Stenseth’s research, published in the latest edition of the Proceedings of the National Academy of Sciences, correlates plague outbreaks in Europe with weather conditions in Europe and Asia, using tree rings and other indicators.
What they found is that while outbreaks showed no correlation with weather conditions in Europe, they were closely correlated with conditions in Central Asia.
“We show that wherever there were good conditions for gerbils and fleas in central Asia, some years later the bacteria shows up in harbour cities in Europe and then spreads across the continent,” he told the BBC.
According to him, great gerbils and ground squirrels living in the steppes of north-West China and Kazakhstan are likely to have been the original sources of the pandemics, with fleas from the animals hopping off onto passing camels and traders ploughing the Silk Route.
Europe’s plagues have previously been attributed to a one-off arrival of infected rats from Asia, after which it persisted in Europe’s rat population, from which it occasionally flared up into epidemics in humans.
Stenseth believes instead that each pandemic originated in Asia in the same way as the Black Death.
“We find plague to have been repeatedly re-imported along the same route as the Black Death was imported, triggered by large-scale climate events in Central Asia,” Boris Schmid, Stenseth co-author, also from the University of Oslo, told The Guardian.
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The team now intends to carry out DNA analysis on samples of the plague bacteria (Yersinia pestis) take from the bodies of European plague victims killed during different outbreaks.
“If the plague that arrived with the Black Death was the ancestor of all the strains of plague in Europe in the centuries afterwards, you will find a different pattern of relatedness than when plague in Europe was repeatedly re-imported from plague reservoirs in Asia,” Schmid told the Guardian.