The Black Plague: Could It Happen Again?
Pastor Dale Morgan
The Black Plague: Could It Happen Again?*
May 24th, 2007 8:40 AM
Megan Sever
Plague ravaged Europe for nearly 500 years, brought about by the
expansion of global travel at the same time climate changed, according
to new research. Given the extent of globalization today, when a person
can fly anywhere in the world in a day or two, and the fact that the
climate is changing, health officials and the public wonder if there is
a risk of history repeating itself.
The 14th through 19th centuries were challenging times in Europe.
Winters were harsh, filled with heavy snowfalls that lasted late into
spring and ice that perpetually covered mountaintops and pushed into
settled valleys. Springs and summers were so cold and wet that crops
would not grow, or became moldy before they could be consumed. People
and livestock starved. Wars were fought over scant resources as people
traveled farther than before, searching for food and better conditions
and colliding with anyone who got in their way.
These desolate conditions forced people to leave their homes and rotting
fields in the countryside and head for cities, where crowding and poor
sanitation were the rule. Meanwhile, international trade greatly
expanded, as ships and caravans brought goods from Asia into European
cities. Trade brought more than just goods, however: It also brought
diseases.
During these 500 years of cold, extreme and unpredictable weather in
Europe, temperatures rose slightly for brief periods of time. But rather
than providing a respite from the cold, the warmer temperatures actually
promoted the proliferation of infectious diseases. Chief among them was
plague. Estimates suggest that up to half of Europe’s already weakened
population was wiped out by devastating epidemics, including the
infamous Black Death that began in 1347 and the Great Plague of London
in 1665, when people died so quickly that bodies piled up on the sidewalks.
In the American Southwest, prairie dogs are often carriers of the plague
bacterium. In Europe, black rats and Norway rats are carriers of bubonic
plague bacterium. These rats are very common in cities, and are widely
blamed for the European plague epidemics in the Middle Ages. Photographs
are courtesy of CDC.
With today’s rapid global transport, infectious vectors, which are more
diverse in many regions than centuries ago, can move much faster. The
SARS virus, for example, spread from China to 30 countries around the
world, from South Africa to Switzerland, in a matter of weeks to months
in 2002 and 2003. And the concern is ever-rising about how quickly and
to where avian flu can spread. Furthermore, with the predicted rising
temperatures and changing precipitation patterns, the optimal climate
conditions for diseases such as plague can now extend into new
locations, intersecting new areas of human populations just as plague
did in the 14th century. In Central Asia alone, for example, a 1 degree
Celsius rise in spring temperatures can lead to a 59 percent increase in
plague prevalence the following summer, according to new research. And
considering the panic incited at the very mention of the word, it’s a
concern.
Today, geoscientists, climatologists, epidemiologists and entomologists
are joining forces to learn more about the plague, to figure out whether
we may be set up for the next great plague pandemic, and to limit the
impact among humans.
“The Plague”
Colloquially, “plague” refers to a pestilence or a pandemic disease that
spreads rapidly between humans, says Kenneth Gage, chief of flea-borne
diseases at the Centers for Disease Control (CDC) in Fort Collins, Colo.
Clinically, however, “plague” refers to a specific disease circulating
primarily among small mammals and their fleas that is caused by a
bacterium called Yersinia pestis, he says.
The three versions of the plague depend on how it affects a person:
Bubonic plague is the most common form and results in flu-like symptoms
and the swelling of lymph nodes in the neck, armpit or groin into large,
painful cysts called buboes. Pneumonic plague is the least common but
most dangerous form of plague. In this form of pneumonia, plague
bacterium gets into the lungs of the victim, causing the coughing up of
bloody sputum. Septicaemic plague occurs primarily as a secondary
disease, Gage says, when pneumonic or bubonic plague is left untreated
and the bacterium gets into the victim’s bloodstream, causing septic
shock or other serious problems.
Unless very promptly treated, pneumonic and septicaemic plague are
almost always fatal, says David Engelthaler, director of TGen North, a
pathogen genomics laboratory in Flagstaff, Ariz., and a branch of the
Translational Genomics Research Institute in Phoenix, Ariz. But bubonic
plague progresses slower than the other two forms of the disease and is
highly treatable with antibiotics if caught early, he says.
Pneumonic plague can be spread from human to human through coughing, but
“you need direct, face-to-face contact” for that to happen, Gage says.
Even though it is an infectious disease, being in the general vicinity
of someone with pneumonic plague is not enough for someone to catch it.
Typically, this type of plague is only seen in developing countries such
as Ecuador, Uganda and India, and even then only among families living
in close quarters, he says. The United States hasn’t seen a case of
pneumonic plague spread from one human to another since 1924, he says.
Occasionally in the United States, a case pops up where pneumonic plague
is spread from a domestic cat to a human, but it’s very rare, he says.
Less than 2 percent of annual cases of plague worldwide are pneumonic
plague spread from human to human or domestic animal to human, he says.
Bubonic plague is much more common, though it is still a rare disease,
Gage says. Worldwide, some 1,000 to 3,000 cases of human infection occur
each year, according to the World Health Organization (WHO). Humans can
acquire the disease by being bitten by an infectious flea, handling an
infected animal or, rarely, inhaling infectious materials, Gage says.
