A Climate-Pandemic Connection?

by Bill Chameides | January 24th, 2012
posted by Erica Rowell (Editor)

Permalink | 1 comment

Regional rainfall changes caused by El Niños shift bird migration, allowing for new strains of the flu to develop. Climate change may alter the oscillations between El Niños and La Niñas, changing the frequency of new flu pandemics. (Dave Gatley/FEMA)

Could changing climate affect the likelihood of contracting a new deadly strain of the flu?

Yes, according to two recent papers, one from last year in the journal Theoretical and Applied Climatology by Adriano Mazzarella of the University of Naples Federico II and colleagues and the other from last week in the Proceedings of the National Academy of Sciences by Jeffery Shaman of the Mailman School of Public Health and Marc Lipsitch of the Harvard School of Public Health. How? Before explaining, let’s review a couple of things.

Seasonal Flu vs. an Influenza Pandemic

Seasonal flu we all know about, and we fervently hope to escape each winter without catching it. Often, seasonal flu can lead to an epidemic in a given area, meaning that more people than normal are laid low. Because we have some amount of immunity to the viruses that cause it, the impact of seasonal flu is limited.

An influenza pandemic is quite distinct from seasonal flu or even a seasonal flu epidemic. Generally, the term pandemic is used by health officials to denote an epidemic that has spread over a large region or even globally. But influenza pandemic has a special meaning: it occurs “when a new influenza A virus emerges for which there is little or no immunity in the human population.” And because of that lack of immunity, influenza pandemics are far more dangerous than seasonal flu. (More on the distinctions [pdf].)

In recent history four influenza pandemics have been documented: 1918–1919 (the Great Pandemic), 1957–1958 (the Asian Flu), 1968–1969 (the Hong Kong Flu), and 2009–2010 (the H1N1 flu). Of these, the 1918–1919 “Spanish Flu” pandemic was by far the worst, leading to an estimated 50 million deaths worldwide.

Pathologists like to use a combination of letters and numbers to identify distinct strains of the influenza virus. The H1N1 strain responsible for 2009’s pandemic was also behind the Spanish Flu; H2N2 was the culprit in the Asian Flu and H3N2 in the Hong Kong Flu. Lurking on the horizon is a potential pandemic from an H5N1 strain that has been highly lethal and contagious in avian populations but has shown limited ability to infect humans. On rare occasions it has infected people, though, and when it has, it has been lethal. In the last few weeks two deaths from H5N1 have been reported in China (see here and here). The fear is that H5N1 will eventually morph into a strain that can more effectively infect humans with devastating effects for the species (and then there’s biomedical manipulation).

Our understanding of how new strains of influenza are generated and begin to infect human populations is not fully developed, but a general picture has emerged. Birds, migratory birds in particular, play a central role, by either passing a flu strain directly to humans (as in the case of H1N1) or indirectly via an intermediate host, giving the virus an additional opportunity to evolve. (More info here and here.)

On antigenic shift. (Link Studio)

The El Niño-Southern Oscillation Connection to the Flu

The El Niño-Southern Oscillation (ENSO) refers to the state of the tropical Pacific Ocean as it sloshes back and forth, like water in a huge bathtub, west and east between Asia and the Americas. This movement affects temperatures and weather patterns worldwide. The oscillation’s El Niño state occurs when the basin’s eastern surface waters are warm and tends to correspond with warm global temperatures, bringing, for example, unusually wet conditions to the southern United States. Hong Kong, where the pandemic of 1968 originated, also tends toward warmer, wetter weather during El Niños in winter. La Niñas, alternately, cool the eastern equatorial Pacific’s surface waters, bringing dry, drought-like conditions to the southern United States while tending to send cooler temps and more hurricanes to Hong Kong in the fall.

About now you might be wondering how could a pandemic caused by a new strain of flu in birds be triggered by an oscillation in the sea surface temperatures in the Pacific? It’s a fair question, but one with a reasonably cogent reply relating to the strange and sometimes counterintuitive ways in which environmental changes can ripple though the system. Here’s the hypothesis:

The El Niño-La Niña oscillations cause significant changes in regional rainfall rates and wind patterns, which in turn affect the migration pattern of birds. And these shifts lead to different groups of of bird species coming into contact with each other in a given region, allowing for new strains of influenza to develop that eventually jump to humans.

Well, that’s the hypothesis; is there any evidence to support it? Both Mazzarella et al and Shaman and Lipsitch used ENSO variations and historical data on the occurrence of pandemics to answer that question. Interestingly, they both came up with positive conclusions: the oscillations did in fact correlate with the onset of influenza pandemics. However, their results were 180 degrees apart: Mazzarella et al concluded that pandemics were triggered by El Niños while Shaman and Lipsitch concluded that La Niñas were the culprit.

Shaman and Lipsitch speculate that the inconsistency can be traced to different methodologies. Focusing on eight pandemics that occurred between 1580 and 1969, Mazzarella et al used data compiled by Quinn et al ($ub req’ed) reconstructed from ship logs etc. to diagnose the presence of El Niños only. Shaman and Lipsitch analyzed pandemics in the 20th and 21st centuries (for which we have much better data) and used actual ocean temperature data to look at the occurrence of both ENSO phases. In all, three pandemics were covered by both research gr

Shaman and Lipsitch argue that because they used modern data, their results are more likely to be correct. I find that rather convincing, but will await further study by other investigators not just to determine if it’s El Ninos or La Ninas that are the culprit but also whether either is a trigger for a pandemic at all.

Regardless, the two studies raise a possibility worth pondering: that climate change may alter the strength and frequency of the oscillations between El Niños and La Niñas and this in turn will significantly change the frequency of new influenza pandemics.

We clearly don’t know how that will all play out. If the frequency increases, the results could be devastating; if it goes the other way, it could be an instance where we catch a lucky break from global warming.

Without realizing it, we’ve already placed a considerable bet on the outcome of this question with the fossil fuel binge of the past two centuries. If, as current trends indicate, we choose to continue on our binge, we will essentially be doubling down on that bet. Anyone feeling lucky?

filed under: climate change, El Nino-Southern Oscillation, ENSO, faculty, global warming, health, La Nina
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1 Comment

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  1. Jane Henry
    Mar 13, 2012

    I know that the weather this winter is having an effect on the life cycle of mosquitos. Could this warm weather have an effect on next fall’s flu season? Could there emerge other viruses that have been long dormant?

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