OSLO Rare outbreaks of plague in the United States seem to match climate shifts over the Pacific Ocean in a hint that global warming may make the region too hot and dry for the disease, scientists said on Wednesday.
Feared as the "Black Death" of the 14th century that killed an estimated 50 million people, plague is still a threat mainly in Africa. The western United States has had 430 cases since 1950, or about seven per year.
The scientists, based in Norway, the United States and Sweden, said the number of U.S. infections seemed to vary with a natural shift between warm and cool ocean conditions known as the Pacific Decadal Oscillation (PDO) that can last 20-30 years.
Until now, researchers had been unable to explain swings in the number of cases -- ranging from 40 in 1983 at a warm part of the PDO to almost none in the 1950s, a cool phase. The plague bacteria is spread to humans by fleas living on rats.
"The cases aren't isolated. You can look at this phenomenon on a larger scale," said Tamara Ben Ari, lead author of the study at the University of Oslo.
The authors, writing in the Royal Society journal Biology Letters, said that warm, wet conditions seemed to favor both rats and fleas. Fewer rodents die off in mild winters and food is more abundant in semi-arid areas when there is more rain.
But future climate change, stoked by human emissions of greenhouse gases, is likely to make the western United States drier, reducing the amount of food for rats. It is also projected to mean more heatwaves that can be deadly for fleas.
"Periods of high plague activity are likely to decrease in the western United States over the coming decades, especially in the active four corners region -- New Mexico, Colorado, Arizona and Utah," the study said.
It also said that plague cases might shift further north and to higher altitudes. The scientists added that there would still be big variations, linked to unpredictable droughts and rains.
By contrast, experts say that global warming could make plague thrive in many other parts of the world, such as central Asia, with a projected shift to moister conditions.
The study focused on states west of a line running through North and South Dakota, Nebraska, Kansas, Oklahoma and Texas. Plague is not normally found to the east.
The plague is little understood -- the disease can vanish for decades in an area and then reappear.
The World Health Organization says that nine countries reported 2,118 cases in 2003 and 182 deaths, with almost 99 percent of both cases and fatalities in Africa.
(Editing by Mark Trevelyan)