Plagues, Pestilences and Diseases
Flea Infested Mice Spread Deadly Black Plague In Prairie Dog Populations
Stanford University postdoc Dan Salkeld has been studying prairie dogs
for years. Credit: Dan Tripp.
by Staff Writers
Stanford CA (SPX) Aug 05, 2010
Prairie dogs, once abundant in the Rocky Mountains and Great Plains,
have been decimated in recent decades by plague - a virulent bacterial
disease spread by fleas.
Plague outbreaks periodically sweep through large prairie dog towns
with thousands of inhabitants, killing virtually the entire population
within months. Other prairie dogs move in and build a new colony, which
eventually is wiped out when the disease returns.
This pattern of re-colonization followed by devastation can occur over
many years. The question for scientists is how does plague persist
after a colony has been wiped out?
"A fundamental question in disease ecology is what happens to pathogens
in between the periods when they cause all of this devastation," said
James Holland Jones, an associate professor of anthropology and a
center fellow at the Woods Institute for the Environment at Stanford
University.
"Previous studies have suggested some sort of magical reservoir for the
plague pathogen," Jones said. "Maybe it gets into the soil and infects
the re-colonized prairie dog town. Or perhaps it's carried in by some
carnivore. Who knows?"
Jones and his colleagues may have finally solved this longstanding
mystery. The likely culprit, they say, is the grasshopper mouse, a
carnivorous rodent that carries plague-infected fleas across the rigid
territorial boundaries separating isolated family groups within the
prairie dog colony.
"We found that when grasshopper mouse density gets high enough, you get
an epizootic - the animal equivalent of an epidemic - and virtually all
of the prairie dogs die," Jones explained.
This finding, published in the online edition of the Proceedings of the
National Academy of Sciences, could have significant implications for
understanding how infectious diseases spread in animals and humans.
"Plague, a disease caused by the bacterium Yersinia pestis and the
causative agent of Black Death that killed 200 million Europeans in the
14th century, arrived in the United States via San Francisco around
1900 and still infects people worldwide," Jones said.
A number of other deadly human diseases - like hantavirus, Ebola and
anthrax - show the same pattern of decimating communities, disappearing
and coming back years later, said Daniel Salkeld, a Stanford
postdoctoral scholar and lead author of the study.
"A key element to their control and eradication is understanding where
they persist in the latent phase and identifying the conditions that
result in sporadic epidemics," Salkeld said.
Territorial families
The research team focused on black-tailed prairie dogs, one of five
prairie dog species whose populations have dwindled over the past
century because of habitat destruction, extermination and plague.
Tracking infectious diseases in prairie dogs is challenging. The
burrowing rodents live in complex underground communities largely
hidden from view.
"Prairie dogs are highly social ground squirrels that occupy colonies,
or towns, which can extend for 500 acres and comprise 5,000 individuals
or more," Salkeld said. "While the town provides a defense against
predators, individual prairie dogs are highly territorial."
Prairie dogs live in small family groups, known as coteries, which they
defend vigorously. A large prairie dog town may have 1,000 coteries.
Together, they form a grid of small, isolated territories within the
town, Jones added.
"The coteries are spread out almost lattice-like," he said. "That has
implications for infectious disease transmission. Each coterie has only
a few neighbors. Because prairie dogs don't venture outside their
territories, they can only infect their immediate neighbors. This
territoriality limits the rate of disease propagation through the
prairie dog town."
Little mouse on the prairie
Plague is not transmitted directly; it is carried by fleas. For the
disease to spread across an entire prairie dog town, infected fleas
must find a host that can freely travel from coterie to coterie. That's
where the tiny grasshopper mouse comes in.
"Grasshopper mice have no respect for prairie dog territories," Jones
said. "They're nasty little beasties, and when they eat the carcass of
a prairie dog that's died of plague, the fleas climb aboard the mice.
The mice then schlep the fleas around to different territories,
connecting family groups that otherwise wouldn't be in contact."
The idea that plague could be facilitated by grasshopper mice was first
proposed by Salkeld and co-author Paul Stapp of California State
University-Fullerton. However, other researchers have been skeptical of
the mouse hypothesis.
"A number of people familiar with prairie dogs say there is no way that
the grasshopper mouse is causing this, because they only trap a few
mice a year," Jones said. "So we decided to write a computer model to
determine if the number of mice being trapped is consistent with
driving these plague epizootics."
Critical threshold
For the PNAS study, Salkeld obtained field data from the Pawnee
National Grasslands in Colorado. Co-author Marcel Salathe, now at
Pennsylvania State University, created a computer model that simulates
plague dynamics in a large prairie dog town.
The results of the simulation revealed that plague is endemic among
prairie dogs.
"Even without grasshopper mice, plague kills one or two prairie dog
families at a time, but it moves very slowly and is extremely hard to
detect," Jones said.
As long as the disease is confined to isolated family groups, the town
can survive. But once the density of susceptible prairie dog families
and grasshopper mice crosses a critical threshold, plague can sweep
through and wipe out virtually the entire town, he said.
"Plague sort of smolders in the prairie dog community for long periods
in between epizootics," Jones said.
"The mouse is like the spark that allows the pathogen to get carried to
a new place where there's more fuel. And then just the right set of
events coalesces, where you have the right mouse densities and the
right spatial pattern of infected prairie dogs. At that point, the mice
trigger the epizootic, and suddenly you get this catastrophic mortality
where nearly all the prairie dogs die of the plague, where before only
a few animals were dying. The territoriality of prairie dogs and the
lack thereof in grasshopper mice is what makes the whole system work."