Global warming increases prevalence of plague
30 August 2006, EC Research
New research from an international team has shown that temperature increases due to global warming increase the populations of plague bacteria by as much as 50 per cent.
The plague ravaged Europe in the Middle Ages, killing up to one-third of the population, and recurring regularly until modern forms of sanitation virtually eliminated the disease. In parts of eastern Europe, the far East, Asia, South America and former Soviet Union, outbreaks of the disease, the Yersina pestis bacterium, are still common; spread via the fleas that live on rodents.

The researchers, based in Oslo, Norway; Liverpool, UK; Antwerp, Belgium; Kongens Lyngby, Denmark; Almaty, Kazakhstan; Iowa City, US; and Birmensdorf, Switzerland examined the prevalence of plague in the fleas that live on populations of great gerbils.

They found that small increases in temperature generate very large increases in plague. 'A temperature increase of one degree Celsius in spring may lead to a 50 per cent increase in the prevalence of the plague bacterium,' said Professor Nils C. Stenseth, lead researcher from Oslo University. 'Climate changes cannot lead to any new Black Death, but it is quite clear that a small increase in temperature may create more cases of bubonic plague than we have today.'

Prof Stenseth had previously written a paper on the bacterium, but had felt there was something missing from his analysis of why plague prevalence varies so much. The team used data from a national Kazakh programme, which has monitored the populations of gerbils since 1949, and compared these data to mean temperatures, estimated from tree growth.

To illustrate the variation, since 1949, cases of plague have collapsed - from more than 100 cases per year to just one or two. Colleagues in Iowa, US, crunched the numbers, and noticed a correlation between temperature and prevalence of bacteria. This correlation explained the variation in cases of plague. 'Samples from the annual rings of trees in Kazakhstan revealed that when the Black Death broke out there in the 14th century, the springs were warm and the summers were wet. Conditions were the same at the onset of the plague of the 1800's in the same region,' he said.

This variation had previously vexed Prof Stenseth. 'But we could have explained it, had we included climate as a cause of variation in the prevalence of this bacteria,' he said. 'The results of this work enabled us to write this article and conclude that climate changes have affected the prevalence of the bacterium which causes plague.'

'In the US, researchers have studied infectious diseases that are passed on among humans, indicating a similar connection between the prevalence of bacteria and climate changes, but this is the first time anyone has found a clear connection between the prevalence of the plague bacteria carried by gerbils and climate change,' he continued.

Prof Stenseth warned that some very successful diseases had their roots in central Asia, and that, 'It was precisely in this area that the genetic and climatic conditions which brought on the Black Death and the Asian flu, emerged', he said.

Now that global warming is slowly cranking up the mean temperature, 'Threats of outbreaks may thus be increasing where humans live in close contact with rodents and fleas (or other wildlife) harbouring endemic plague,' warns the paper.