Washington: Influenza pandemics that caused death and illness worldwide in the past were preceded by La Nina conditions in the equatorial Pacific, a study has found.
The study examined weather patterns during the devastating flu pandemics in 1918, 1957, 1968 and 2009, Xinhua reported.
The La Nina pattern is known to alter the migratory patterns of birds, which are thought to be a primary reservoir of human influenza. Experts theorise that altered migration patterns promote the development of dangerous new strains of influenza.
To examine the relationship between weather patterns and influenza pandemics, the researchers studied records of ocean temperatures in the equatorial Pacific before the four most recent flu pandemics emerged.
They found that all four pandemics were preceded by below-normal sea surface temperatures -- consistent with the La Nina phase.
The findings were published online Monday in the Proceedings of the National Academy of Science.
The authors cite other research showing that the La Nina pattern alters the migration, stopover time, fitness and inter-species mixing of migratory birds.
Jeffrey Shaman of Columbia University`s Mailman School of Public Health and Marc Lipsitch of the Harvard School of Public Health co-authored the study.
"We know that pandemics arise from dramatic changes in the influenza genome. Our hypothesis is that La Nina sets the stage for these changes by reshuffling the mixing patterns of migratory birds, which are a major reservoir for influenza," Shaman said.
Changes in migration not only alter the pattern of contact among bird species, they could also change the way birds come into contact with domestic animals like pigs.
Gene-swapping between avian and pig influenza viruses was a factor in the 2009 swine flu pandemic.