Floodwaters rise in Clarksville, Missouri June 19, 2008. (Eric Thayer/Reuters)

CHICAGO (Reuters) -
Extreme floods and droughts brought on
by climate change can turn normally harmless infections into
significant threats, international researchers said on Tuesday.

They said weather extremes can create conditions in which
several fairly harmless diseases converge at once, creating a
“one-two punch” that can devastate populations of wildlife or
livestock.

“When you have these extreme swings it will tend to
synchronize these kinds of co-infections, which are likely to
be more common with climate change,” said Craig Packer of the
University of Minnesota, whose study appears in the Public
Library of Science journal PLoS ONE.

Many researchers have predicted that climate changes
brought on by heat-trapping carbon dioxide emissions could
alter traditional relationships between pathogens and their
hosts, making normally benign diseases more deadly.

Packer said his team has found a real-world example.

The researchers studied two unusually lethal outbreaks of
canine distemper virus or CDV that occurred in 1994 and 2001 in
a population of lions in Tanzania's Serengeti National Park and
Ngorongoro Crater.

Most canine distemper outbreaks in the past have caused
little or no harm to lions in the region, Packer said.

“It turns out that the lethal outbreaks had immediately
followed severe droughts within the country, which had a very
interesting effect on the ecosystem,” Packer said in a
telephone interview.

He said the droughts weakened local populations of Cape
Buffalo, which were then infested with ticks. “Those buffalo
had been weakened to the extent that they could no longer fight
off infections from the ticks,” Packer said.

So, when the lions feasted on this rich source of meat,
they became infected with tick-borne blood parasites.

The lions, meanwhile, had been fighting off an outbreak of
the canine distemper virus, which had suppressed their immune
systems. “That one-two punch is what killed them,” Packer said.

“A distemper infection is like having a short, sharp bout
of AIDS,” Packer explained.

“It's an immunosuppressive virus. If you are being
challenged, it now allows those other diseases to completely
take over. That's what happened.”

Packer said the study suggests extreme climate conditions
can change traditional relationships between pathogens and
their hosts.

“That could be a concern not just for wild animals like
lions, but also for livestock, people, you name it,” he said.

The study is available on the Internet at
http://www.plosone.org/doi/pone.0002545.

(Editing by Maggie Fox and Todd Eastham)