Normally percolation theory is used to describe the movement of liquid through porous material. A good example of percolation is when hot water is forced through ground coffee in an espresso or Senseo machine.
By moving through the coffee via the empty spaces between the ground coffee particles, the water picks up the flavour of the coffee. Stephen Davis and colleagues at the Faculty of Veterinary Medicine, Utrecht University, report in Nature their discovery that the spread of the bubonic plague bacteria in Central Asia by gerbils, works much the same way.
Plague bacteria percolate through the landscape transmitted by fleas from one great gerbil family to the next, from burrow system to burrow system. It’s the first time percolation theory is used to accurately describe the natural dynamics of an infectious disease. The discovery might be helpful to understand how outbreaks of disease occur in other populations. It may, for example, shed new light on spread of bovine tuberculosis in badgers, and spread of viral diseases in populations of African lions.
Spread of infection predictable
Like making a good cup of coffee, there are conditions for successful percolation of the plague through gerbils. In the case of coffee, if you pack it too tightly the water will not move through and the coffee just burns.
In the case of plague, if there are too few gerbils, the fleas won’t be able to move from one burrow to another, and the spread halts. The use of percolation theory by Davis can predict the final size of an outbreak of plague, starting with one infected family group. It has also determined the so called ‘abundance threshold’ necessary to be exceeded before percolation occurs, in this case the percentage of available burrows populated by gerbils.
Russian ‘Plague Archives’ show plague dynamics
In Kazakhstan and elsewhere in central Asia, the plague bacterium Yersinia pestis, source of human cases of bubonic plague, still circulates in wild populations of gerbils and other small mammals. Intensive surveillance systems established by Soviet scientists in Kazakhstan at the end of World War II appear to have successfully reduced human cases. These programs also resulted in vast amounts of data, referred to as ‘The Plague Archives’, recording the dynamics of plague in wild animals.
The data for the research done by Davis comes from one of the desert plague foci in Kazakhstan where great gerbils play a central role as a plague host. A remarkable feature of these gerbils is their complex burrow systems that create discs of bare earth, typically 30 metres in diameter and visible from space on satellite images like the ones from Google Earth. The vast tracts of land inhabited by connecting burrows of great gerbils contrast starkly with the relatively short distances of about 200 metres travelled by fleas responsible for transmission of plague. This difference in scales means that connectivity of the population is key.
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