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Gerbil population boom predicts plague

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  • sonya
    Gerbil population boom predicts plague 12:05 30 April 04 NewScientist.com news service Gerbil population boom predicts plague Plague outbreaks can be predicted
    Message 1 of 1 , May 1, 2004
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      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
      pestis.

      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
      appears."


      Fleas on the great gerbils may transmit plague to cattle or humans
      (Image: Science)
      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
      Leirs.


      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
      people.

      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
      smaller."


      Time delay


      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
      Leirs.

      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
      suggests.

      Journal

      http://www.newscientist.com/news/news.jsp?id=ns99994945
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