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Does La Nina Fuel Flu Pandemics?

It often seems like weather forecasters blame everything unusual on El Niño or La Niña, be it a drought, a heat wave, or a snowless winter. But this natural climate cycle in the equatorial Pacific Ocean may actually have much greater — and far deadlier — impacts. A new study published in the journal Proceedings of the National Academy of Sciences explores the possibility that La Niña helps make conditions more favorable for deadly global flu pandemics. The study finds that the past four flu pandemics, including the Spanish Flu of 1918, the Asian Flu of 1957, the Hong Kong Flu of 1958, and the Swine Flu of 2009, were all preceded by La Niña conditions. The Spanish Flu alone killed tens of millions.

Not to worry you, but we are currently in the midst of our second year of La Niña conditions, with tropical Pacific sea surface temperatures running cooler than average. La Niña conditions are expected to continue through the winter before weakening during the spring or summer, according to recent forecasts.

Monthly sea surface temperature departures from average during December, 2011. The cool waters (blue) in the tropical Pacific indicate La Nina conditions. Credit: IRI.

But the study doesn't say that La Niña causes flu pandemics. It does, however, provide insight into a relationship that could help scientists improve their public health monitoring and prediction efforts.

The study, by Jeffrey Shaman of Columbia University's Mailman School of Public Health and Marc Lipsitch of Harvard's School of Public Health, proposes that La Niña-related weather pattern shifts in turn alter bird migrations, thereby changing the interactions between bird species and between bird species and domestic animals. Birds are a main reservoir for human influenza.

According to a Columbia University press release, migration changes can influence contact patterns between birds and domestic animals, including pigs. Gene-swapping between bird and pig flu viruses was a key factor in the 2009 swine flu epidemic.

As Bryan Walsh of Time Magazine writes, "Previous studies have also shown that La Niña can alter the migration patterns, stopover points and interspecies mixing of migratory birds, all of which could favor the kind of gene swapping that can lead to new flu viruses — and those microbes, if they hit the genetic lottery, can then trigger pandemics..."

It should be noted that the study may also have just stumbled upon a coincidence, and there may not be a real relationship between La Niña and flu pandemics at all. Still, it definitely offers something to think about during this flu season. 

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