Espresso science sheds light on Kazakhstan's bubonic gerbils

18 September 2008

Coffee granules reveal how plague spreads in wild animals.

giant gerbil

A bubonic plague-harbouring giant gerbil

Disease outbreaks are usually explained by abundance theory - as populations grow, more individuals are likely to bump into one another and so disease will spread.

But when it comes to the bubonic plague-harbouring giant gerbils of Kazakhstan, it seems another mathematical concept - percolation theory - may better explain outbreaks.

In Kazakhstan and elsewhere in central Asia, the plague bacterium Yersinia pestis - the source of human cases of bubonic plague - still circulates in wild populations of gerbils and other small mammals.

At a workshop on plague thresholds funded by NERC for the Environment & Human Health programme, researchers wondered aloud whether the key for disease outbreaks was not abundance thresholds, but percolation thresholds.

Percolation theory allows mathematicians and material scientists to work out how something slips through a porous material, like hot water through coffee granules in an espresso machine.

After the meeting, the researchers contacted percolation expert Peter Trapman from the University Medical Center in Utrecht, Netherlands. This new line of enquiry led to a paper in the journal Nature.

Co-author and Deputy Head of the School of Biological Sciences Michael Begon was at the workshop. "Instead of thinking of populations in terms of critical densities of individuals, percolation theory imagines points fixed in space and that there are paths between them," he said. "The points are the gerbil colonies."

Watching gerbils from space

Gerbil burrow systems in central Asia interconnect and spread from the Chinese borders to the Caspian Sea across vast tracts of desert. Earth observation satellites can pick them out because inhabited burrows create great discs of bare earth above, 30 metres in diameter. Scientists can use these to make maps of the various clusters.

Spraying against bubonic plague in Kazakhstan

Spraying against bubonic plague in Kazakhstan

"Imagine a path from colony to colony," said Begon. "What we've got is a lot of addresses, but not all are inhabited. Inhabited burrows stand out because the vegetation surrounding them has been kept back by the gerbils."

"What we've found is that, up to a point, it does not matter about the size of a family of gerbils, what matters is how connected they are to other families," he added.

Below the threshold, plague may pass from family to family but is always aborted. Above the threshold, plague sweeps through the whole lot of interlinked burrows.

It turns out that outbreaks only occur when family groups of gerbils live in more than one third of burrow systems. Below this number - or percolation threshold - plague remains contained.

The scientists believe this is the first time percolation theory has successfully described the natural dynamics of an infectious disease. The team was greatly aided by the so-called Plague Archive, the Soviet Union's meticulous record of plague in wild animals kept after World War II.