La Nina may increase likelihood of influenza pandemics

The La Nina phenomenon may increase the likelihood of influenza pandemics.

Researchers in the United States recently found that the last four major flu pandemics occurred after La Nina events, which bring colder water than normal to the surface off the Pacific coast of South America, according to the BBC.

The scientists, whose study appears in the Proceedings of the National Academy of Sciences, concluded that birds carrying influenza alter their migratory patterns. They caution, however, that not all La Nina events have promoted the spread of novel influenza strains.

While the climatic phenomenon, known collectively as the El Nino Southern Oscillation, may make a pandemic more likely, it is not a guarantee that one will occur.

"Certainly ENSO affects weather and precipitation and humidity around the world," Jeffrey Shaman from Columbia University in New York said, the BBC reports. "But the effects are very varied around the world - there's no coherent picture."

Nonetheless, the last four major influenza pandemics - the Spanish Flu of 1918, the Asian Flu of 1957, the Hong Kong Flu of 1958 and the swine flu of 2009 - all occurred soon after periods of La Nina conditions.

"Our best guess is this brings together birds [in La Nina conditions] that don't otherwise mix, and that allows the genetic reassortment to take place," Shaman said, according to the BBC.