Infectious disease experts are constantly on the lookout for signs of the next flu pandemic.
Now, two U.S. scientists – Jeffrey Shaman at Columbia University and Marc Lipsitch at Harvard University's school of public health – have identified a climatic pattern that could set the stage for the emergence of a new and deadly strain of influenza.
Each of the past four flu pandemics – in 1918, 1957, 1968 and 2009 – were preceded by La Nina conditions in the equatorial Pacific, according to their study published this week in the Proceedings of the National Academy of Sciences.
La Nina is part of the El Nino-Southern Oscillation, a climatic phenomenon involving major swings in ocean temperatures. La Nina is characterized by cooler that normal water in the eastern and central Pacific and often has a disruptive effect on weather, trade winds and wildlife.
La Nina is also known to alter the migratory patterns of birds, which are major carriers of the flu.
"It brings together bird species that otherwise wouldn't be mixing," Dr. Shaman said.
The researchers speculate the birds infect each other with diverse flu strains. And once a bird is carrying several strains, there is a risk influenza genes will recombine into a new flu bug that's radically different from previous types. "Pandemics arise from dramatic changes in the influenza genome," Dr. Shaman said.
He noted that La Nina occurs every two to seven years. Its arrival does not necessarily mean a pandemic is certain to erupt. "Many other things have to come into play," he emphasized. "But, when you have these conditions, there is an increased likelihood that you could get a human pandemic."
He added that this hypothesis would be easy to verify through field studies of migratory birds and the germs they carry. If their hunch is correct, public health officials could soon have a new factor to consider when tracking impending pandemics.