[Vietnam Public Health] 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."

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.

If so, global warming may exacerbate the threat.

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Posted By Kingdragon to Vietnam Public Health on 2/14/2008 02:48:00 PM