Seventy to 80 percent of human plague cases worldwide each year are
acquired by flea bites, he says. Around 20 percent of cases are caused
by handling infected animals. Occasionally, so-called human fleas, Pulex
irritans, can become carriers of the bacterium and can thus transmit it
to humans, Engelthaler says, which may have been the cause of the great
pandemics of the past. Now, he says, that situation is extremely rare,
thanks to modern sanitation.
Today, plague still exists in many African countries, East and Central
Asia and the Americas. The overwhelming majority of outbreaks occur in
Africa, especially in Madagascar, according to WHO: In 2003, nine
countries reported 2,118 cases of plague and 182 deaths from the
disease. Of those, 98.7 percent of the cases were in Africa, and 98.9
percent of the deaths occurred in Africa. Still, the disease could
spread with climate change both geographically and in prevalence,
according to new research.
The environmental link
Bubonic plague is “a disease of nature,” Engelthaler says, meaning that
climate and landscape play a vital role in the survival and spread of
the bacterium that causes the disease. Rodent and flea population
dynamics are driven by many factors, Gage adds, including food
availability, disease and climate variables, namely precipitation and
temperature. In studies published over the last five years, models and
observations have shown that precipitation and temperature strongly
influence the spread of plague.
The most important factor in the disease, besides the bacterium,
Engelthaler says, is the flea that carries and transmits the disease.
Not all species of flea will transfer or maintain the bacterium, and
some transmit it better than others. The type of flea that lives on
cats, for example, is not a good vector, he says. But the fleas that
live on black rats and ground squirrels are great vectors. Furthermore,
the fleas that carry Yersinia pestis can only survive for long periods
in “optimal” conditions, including warm but not hot temperatures and wet
environments. And they can only transmit the bacterium under even more
specific conditions, he says. If temperatures get too hot, the biology
of the bacterium stops it from spreading, by breaking down the bacterial
blockages that have built up in the flea vector’s gut and are considered
essential for efficient transmission.
In addition to needing the right type of flea, the right type of host
needs to be present to keep the cycle of transfer from flea to host and
back to flea going, Engelthaler says. Black rats and prairie dogs die
within days of being infected, so they might not be the best hosts, he
says. Although ground squirrels also often die from plague, they can
carry the bacterium around for months, allowing fleas to transfer the
plague bacterium from their dying host to another unsuspecting host.
To get widespread epidemics of the disease, the density of host rodents
must first reach a threshold level in a region, Gage says. Then the
weather has to cooperate to keep it going and to increase the number of
human cases, he says.
In the American Southwest, where plague is prevalent in wild rodents and
an average of five to 15 people contract the disease each year,
increasing rainfall in late winter and early spring leads to a sizable
increase in plague 15 months later, Gage says, as seen in models and
observations over the past 50 years. It works in a sort of “trophic
cascade,” he says: “Heavy precipitation in early spring leads to more
plant growth and more insects, which means more food for the rodents,
which leads to more hosts for the [plague-bearing] fleas, and thus more
plague.” The other important factor, he says, is lower summer temperatures.
The story is similar in Central Asia, says Nils Chr. Stenseth of the
University of Oslo. Infection rates and climate data from 1949 to 1995
in Kazakhstan showed that with just a 1 degree Celsius increase in
spring temperatures, plague prevalence in gerbils more than doubled a
year or two later, Stenseth says. Wetter summers also led to an increase
in plague prevalence the following fall, he says.
Ongoing research in China and other parts of the world is finding a
similar trend, Stenseth says, though the exact mechanisms may be
slightly different, such as whether spring or summer precipitation or
temperature is the driving factor. And models are agreeing with the
data. “The general message we’re seeing all over the world is that
climate is important,” he says. “Furthermore, climate is changing in a
way that will affect human plague cases.”
The future
“You can make four generalities about climate change’s effects on
biodiversity,” including everything from pathogens to fleas, rodents to
humans, says Townsend Peterson of the University of Kansas. First, when
temperatures rise, species’ distributions tend to shift toward poles.
Secondly, species’ distributions tend to shift upward in elevation.
Third, “complex topography makes for smaller-scale, less dramatic
effects: For example, you’ll see more dramatic shifts in the Great
Plains, which are flatter, than in the Rockies,” he says. “And finally,
the kicker is that every species is an individual and it’s pretty darn
hard to predict what they are going to do as the climate changes,” he says.
Nonetheless, Peterson says, “the link between plague and climate change
opens all sorts of doors and prompts all sorts of questions.” The two
primary questions for the future, he says, are whether or not the
disease’s range will spread or migrate, and if its prevalence will increase.
Peterson and colleagues modeled these possible changes for plague and
tularemia in the United States in coming decades. Using several
different climate models, including the Hadley Centre and the Canadian
models, and running models for both extreme-case scenarios of extensive
climate change and minimal climate change, the team found that plague
will not likely shift its range extensively in North America, although
tularemia will likely shift somewhat northward. Plague is likely not
showing dramatic geographic shifts in the American Southwest because of
the complex topography.
