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
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.