[Understanding how human behaviors affect the spread and virulence
of a disease can inform preparations for the future.]
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PLAGUE WAS AROUND FOR MILLENNIA BEFORE EPIDEMICS TOOK HOLD – AND
THE WAY PEOPLE LIVED MIGHT BE WHAT PROTECTED THEM  
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Sonja Eliason, Bridget Alex
November 7, 2019
The Conversation
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_ Understanding how human behaviors affect the spread and virulence
of a disease can inform preparations for the future. _

What happened to make plague able to cause devastating epidemics, as
in this depiction from 1349?, Pierart dou Tielt/Wikimedia

 

One of civilization’s most prolific killers shadowed humans for
thousands of years without their knowledge.

The bacteria _Yersinia pestis_, which causes the plague, is thought to
be responsible for up to 200 million deaths
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history — more than twice the casualties of World War II
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The _Y. pestis_ death toll comes from three widespread disease
outbreaks, known as epidemics
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sixth century Justinianic Plague that ravaged the Eastern Roman
Empire; the 14th century Black Death that killed somewhere between 40%
and 60% of the European population; and the ongoing Third Pandemic,
which began in China [[link removed]] in
the mid-19th century and currently afflicts thousands worldwide
[[link removed]].

Scientists long assumed that the deadly disease began infecting humans
just before the earliest epidemic, the Justinianic Plague.

But recent paleogenetics research reveals
[[link removed]] that plague has been with
us for millennia longer: Ancient DNA (aDNA) from the bacteria was
recovered from human skeletons as old as 4,900 years
[[link removed]]. This means people were
contracting and dying from plague at least 3,000 years before
there’s any archaeological or historical evidence for an epidemic.

Why didn’t these earlier infections lead to devastating outbreaks
like the Black Death? It seems the answer is part biological —
genetic mutations to the bacteria itself — and part cultural —
changes to human lifestyles that encouraged the spread of the disease.

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Electron micrograph scan of _Yersinia pestis_ bacteria. NIAID/Flickr
[[link removed]], CC BY
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New proof of ancient plague

To identify cases
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plague, researchers extract aDNA from a skeleton’s dental pulp
chamber and search for genetic code from _Y. pestis_ bacteria. If
fossil teeth contain _Y. pestis_ DNA, it’s safe to assume that
person died from plague.

Several studies [[link removed]] have
found [[link removed]] plague victims
[[link removed]] who lived nearly 5,000
years ago — more than three millennia before the first known plague
epidemic.

Pathogen aDNA analysis also revealed how _Y. pestis_ bacteria have
evolved over time. The oldest genomes recovered belong to a
now-extinct lineage, which was missing certain mutations that make
plague so contagious for humans. For example, later _Y. pestis_
strains evolved a gene that allows the bacteria to efficiently infect
fleas [[link removed]], the main carriers
of the disease in recent times. More ancient _Y. pestis_ samples lack
the gene.

So far, the earliest plague genome
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mutations dates to around 1800 BC from the Samara Valley, Russia. The
mutations were also identified in a skeleton from Iron Age Armenia
that was dated to around 950 BC
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Missing evidence for epidemics

It seems the more contagious form of plague has been infecting humans
for nearly 4,000 years.

But there are no indications in the archaeological record of epidemics
in the ancient societies in Russia and Armenia — despite the fact
that some individuals died from the highly contagious plague strain.

It’s possible outbreaks occurred but the evidence simply hasn’t
been found yet. For example, if future excavations were to uncover a
series of mass graves that differed from the usual burial customs of
those cultures, this could suggest societal disruption consistent with
an epidemic.

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A mass grave of plague victims from the early 18th C in Martigues,
France. S. Tzortzis
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CC BY [[link removed]]

Or perhaps the bacterial strains, though genetically similar to the
plagues of Justinian and the Black Death, lacked some other critical
mutation, still unidentified.

Alternatively, there could be another explanation, related to the
behavior of the people being infected. Did the ancient people of the
Samara Valley and Armenia live in a way that protected them from
plague — perhaps without even knowing it?

