The plague is back, and this time
it’s not thanks to far-voyaging ships or caravans traversing some distant trade
routes, but to corn. This disease, caused by one of man’s oldest bacterial
foes, Yersinia pestis, and spread by flea-infested rodents, is often
overlooked in modern times in favor of more headline-grabbing epidemics like
Ebola, HIV, and antibiotic-resistant STDs. But the plague has always kept close
quarters with mankind and continues to surprise us with its adaptability.
Historically, the plague has long
been associated with poor hygiene, cramped living situations, and substandard
housing, with outbreaks blooming in the cities that make such fine homes to
hordes of the rodent vectors. Yersinia has, for the most part, lost its
edge in our modern society thanks to widespread advancements in urban hygiene
and sanitation. Good housing and sound infrastructure largely bar the entry of
rodents into our homes, hindering human exposure to microbe-bearing fleas.
But the plague is a tenacious enemy,
and its dynamics have changed to counter these hygienic advancements. As I
wrote a few years ago,
Yersinia has established an endemic presence in the African continent,
where cases of the infection comprise over 97% of the world’s 20,000-odd cases
in the past fifteen years. Plague is a major public health concern
throughout the continent, but particularly so in east Africa’s Tanzania, where
outbreaks have been pulsing through the region in time with the cycle of the
seasons since 1980. Since then, the country has tallied just under
9,000 cases and 675 deaths due to plague outbreaks.
The public health community has
struggled to identify why this ancient microbe has so firmly established itself
in the region, but research published yesterday in the American Journal of
Tropical Medicine and Hygiene (AJTMH) is making headway in shedding some
light on the nature and methods of the disease. Of key importance is the fact
that in Tanzania, the plague is taking root in an entirely different setting
than what we associate with the Black Death. Rodents, their fleas, and the
dangerous microbial cargo they carry are emerging not in cities heaving with
people and sewage, but in areas where the natural forest terrain is eroding as
acres of maize crops are seeded, grown, and harvested in its place (1).
Over the last few decades, croplands
in northern Tanzania have expanded by 70 percent, driven by heaving food
production needs. As crops expand and nibble away at existing wild
environments, native animals are forced from their habitats. This situation is
complicated in Tanzania, where the population receives up to one third of its
daily caloric intake from maize.
This plant, a staple crop for Tanzanian smallholder farmers and large-scale
growers alike, just so happens to hold a status not far below manna to rodents.
In regions where maize farming has become the norm, the population of African
rat (Mastomys natalensis) has exploded 20-fold. Hillary Young, PhD, a
community ecologist at the University of California, Santa Barbara and one of
the lead authors of the study, explains the deleterious effects of increased
maize production:
Introducing maize production in
natural areas appears to create a perfect storm for plague transmission. The
presence of the crop as a food source caused a surge in the population of a rat
species known to carry plague. Local farmers often then store this harvested
corn next to or inside their homes – baiting in the hungry field rats and
increasing opportunities for human infection. (2)
Agricultural expansion in Tanzania
is not only transforming the ecological landscape, but upending the delicate
balance between good public health and exposure to zoonotic diseases.
Previously, forest regions had formed protective barriers between human
populations and the farmland that attracts harmful pests, much like the sturdy
walls of houses and enclosed toilets of today’s society shield us from rodents
that crawled through the gutters of premodern cities. But the felling of these
“green walls,” these natural barriers and buffers to disease, is putting the
farming community in Tanzania at risk of contracting a disease that boasts a
staggeringly high fatality rate.
“When we change a forest into something else,
we wholly rewrite the rules for the way that life on that kilometer-square
space works,” says Douglas McCauley, PhD, an ecologist at the University of
California, Santa Barbara, and one of the lead authors of the study. (3) “In
this case, the change that we engineer by changing a forest into a cornfield
causes these eruptive dynamics that really amplify the risk for plague.”
The emergence of plague in this
country has important implications for the global food trade and security.
Sub-Saharan Africa contains 60% of the
world’s uncultivated soil that may
be used for crop production, and yet the region continually experiences
crippling food security instabilities. Agricultural expansion in the region is
inevitable and, as McCauley points out, even desirable: “Some degree of
conversion has to absolutely happen to prevent destabilizing food security in
this part of the world.” (3) However, these needs must be balanced with a
growing consideration of the disease ecology and epidemiology in eastern
Africa.
As studies like these continue
to decipher the kinetics between agricultural expansion and disease ecology,
the intricate consequences of man’s manipulation of his natural environment are
further exposed. The dynamics of plague in Tanzania have become
clearer, but these findings also offer a sobering reminder that progress
also presents us with new challenges, especially in less stable regions of the
world.
Previously on Body Horrors
“What’s the scoop on plague in the
21st century? Is “ring-around-the-rosie” still relevant in this modern era of
twerking?” Learn more about plague’s new dirty tricks in the modern era: Plague It Again, Sam: Plague in the Twenty-First
Century
Farming also played a role in the
emergence of a novel and deadly disease nearly twenty years ago: The Climatic Origins of the Malaysian Nipah Virus
Outbreak
Resources
References
1) DJ McCauley et al.
(2015) Effects of Land Use on Plague (Yersinia pestis) Activity in Rodents
in Tanzania. Am J Trop Med Hyg: 14-0504
2) American Journal of Tropical
Medicine and Hygiene. (2015). Researchers Find Agriculture Expansion in
Tanzania May Greatly Increase Risks of Plague in Humans [Press release].
Retrieved
from https://astmhpressroom.files.wordpress.com/2015/02/ajtmh-plague-agriculture-press-release_final.pdf
3) DJ McCauley, personal
correspondence, Feb 22, 2015
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