses.

5. It remained active in France year on year when 'by all the rules of infectious diseases when the Black Death finished it should have disappeared'. This seems to have been the reservoir country - outbreaks in all other countries after the initial Black Death arose from the visits of strangers from 'far away'.    Once it disappeared from France due to a cold spell then it disappeared from Europe for good since the reservoir was gone.

6.  In Italy and elsewhere it was established that only a quarantine period of 40 days would guarantee safety from infection.  This again supports the idea of a long incubation period.

7.  It was generally acknowledged by everyone at the time that the disease was transmitted from person to person.

8.  If the plague was really dependent on rats and fleas then establishing cordon sanitaire as they did in Eyam should have been ineffective since rats would have breeched it in summer - but it famously was.

9.  It has been found that there is a mutation among Europeans which makes white blood cells resistant to being infected by HIV (one of the gateways into the cell, the CCR 5, is mutated).  This mutation came about 2,000 years ago and was selected for an event 700 years ago i.e. the time of the Black Death.  It is postulated that this mutation (CCR 5 - delta32)  also guards against plague and therefore waves of plague have ensured this mutation's survival and increase in the population.  It has been found in the descendent of one of the plague survivors at Eyam.  The mutation is not found in non-Europeans - and it is only outside Europe that the bubonic plague has been rampant.

I found it very convincing and very interesting.  The last couple of chapters go on to describe that if the Black Death was viral rather than bacterial then we still cannot do anything to cure it.  The best we could hope for would be to find a vaccination.  I suppose this would be effective once it was found because, unlike influenza, the microorganism causing the plague was stable over 300 years - or at least its symptoms were.

So...I was convinced that The Black Death was caused by a virus similar to Ebola and arising in east Africa, but then I came across a recent letter in Nature, which seems to establish that The Black Death was caused by the bubonic plague bacteria Yersinia pestis after all.  It was an ancient ancestor of present day bacteria and 'our data suggest that few changes in the known virulence-associated genes have accrued in the organism's 660 years of evolution as a human pathogen, further suggesting that its perceived increased virulence in history may not be due to novel fixed point mutations detectable via the analytical approach described here.  At our current resolution. we posit that molecular changes in pathogens are but one component of a constellation of factors contributing to changing infectious disease prevalence and severity, where genetics of the host population, climate, vector dynamics, social conditions and synergic interactions with concurrent diseases should be foremost in discussion of population susceptibility to infectious disease and host-pathogen relationships with reference to Y pestis infections.' 

The paper gives references for each of these factors.  In other words the Black Death was bubonic plague - a bacterial infection made more severe by various unfavourable factors of the time including the the climate, changes in the genetics of humans and their interactions with rats and fleas, and perhaps, most importantly other 'concurrent diseases'   that I guess we may never know about.


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