Climate-change-induced extreme floods and droughts can change infections that are usually harmless into significant threats, international researchers in Chicago said 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.

An example is 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.

(Agencies via Xinhua June 25, 2008)