Baku, January 18 (AZERTAC). A new study examining weather patterns around the worldwide pandemics of influenza, which caused widespread death and illness in 1918, 1957, 1968 and 2009, finds that each of them was preceded by La Nina conditions in the equatorial Pacific. The study findings are published online Monday in the Proceedings of the National Academy of Science.

The study`s authors — Jeffrey Shaman of Columbia University`s Mailman School of Public Health and Marc Lipsitch of the Harvard School of Public Health — note that the La Nina pattern is known to alter the migratory patterns of birds, which are thought to be a primary reservoir of human influenza. The scientists theorize 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 in the fall and winter 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 of the El Nino- Southern Oscillation.

The authors cite other research showing that the La Nina pattern alters the migration, stopover time, fitness and interspecies mixing of migratory birds. These conditions could favor the kind of gene swapping or genetic reassortment that creates novel and therefore potentially more variations of the influenza virus.

“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,” says Shaman.

Changes in migration not only alter the pattern of contact among bird species, they could also change the ways that 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.