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Global warming and the return of plague


Return of the Plague

Tuesday, Feb. 12, 2008 By LAURA BLUE


Like no other disease, plague evokes terror. One of the most lethal illnesses in human history, it killed


probably a third of Europe's population in the  14th century. It may also have been one of the first 


agents of biological  warfare: It's said that in the 1340s, invading Mongols catapulted their plague
dead over the city wall into Kaffa in the Crimea.

Yet the plague is not just a disease of the distant past. While cases tapered off in the mid-20th century,


the World Health Organization (WHO) now  classifies plague as "re-emerging." No one is predicting 


another pandemic like the  Black Death that devastated Europe. The WHO now records at most only a 


few thousand cases worldwide per year; and, if detected early, the disease can be  treated effectively with 


antibiotics. But since the early 1990s, plague has returned to places — including India, Zambia, 


Mozambique, Algeria and parts of  China — that had not seen it in many years or even decades. 


Its global footprint has also shifted, according to a paper published last month in the journal PLoS  


Medicine.

In the 1970s, most plague cases were in Asia; today, more than 90% are in Africa. The conundrum


for epidemiologists: Why is human plague  reappearing now, even though nearby animal populations 


have likely harbored the culprit  Yersinia pestis bacteria all along?

Plague lives in many rodent species, and is most often transferred to humans by the animals' fleas.


Scientists know which regions of the world harbor  infected animals, but they are only just beginning 


to understand the dynamics of  plague infection. Its spread depends not just on Yersinia pestis but also 


on interactions among rodents and, crucially, on contact between humans  and wildlife. Madagascar is 


a good example. For decades, plague was  restricted to the highlands, according to a 2004 paper by 


researchers in Madagascar,  Senegal and France. But it showed up on the coast in 1991, when the Asian 


shrew  somehow picked up infected fleas. The plague's earlier comeback in the inland  capital, 


Antananarivo, arose as city sprawl and shoddy housing put residents in  closer contact with black rats. 


In 1998, inland villages reported cases, too,  perhaps caused by rats displaced through deforestation.

Even in the antibiotic age, then, containing plague requires monitoring more than human cases, says


Nils Christian Stenseth, head of the Center for Ecological and Evolutionary Synthesis in Oslo, and 


lead author of the  PLoS Medicine paper. Working with nearly 50 years of animal, human and 
bacteriological statistics from the former Soviet Union, his team found that human plague in Kazakhstan


occurs only when the local gerbil population  reaches a certain threshold in winter. Warmer winters 


mean more gerbils. That,  says Stenseth, suggests plague's "re-emergence might have a climate
component."





If so, global warming may exacerbate the threat — an unsettling thought, given the viciousness of


the disease. "The plague bacillus is probably the  most pathogenic infectious agent on the planet right 


now, and we still don't  know why it's so virulent," says Elisabeth Carniel, a plague expert at the
Institut Pasteur in Paris. It may no longer make history, but plague hasn't lost its terrifying power.