Under the Weather: How La Niña May Influence the Outbreak of Flu Pandemics

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Manoj Shah

A new study indicates that La Niña weather effects could change the flight patterns of migratory birds — possibly triggering flu pandemics

There are two things you can be certain about when it comes to flu pandemics: they’re inevitable and you never know when one will strike. That unpredictability makes it difficult to prepare for a pandemic — some 40 years passed between the pandemic of 1968 and 2009, yet only a little more than a decade passed between the pandemic of 1957 and 1968. Scientists know that wild migratory birds are the primary reservoir for human flu, and that a pandemic ignites when a new flu virus makes its way into the human population. But they can’t predict when it will happen.

A new study by researchers at Columbia and Harvard, however, may shed some light on that on the pandemic mystery. Writing in the Proceedings of the National Academy of Sciences (PNAS), Jeffrey Shaman of Columbia’s Mailman School of Public Health and Marc Lipsitch of the Harvard School of Public Health theorize that the recurring La Niña weather pattern — which happens when sea surface temperatures in the central Pacific Ocean become colder than normal — may help trigger pandemics by altering the flight paths of those migratory birds, which may lead to the rise of new flu viruses.

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From the PNAS paper:

Our findings indicate a possible association between the emergence of pandemic influenza and the phase of ENSO [La Niña]. Whether this ENSO–pandemic influenza association is causal or merely coincidental is unclear; however, the effect of ENSO on migratory bird health and behavior could be one means by which the large-scale environment alters the likelihood of influenza virus reassortment events and cross-over to human hosts. It has been suggested that the risk of pandemic influenza emergence may vary in time because of changes in the population genetics and ecology of influenza in reservoir species (26). Our findings here lead us to hypothesize that the likelihood of such emergence events is greater during La Niña events.

Shaman and Lipsitch looked at the last four pandemics — 1917/18, 1957, 1968 and 2009 — then retraced the weather conditions in the months before the flu outbreaks. They found that all four pandemics were preceded by colder than normal sea-surface temperatures, which signifies the La Niña effect. Previous studies have also shown that La Niña can alter the migration patterns, stopover points and interspecies mixing of migratory birds, all of which could favor the kind of gene swapping that can lead to new flu viruses — and those microbes, if they hit the genetic lottery, can then trigger pandemics. As Shaman put it:

We know that  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.

The La Niña theory is far from bulletproof, however — we’ve had many more La Niña events than we have had flu pandemics, so the data in the PNAS study may simply be a coincidence. Other studies have also linked pandemics to El Niño events — the flip slide of the La Niña, when ocean-surface temperatures are warmer than normal. But the PNAS study offers a new way of looking at the causes of flu pandemics — and serves as a reminder that the weather might be responsible for even more than we think.

P.S. My apologies for the lack of posts the past few days. I was on a crashed deadline for a magazine story. Back to normal now.

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