Gerbils and squirrels implicated in the spread of Black Death

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Most British school children are familiar with the story of the Black Death and subsequent plague epidemics that decimated the population of Europe in the Middle Ages. It now seems that the villain of the plot, the black rat, may have been wrongly accused of acting as a European reservoir for the plague bacteria and initiating sporadic disease outbreaks for 400 years.

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One of the places where the plague bacterium persists is in the Central Asian Deserts, where the great gerbil, Rhombomys opimus, is the primary host. Gerbils are social animals, living in burrows infested with several parasitic arthropods, including the fleas that act as vectors for the bacterium. Gerbil populations fluctuate and the sylvatic plague bacterium will spread in the population when it exceeds an upper threshold level. Work published in 2007 by Nils Christian Stenseth and colleagues, who were studying the gerbil populations in Kazakhstan, suggested a co-dependency exists between climatic conditions, vegetation cover, synchronised increase in gerbil densities and plague transmission. Warm, moist conditions promote vegetation growth in these arid areas, giving rise to large gerbil populations and large plague epizootics. As gerbil populations crash, flea populations rise, the plague bacteria spread and infected fleas spill-over onto domestic animals and humans.

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Using the same criteria to select locations, they were able to detect 61 potential maritime introductions that occurred between 1346 and 1856 in 17 harbours around the Mediterranean and the Black Sea coasts. These sea routes linked Europe with the overland routes to Asia. Historical reports were used to identify 16 potential introductions from Asia. Periods of climatic fluctuations were identified from tree ring analysis at sites on these ancient trade routes to Central Asia. Here populations of long tailed ground squirrels and Altai marmots are the rodent reservoir of plague.

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