Gerbil population boom predicts plague
- Gerbil population boom predicts plague
12:05 30 April 04
NewScientist.com news service
Gerbil population boom predicts plague
Plague outbreaks can be predicted by the rise and fall of the wild
gerbil population in Kazakhstan, researchers have discovered. Their
plague predicting model might also be applicable to other parts of
the world where outbreaks are irregular.
In Kazakhstan, wild great gerbil colonies are the natural reservoir
for the deadly disease, which is caused by the bacterium Yersinia
The gerbil population swings markedly from year to year, but Herwig
Leirs, at the University of Antwerp, Belgium, says: "We found a
fairly simple pattern - if the population of gerbils increases in
density, when they reach a certain level, two years after that plague
Fleas on the great gerbils may transmit plague to cattle or humans
He says his team's model cannot predict with certainty when an
outbreak will occur, but it can predict the years when there is no
chance of an outbreak. This could help public health teams to target
their efforts to prevent plague on risky years only, says Leirs, who
also works at the Danish Pest Infestation Laboratory in Copenhagen.
Leirs' team was able to model the dynamics of the disease only
because of the "huge goldmine" of data left by Soviet scientists who
meticulously monitored the gerbils and their plague-ridden fleas from
1947 until the collapse of the Soviet Union. Western powers were
concerned the information might be used in biowarfare research, but
ironically the vast database may actually help prevent plague, says
High density homes
The team used data from two of the 120 sites monitored daily for
decades by Soviet scientists. Great gerbils (Rhombomys opimus) carry
plague but are not made particularly ill by it. However fleas on the
gerbils can transmit the deadly bug between colonies and to cattle or
Families of gerbils live in permanent burrow systems. The team
noticed that when the gerbil density rose above the point when about
40 per cent of these underground homes were occupied, plague followed
two years later.
Leirs says their model "fits nicely" into epidemiological theories
about population dynamics and disease, which until now had neither
been contested nor proven.
"If a population is dense enough then the disease can persist because
there is frequent contact between infected and susceptible
individuals," he told New Scientist. "But when the population density
drops the disease is less likely to persist because the chance for
infected individuals to meet susceptible individuals becomes much
The two-year delay between population peaks and plague is partly due
to the time the disease takes to spread, says Leirs, although the
exact reasons are not clear.
The link between outbreaks in rodents and humans is also not
necessarily straightforward, notes bacteriologist Elisabeth Carniel,
who has worked with the Leirs's team and is director of the World
Health Organization centre for Yersinia in Paris, France.
"Without a rodent outbreak, there would probably not have been human
outbreaks. But the fact there are rodent outbreaks doesn't mean there
will be human outbreaks - it depends on the context," she says.
Since Soviet intervention in the late 1940s, large plagues in
Kazakhstan have subsided from affecting hundreds of people every year
to only a handful of cases every year. But it remains a threat in
central Asia, Africa, South America and in some southern US regions.
Leirs and Carniel say the idea of using wildlife as an early warning
system for plague could be tested in other regions but would have to
take account of any differing factors such as primary host, soil
type, climate and type of flea.
What remains a complete mystery is where the plague bacteria go when
they disappear from the gerbil population. Soviet scientists believed
they survive in some kind of quiescent form, but this would imply a
much shorter time delay between population peaks and outbreaks, says
He believes a more likely explanation is that the plague genuinely
disappears but is shipped back in from outside the region by other
animals. For example, a species of migratory bird called wheat-ear
nests in the gerbils' burrows and could plague-ridden fleas, he