La Niña weather pattern linked to influenza pandemics

Shaman J. Proc Natl Acad Sci U S A. 2012;doi:10.1073/pnas.1107485109.

Weather patterns exhibited by La Niña in the equatorial Pacific drastically alter the patterns of migratory birds, which are considered to be primary carriers of human influenza, according to study results published in Proceedings of the National Academy of Sciences.

Examining the four most recent human influenza pandemics (1918, 1957, 1968 and 2009), researchers found that each pandemic occurred in spring or early summer and was preceded by below-normal sea surface temperatures — indicative of the La Niña phase of the El Niño–Southern Oscillation weather pattern.

Prior studies have reported that the El Niño–Southern Oscillation drastically affects weather conditions, including temperatures, precipitation, wind speed and direction, which can in turn influence the behavior of migratory birds by altering flight and stopover patterns. Researchers theorized that this alteration in typical migratory patterns is significant in blending divergent influenza subtypes together and generating novel pandemic strains.

“We know that pandemics arise from dramatic changes in the influenza genome. Our hypothesis is that La Niña sets the stage for these changes by reshuffling the mixing patterns of migratory birds, which are a major reservoir for influenza,” study researcher Jeffrey Shaman, PhD, assistant professor of environmental health sciences at Columbia University’s Mailman School of Public Health, said in a press release.

Although findings from the study indicate a link between the emergence of pandemic influenza and the La Niña phase of the El Niño–Southern Oscillation, it has not been determined whether association is causal or merely coincidental. Researchers have demonstrated that the effect of El Niño–Southern Oscillation on migratory bird health and behavior could be one avenue by which influenza viruses undergo reassortment events and crossover to human hosts.

“The most plausible biological explanation for the La Niña–pandemic influenza association identified here involves [El Niño–Southern Oscillation] mediated changes to bird migration,” the researchers said. “The 1957 and 1968 pandemics are thought to have originated from the reassortment of avian influenza viruses; however, the 2009 pandemic is thought to have arisen from the reassortment of two swine influenza viruses.”

Disclosure: Study researcher Marc Lipsitch, PhD, reports consulting and honorarium support from the Avian/Pandemic Flu Registry, Air Worldwide, Pfizer and Novartis.

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