Certain disease outbreaks, including some of the worst pandemics of the 20th century, are linked to weather patterns in the Pacific Ocean, according to new research. Scientists said tracking these climate changes can help officials anticipate and plan for surges in illnesses.

The El Niño-Southern Oscillation (ENSO) cycle causes ripples through the global climate, changing rainfall and air currents. These shifts, in turn, can cause disease carriers to interact in new ways, creating novel pathogens. Weather changes can also increase the number of people exposed to a disease, increasing the likelihood of an outbreak.

El Niño is the warm phase of the ENSO, characterized by unusually high sea surface temperatures along the equator in the Pacific, lasting between nine months and two years, according to the National Oceanic and Atmospheric Administration.

The temperature changes seem small -- usually 0.5 to 1 degree Celsius -- but they can alter rainfall patterns all over the world. "It's a shift in the large-scale dynamics of the tropical atmosphere," said Jon Gottschalck, head of forecast operations at NOAA's Climate Prediction Center. "It's not regular, and it's not predictable. They call it an oscillation, but it's an aperiodic oscillation."

After three to five years, the ENSO usually cycles to La Niña, when sea surface temperatures reach a low point. Gottschalck said this leads to more rainfall over Indonesia while weakening jet streams -- fast-moving, high-altitude air currents -- and pushing them further north. This leads to drier conditions in the southern United States.

Currently, the planet is in the middle of a La Niña phase, and parts of the American South, like Texas, experienced record drought last year.

Shifts in bird migrations create flu strains
These regular precipitation patterns change how migratory birds interact. The birds are vectors for the influenza virus. As their populations split off and reunite every few years due to ENSO, new flu strains emerge. "Flu is interesting because it mutates all the time," explained Jeffrey Shaman, an assistant professor in the Environmental Health Sciences department at the Mailman School of Public Health at Columbia University. The illness is transmitted through feces, fluids and aerosols.

As the flu virus reproduces in its host, it changes over time. When carriers are separated, the virus mutates differently in the two groups. Once La Niña comes around, disparate bird populations start to mingle again, increasing the likelihood that a carrier is infected with more than one variety of the virus.

These two versions can then form hybrid viruses in a process known as reassortment. "This reassortment happens more often than there are pandemics. In order for there to be a pandemic, there needs to be a radical change," said Shaman.

The new virus can then spread to humans, rapidly infecting the population since few people are immune, with devastating results. For example, the 1918 global flu pandemic spread as far as the Arctic Circle and to isolated islands, killing up to 50 million people around the world, according to the U.S. Department of Health and Human Services. Shaman said this pandemic was preceded by La Niña conditions.

The other global flu pandemics over the past century -- in 1957, 1968 and 2009 -- also followed cooler sea surface temperatures in the Pacific Ocean. Shaman and Marc Lipsitch at the Center for Communicable Disease Dynamics at the Harvard School of Public Health published their findings earlier this month in the Proceedings of the National Academy of Sciences.

Shaman said these pandemics don't emerge from La Niña itself, but from cyclical changes in the climate. "Really, what we're talking about is the segregation of these birds and then reaggregating. If you were to lock in one phase, you would not see this pattern. I think it's the variability is quite important," he said.

Predicting cholera outbreaks in Dhaka
Though the ENSO is hard to predict, there is some lag between a La Niña onset and an influenza pandemic, the former preceding the latter by three to six months. Shaman said this could help public health officials anticipate vaccination needs and increase their vigilance accordingly.

Researchers have also found that ENSO patterns can be used to forecast cholera outbreaks 11 months in advance in Bangladesh's capital. "Cholera is sort of a poster child for a climate variability-sensitive disease," said Aaron King, a professor of ecology and evolutionary biology and of mathematics at the University of Michigan. The disease lives in brackish water and is closely associated with rainfall linked to the ENSO, spreading through contaminated water and shellfish.

Looking at 12 years of data from Dhaka, the scientists examined where and when cholera was spreading. King said the city makes a good case study, since cholera originated in the region. The disease is endemic in Dhaka, which has 7 million densely packed residents and 12 million in its metropolitan area, according to the Bangladesh Bureau of Statistics.

The disease was studied in the region for a long time, giving researchers a great deal of information to work with. "Really, the reason why we focused on Dhaka was because that's where the data were the best," said King.

The team found that even though El Niño starts halfway around the world, its effects are very localized in the city, changing from one district to another on the scale of kilometers. "It's based on where the water comes from for the different regions," said Robert Reiner Jr., a postdoctoral researcher in the department of entomology at the University of California, Davis. "It seems that flooding itself correlates with El Niño effects [in] urban areas differently from less urban areas."

Cholera patterns emerged, showing a higher incidence in more densely populated areas in the city's center compared to its outer regions in El Niño conditions. "We weren't anticipating that there were going to be these two distinct regions," said Reiner, who was surprised to see that a global climate driver had such fine-scale consequences.

From the data, the researchers created and validated a model that links the ENSO to cholera. "The purpose of the model is to predict the likelihood of an outbreak," said Reiner. "Public health officials can ensure they have enough supplies on hand. Even in a city where cholera is not rare, they run out of supplies."

However, the planet's rising average temperature may alter these predictions. "It's important to distinguish this stuff from climate change. There, the whole system is moving into uncharted territory," said King. Reiner, King and the rest of their team also published their findings last week in the Proceedings of the National Academy of Sciences. The researchers now plan to construct similar models for other cities throughout Southeast Asia.

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500