La Nina 'linked' to flu pandemics

Richard Black BBC News 16 Jan 12;

La Nina events may make flu pandemics more likely, research suggests.

US-based scientists found that the last four pandemics all occurred after La Nina events, which bring cool waters to the surface of the eastern Pacific.

In Proceedings of the National Academy of Sciences (PNAS), they say that flu-carrying birds may change migratory patterns during La Nina conditions.

However, many other La Nina events have not seen novel flu strains spread around the world, they caution.

So while the climatic phenomenon may make a pandemic more likely, they say, it is not sufficient on its own - and may not be necessary either.

La Nina is the cold cousin of El Nino - the two collectively making up the El Nino Southern Oscillation (ENSO).

"Certainly ENSO affects weather and precipitation and humidity around the world," said Jeffrey Shaman from Columbia University in New York.

"But the effects are very varied around the world - there's no coherent picture."

Nevertheless, the last four pandemics - the Spanish Flu that began in 1918, the Asian Flu of 1957, the Hong Kong Flu of 1958 and the swine flu of 2009 - were all preceded by periods of La Nina conditions.

What pandemics have in common is that they all feature novel strains of the virus to which people have not developed immunity.

Typically these are created when two existing strains infecting an animal such as a bird or a pig exchange genetic material.

The link to La Nina events is not clear. But recent research has shown that some wild birds' patterns of flights and stopovers during migrations, or moulting times, differ between El Nino and La Nina years.

"Our best guess is this brings together birds [in La Nina conditions] that don't otherwise mix, and that allows the genetic reassortment to take place," Professor Shaman told BBC News.

Yet the fact that many other La Nina periods have not been followed by a pandemic indicate that other factors must also be involved.

If the swine flu pandemic of 2009-10 was part of this pattern, the crossing of viral strains must have had something to do with birds as well as pigs.

As wild migratory birds will sometimes visit farms and as domestic flocks of ducks or chickens often live alongside pigs, especially in developing countries, this is quite feasible.

Professor Shaman cautions that the link is far from being firm enough that it could be used as a tool to forecast pandemics.

But the monitoring of birds, pigs, people and the genetics of the influenza virus have all been stepped up in response to recent outbreaks of both swine flu and bird flu.

And this, he believes, should in time show whether the theory is correct.

"Now we can look at viral gene flow in a number of birds, pigs and people - and we might be able to get something more statistically robust, to get a better sense of the mechanisms."

Flu Pandemics Linked to Ocean's Cooling Cycle
Wynne Parry Yahoo News 19 Jan 12;

Flu pandemics have been linked to fluctuations in climate, and new research connects the world's four most recent pandemics to the cyclical cooling of the Pacific Ocean near the equator.

The connection? Changes in ocean temperature affect migrating birds, which are major contributors to the spread and mixing of flu viruses.

An earlier study had linked flu pandemics to ocean warming, rather than cooling, but public health researchers Jeffrey Shaman of Columbia University and Marc Lipsitch of Harvard University dispute these findings, saying this analysis relies on flawed data, such as records of older pandemics and climate fluctuations, which are less precise and reliable.

The researchers found that the four flu outbreaks that swept the world in the past 100 years — in 1918, 1957, 1968 and 2009 (in this last instance, the H1N1 "swine flu") — followed a fall or winter when sea-surface temperatures sank abnormally low. This cooling is associated with La Niña, a phase in a larger climate pattern. La Niña is the cool counterpart to El Niño, which is marked by unusually warm temperatures in the equatorial Pacific; both alter weather patterns around the globe.

There are three types of influenza viruses. One of them, type A, is naturally carried by wild aquatic birds. Type A viruses, which include swine flu, can infect not only people but other mammals, such as pigs and dogs, as well as other birds, according to the United States Centers for Disease Control and Prevention.

The long flights and many stopovers made by migratory birds turn these viruses into globe trotters and allow them to mix with each other. This process, called re-assortment, occurs when an animal or person carries multiple strains of the virus that swap genetic information. It can lead to the emergence of new viruses that are potentially capable of causing pandemics. "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 Shaman in a press release issued by Columbia University's Mailman School of Public Health.

Previous research showed the El Niño/La Niña climate pattern affects the health and behavior of migratory birds, including their population density and flight and stopover patterns, the researchers noted in their study.

The researchers note that the 2009 pandemic is believed to have arisen from swine flu viruses. It is possible birds carried one or more of the ancestors of the 2009 pandemic virus, they write Monday (Jan. 16) in the journal the Proceedings of the National Academy of Sciences.

Their results contradict earlier research that showed a link between flu pandemics and moderate to strong El Niño events.

Flu isn't the only misery that has been linked to fluctuations in this climate cycle. Past research has connected El Niño to civil conflict, and La Niña to drought that gripped Texas, Oklahoma and parts of New Mexico.