Flu pandemics may be linked to La Niña weather conditions that alter bird migration patterns and can promote dangerous new strains of the human influenza virus, according to American scientists in a statement on Monday. 

Researchers from Columbia University and Harvard University examined past records of ocean temperatures in the fall and winter before the last four flu pandemics, the 1918 Spanish Flu, the 1957 Asian Flu, the 1958 Hong Kong Flu and the 2009 swine flu were all preceded by periods of La Nina conditions.

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, said co-author Jeffrey Shaman, and assistant professor of Environmental Health Sciences at Columbia University.

However, the scientists noted that while the climatic phenomenon correlated with pandemics, the evidence is not sufficient to say that the weather patterns were the cause of novel strains of the flu virus to which people have not developed immunity. 

La Niña is the phase of the El Niño-Southern Oscillation that occurs in the tropical Pacific Ocean every two to seven years and during this face the sea surface temperature is lower than normal which is opposite of the El Niño phase then the sea surface temperature becomes higher than normal.

Researchers said that La Niña alters flying routes, stopover times, fitness and interspecies mixing of migrating birds which the study authors predict could create ideal conditions for gene swapping and can lead to new flu strains.

However researchers also noted that not all periods of La Niña have been followed by pandemics, which suggested that other factors play a part in spawning new influenza strains.

Researchers said that changes in migration also alter the way birds come into contact with animals other than birds, like domestic animals.  The gene-swapping between avian and pig influenza viruses was a factor in the 2009 swine flu pandemic. 

The study findings are currently published online in the Proceedings of the National Academy of Sciences.