Satellite images reveal key to predicting outbreaks of plague

[Giuseppe]

The abundance threshold for plague as a critical percolation phenomenon

July 30 2008, 21.32.49 | S. Davis

The abundance threshold for plague as a critical percolation phenomenon
Nature 454, 634 (2008). doi:10.1038/nature07053

Authors: S. Davis, P. Trapman, H. Leirs, M. Begon & J. A. P. Heesterbeek

Percolation theory is most commonly associated with the slow flow of liquid through a porous medium, with applications to the physical sciences.

Epidemiological applications have been anticipated for disease systems where the host is a plant or volume of soil, and hence is fixed in space.

However, no natural examples have been reported.

The central question of interest in percolation theory, the possibility of an infinite connected cluster, corresponds in infectious disease to a positive probability of an epidemic.

Archived records of plague (infection with Yersinia pestis) in populations of great gerbils (Rhombomys opimus) in Kazakhstan have been used to show that epizootics only occur when more than about 0.33 of the burrow systems built by the host are occupied by family groups.

The underlying mechanism for this abundance threshold is unknown.

Here we present evidence that it is a percolation threshold, which arises from the difference in scale between the movements that transport infectious fleas between family groups and the vast size of contiguous landscapes colonized by gerbils.

Conventional theory predicts that abundance thresholds for the spread of infectious disease arise when transmission between hosts is density dependent such that the basic reproduction number (R0) increases with abundance, attaining 1 at the threshold.

Percolation thresholds, however, are separate, spatially explicit thresholds that indicate long-range connectivity in a system and do not coincide with R0 = 1.

Abundance thresholds are the theoretical basis for attempts to manage infectious disease by reducing the abundance of susceptibles, including vaccination and the culling of wildlife.

This first natural example of a percolation threshold in a disease system invites a re-appraisal of other invasion thresholds, such as those for epidemic viral infections in African lions (Panthera leo), and of other disease systems such as bovine tuberculosis (caused by Mycobacterium bovis) in badgers (Meles meles).
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http://dx.doi.org/10.1038/nature07053
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[Sally Furniss]

Satellite images reveal key to predicting outbreaks of plague

Satellite images reveal key to predicting outbreaks of plague

01.08.2008

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.

http://www.innovations-report.de/html/berichte/umwelt_naturschutz/bericht-115350.html

 

[gsgs]

Gerbils rather than rats are responsible for European black death
http://www.washingtonpost.com/news/...-in-spread-of-black-death-gerbils-implicated/