Investigating plague protections

We sought to answer this by investigating whether the populations of
1800s BC Samara Valley and Iron Age Armenia behaved differently from
people in Justinian’s Empire in crucial ways.

First we established conditions that make a population more or less
vulnerable to an outbreak. We identified criteria known to be
associated with plague virulence, or how infectious the bacterium is.

Population density [[link removed]] is
important; the number of people in contact with an infected individual
affects the rate of disease spread.

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Fleas spread bubonic plague, and tend to proliferate where rodents do.
CDC/Ken Gage [[link removed]], CC BY
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Permanent [[link removed]] agricultural
[[link removed]] settlements accumulate
food storage and waste, which supports co-habitating rodent species
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hosts for fleas that harbor plague bacteria.

As East Asia is the likely geographic source of plague
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regular trade [[link removed]] with the
region is another factor.

And we examined reliance on horses
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because some scholars suggest — though it’s not yet biologically
tested — that the animals carry natural immunity to plague. Regular
contact with horses could reduce a population’s susceptibility to
the disease.

Comparing cultures point by point

We then compared three populations on these six criteria using
archaeological and historical data.

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Byzantine Emperor Justinian presided over a booming metropolis in
Constantinople. Hein Nouwens/Shutterstock.com
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For the Justinianic Plague, we focused on Constantinople, the capital
of Justinian’s Empire and an epicenter of the outbreak.
Constantinople’s culture created a perfect storm of conditions for
an epidemic.

It was a congested urban center with a population of over 500,000
people, or 140 individuals per acre
[[link removed]]. All of Constantinople’s
staple foods, including grain, were shipped from surrounding areas
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and stored in large warehouses, creating ideal breeding grounds for
rodents. Flourishing trade also introduced the rat species — _Rattus
rattus_ — from India
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that would later be identified as the main carrier of fleas harboring
plague.

In contrast, lifestyles in Samara and Armenia may have kept the
epidemic at bay.

These populations were significantly more mobile and less congested
than the urban population of Constantinople. The Samara population
shows little evidence for agriculture
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settlements of extended families. These communities managed shared
herds, and horse tools found in their characteristic burial mounds
[[link removed]] suggest the animals were highly
valued. Remember, horses may have had some natural immunity to the
disease.

Due to shifting local powers, Early Iron Age Armenia
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appears to have been home to farmers as well as nomadic pastoralists.
Generally, though, archaeologists presume the populations practiced
cattle farming
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which would have made people substantially more mobile and dispersed
than inhabitants of Constantinople.

Less congestion would have made contaminating nearby villages more
difficult. Lacking agriculture, Samara could not have supported
human-dependent rodents, the way Constantinople did. Both populations
potentially benefited from a high ratio of horses to people.

While Samara and Armenia saw occasional plague victims, the structure
of their societies likely protected them from the devastation wrought
in Constantinople.

Cultural perspectives on disease

While encouraging economic and technological gains, urban development
and trade created ideal conditions for an epidemic in Constantinople.
Vulnerability to plague was an unintended consequence of this
society’s lifestyle.

Meanwhile, it seems earlier cultures unwittingly shielded themselves
from the same threat.

The harsh reality is that it’s exceedingly difficult, if not
impossible, to control a pathogen, its possible mutations or its next
outbreak. But understanding how human behaviors affect the spread and
virulence of a disease can inform preparations for the future.

As a society, we can take organized measures to reduce the spread of
infection, whether by limiting over-congestion, controlling food
waste, or restricting access to contaminated areas. Human behaviors
are just as critical to our disease susceptibility as are the
characteristics of the pathogen itself.

_This article has been updated to include a lead image that depicts a
plague outbreak._

[ _Like what you’ve read? Want more?_ Sign up for The
Conversation’s daily newsletter
[[link removed]].
][The Conversation]

Sonja Eliason
[[link removed]], MPhil
Candidate in Bioscience Enterprise, _University of Cambridge
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and Bridget Alex
[[link removed]], Lecturer,
_California State University, Long Beach
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This article is republished from The Conversation
[[link removed]] under a Creative Commons license. Read
the original article
[[link removed]].

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