That doesn’t speak to prevalence, however. Climate models suggest that
the Southwest may see more varied weather patterns, such as more El Niño
years. And El Niño years bring the precise cooler, wetter temperatures
to the region that the plague loves, Gage says. The strong El Niño in
1982 was followed by a two-year spike in human plague cases, he says.
Tularemia and hantavirus pulmonary syndrome (spread by field mice) cases
have also spiked in the wake of El Niño events, Peterson adds. “We saw
the same thing happen after the 1992 El Niño,” Gage says. First, a
hantavirus outbreak erupted in 1993 and then plague cases spiked the
following year. Human plague cases then decreased drastically in the
summer of 1994, he says, likely because the summer was “incredibly hot
with 100-degree-Fahrenheit-plus temperatures.” That likely killed off
the fleas, he says.
In Central Asia, climate models suggest that the precise weather
conditions conducive to plague — warmer and wetter springs and summers —
will grow more frequent in coming decades, Stenseth says. Those
conditions would lead to an increase in rodent populations and thus an
increase in plague, he says.
Pandemics
“I fully expect we will see an increase in the number of humans affected
by plague” in coming decades, Stenseth says. “But there’s no reason to
suggest we will see another Black Death.”
Indeed, climatic changes could very easily lead to an increase in
conditions conducive to rodent (host) population expansion and more
human plague cases in some areas, Gage says. Furthermore, humans, and
Americans especially, are moving farther and farther out of cities and
into suburbs and rural areas where they are more likely to encounter
plague-ridden rodents. But climatic changes could also cause a decrease
in human plague cases in other regions, he says, such as the American
Southwest, if it gets too hot and dry.
The research linking climate change and plague is “really early in the
process and there is a lot more to be done,” Peterson adds. “But what I
can say is that the likelihood of a pandemic” of bubonic plague “is
incredibly slim,” as most people don’t live on top of one another in
squalor and surrounded by rats like they used to, and modern healthcare
can cure the disease once it strikes, he says.
Still, questions remain. Overall, whether or not more people will get
the plague is “just really hard to predict,” Gage says. Although
temperatures were much colder in Europe during the Little Ice Age
(roughly 1300 to 1850), those brief periods of warming around the
mid-1300s, mid-1600s and mid-1800s, as recorded in tree-rings, occurred
at the same time as the plague pandemics, Stenseth says. The Black Death
struck Europe in 1347. The Great Plague of London struck in 1665. The
Third Pandemic began around 1855 in China.
Overall, so many factors are involved in the rise and spread of bubonic
plague that causation between climate and the disease is not conclusive,
Stenseth says, but it’s certainly a possibility.
Graver threats
With warming temperatures and changes in precipitation patterns come
changing threats to human health. Recent research linking historic
outbreaks of bubonic plague to climate change have led some people to
wonder if another plague pandemic is on the horizon. But climate and
health researchers say that a resurgent bubonic plague is probably the
least of our worries.
Malaria. Dengue fever. Filariasis. Giardia. Cryptosporidiosis. West Nile
virus. Avian Flu. Giardia. Hantavirus pulmonary syndrome. Bubonic
plague. These, among other, vector-borne and waterborne diseases are
“highly sensitive to climate,” says Jonathan Patz, an associate
professor of Environmental Studies and Population Health Sciences at the
University of Wisconsin-Madison.
Some of these diseases are more affected by temperatures and others by
precipitation changes, Patz says. Cryptosporidiosis and giardia
outbreaks, for example, can be tied directly to extremes in the
hydrologic cycle, he says. In 1993, following the heaviest rainfall in
the previous 50 years, a cryptosporidiosis outbreak struck Milwaukee,
Wis., sickening 400,000 people through drinking water. Hantavirus and
plague are also tied to hydrologic extremes, such as strong El Niños
(see main story).
In recent years, the geographic extents of outbreaks of West Nile,
malaria and dengue fever have expanded, thanks to warming temperatures,
Patz says. Further warming may exacerbate the situation, sending malaria
back into Europe, where it hasn’t been common for centuries (see
Geotimes, May 2005).
Still another human health challenge may arise from rising sea levels,
Patz says. In developing countries, as people migrate inland to escape
rising sea levels and storms, they may be exposed to new diseases, or
they may end up living in poor, crowded shelters with little healthcare
— similar to the type of unsanitary conditions Europeans faced in the
Middle Ages when the plague ravaged the populations. This “is a very
difficult risk to study, but it could be the biggest problem, the large
mass under the tip of the iceberg,” Patz says.
The risks human face most directly from climate changes, however, Patz
says, are extremes such as heat waves like the one that struck Europe in
2003 and killed thousands of people.
Regardless of the exact form the threats will take, warmer temperatures
and wetter weather influence infectious diseases, according to the World
Health Organization (WHO). And though it is quite difficult to quantify
disease changes, “changes in infectious disease transmission patterns
are a likely major consequence” of such climate changes, according to WHO.
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© 2007 American Geological Institute. All rights reserved.