US scientists propose that flu pandemics follow La Niña weather conditions in the equatorial Pacific. The conditions alter bird migration patterns and this promotes new strains of flu (migrating birds are known to be primary pools of human influenza virus). However, since La Niña occurs more frequently than global flu pandemics, the researchers suggest other factors must also come into it, and their findings are just one piece of the puzzle.
Jeffrey Shaman of the Mailman School of Public Health at Columbia University, and Marc Lipsitch of the Harvard School of Public Health, write about their findings in a paper due to be published in PNAS this week.
Shaman is assistant professor of Environmental Health Sciences at the Mailman School. He said in a statement that:
“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.”
La Niña is a particular ocean temperature pattern that occurs in the tropical Pacific Ocean every two and seven years approximately. During La Niña, the sea surface temperature is lower than normal. It is the opposite of El Niño, when the sea surface temperature becomes higher than normal.
For their study, Shaman and Lipsitch examined records of the ocean temperatures in the equatorial Pacific in the fall and winter before the four most recent global flu pandemics: the Spanish Flu of 1918, the Asian Flu of 1957, the Hong Kong Flu of 1968 and the Swine Flu of 2009.
They found they were all preceded by below-normal sea surface temperatures, consistent with a La Niña weather pattern.
With reference to other studies that show how La Niña changes the flying routes, stopover times, fitness and interspecies mixing of migrating birds, the authors suggest this could create ideal conditions for the gene swapping or genetic reassortment that leads to new flu strains.
But, not all episodes of La Niña have been followed by flu pandemics, which suggests other factors must also play a part.
Changes to migration patterns could also affect gene-swapping in other ways. For instance, migrating birds sometimes stop over at farms, where they come into contact with domesticated birds and animals, such as chickens and pigs. The 2009 swine flu virus showed signs that gene-swapping had taken place between avian and pig flu viruses.
Written by Catharine Paddock PhD