La Niña could set the stage for flu pandemics
In 1918, the Spanish flu spread around the world, claiming between 50 million and 100 million lives — more than 3 percent of the world’s population. The previous fall and winter, La Niña had brought cooler-than-normal sea-surface temperatures to the central and eastern equatorial Pacific. More recently in 2009, swine flu swept across the planet. Again, the widespread outbreak was preceded by La Niña conditions. This link might be more than coincidental, according to new research, and could lead to improved predictions of future pandemics.
In a new study in Proceedings of the National Academy of Sciences, Jeffrey Shaman of Columbia University and Marc Lipsitch of Harvard University note that La Niña can alter weather patterns and shift the migration patterns of birds. Such a rerouting of avian flu carriers could set the stage for the rise of novel — and deadly — strains of the virus, they say.
To examine the connection between La Niña and flu pandemics, Shaman and Lipsitch looked at the four most recent, well-dated human influenza pandemics — 1918, 1957, 1968 and 2009 — and then looked at El Niño Southern Oscillation (ENSO) conditions during the fall and winter immediately preceding the emergence of the outbreaks. The researchers found that, indeed, all four pandemics were preceded by La Niña.
The study is limited, however, by the paucity of data, Shaman says. With just four well-dated flu pandemics, the scientists say, it’s unclear whether the relationship is causal, and La Niña affects flu, or coincidental, and the association arose by chance.
If it is causal and La Niña affects the development and onset of pandemic flus, one scenario according to the team is as follows: La Niña brings about cooler sea-surface temperatures and shifts wind and precipitation patterns across the expanse of the equatorial Pacific. Those local changes impact global weather conditions, which in turn affect the resources and habitability along the flyways of migratory birds. Previous research, for example, has found impacts on migratory birds’ health and fitness, molting times, stopover patterns and contact with other bird species.
Like a burned-out traveler on an international flight, stressed birds in the presence of new company are more susceptible to picking up viruses. Conditions for a pandemic arise then, when different strains of influenza simultaneously infect a single host in a process called reassortment.
“Like all viruses, influenza hijacks the cellular machinery of its host and uses it to make copies of itself,” Shaman says. Introduce two or more strains and the host’s cells can make a hybrid strain that’s “radically different, and which the world’s population has no prior exposure to and little immunity against. This new hybrid strain can spread very efficiently around the planet,” he says. “This is a pandemic.”
Shaman notes, however, that “the jury is still out” and more work needs to be done, such as examining the mixing between bird species, and rates of reassortment in response to ENSO. “But the hypothesis we use to explain the relationship makes sense and is testable,” he says. “If evidence from future studies supports this hypothesis — if the relationship is shown to be robust — then we have a framework for developing advanced predictions of pandemic influenza risk. This would enable governments and public health officials to time the allocation of influenza resources better.”
For now, only time will tell what, if anything, follows the 2011–2012 La Niña. “La Niña by itself is not sufficient, it’s just a state that may increase the likelihood of reassortment,” Shaman says. “Many downstream effects, such as changes in weather, in bird behavior, and convergence of divergent flu subtypes, would be needed to facilitate the reassortment that generates the novel pandemic flu strain.”