A new study has found a link between the La Nina weather pattern and the worldwide pandemics of influenza in 1918, 1957, 1968 and 2009.

Each of those pandemics was preceded by La Nina conditions in the equatorial Pacific, found the study published online Monday in PNAS (Proceedings of the National Academy of Sciences).

The La Nina pattern alters the migratory patterns of birds, which may in turn promote the development of dangerous new strains of flu, said study co-author Jeffrey Shaman of Columbia University's Mailman School of Public Health.

"We know that pandemics arise from dramatic changes in the influenza genome," Shaman said. "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."

For the study, public health scientists at Columbia and Harvard University studied records of ocean temperatures in the equatorial Pacific in the fall and winter before the emergence of the four most recent flu pandemics.

They found all four pandemics were preceded by below-normal sea surface temperatures, which is consistent with the La Nina phase of the El Nino-Southern Oscillation. This weather pattern develops in the tropical Pacific Ocean approximately every two to seven years.

The study authors also noted research that showed the La Nina pattern alters the migration, stopover time, fitness and interspecies mixing of migratory birds.

Such conditions may favour the kind of gene swapping, or genetic reassortment, that creates new and potentially more variations of the influenza virus.

Altered migration patterns not only change the contact among bird species but they may also change the ways that birds come into contact with domestic animals such as pigs.

Gene-swapping between bird and pig influenza viruses was a factor in the 2009 swine flu pandemic, researchers noted.