PAGE 5 OF 6
From Michigan, Pascual continued her cholera studies, following up in 2005 with a paper in Nature that teased ENSO's role apart from the contributions of population immunity, which also cycles over multiyear time frames. “I wanted to approach the role of climate with a better understanding of what was happening with respect to the disease itself,” Pascual says.
Doing that wouldn't be straightforward. To control for host immunity, Pascual had to know the fraction of susceptible people in the Bangladeshi population over time. Those data weren't available, so she used population data supplied by her collaborators at ICDDR. From 1966 to 2002, ICDDR staff had followed cases among roughly 200,000 people living in Matlab, a rural area just south of Dhaka. This defined population gave Pascual a denominator with which—by applying statistical wizardry—she could estimate the percentage of immune individuals living in the area from one year to the next. With that knowledge, Pascual was able to remove the influence of host immunity from her investigation of ENSO's effects on cholera transmission. She found that ENSO's effects still held up, meaning that the spike in cases couldn't be attributed solely to declines in immunity.
What those results didn't explain, however, was how ENSO held sway over cholera dynamics in Bangladesh. To answer that question, Pascual collaborated with scientists at the Climate Research Laboratory in Barcelona, Spain, and the Center for Ocean-Land-Atmosphere Studies (COLA) in Calverton, Maryland. Supported by the National Oceanic and Atmospheric Administration, the National Science Foundation, and the National Aeronautics and Space Administration, COLA scientists believe that the global climate—while chaotic—has predictable elements that allow for accurate forecasting on both short- and long-term timescales.
Crucial to their analyses is a 3,000-mile-long rectangular swath of Eastern and Central Pacific Ocean dubbed “Index 34.” COLA research scientist Benjamin Cash says that during ENSO periods, sea-surface temperatures throughout Index 34 can rise by up to 2.5°C, producing a general warming of the tropical atmosphere. “And if that warming persists long enough to influence monsoon circulation patterns, you see increased rainfall over Bangladesh,” he explains. “And Bangladesh is a low-lying country where floods lead to a breakdown in sanitation.”
Pascual agrees that heightened rainfall may be the culprit behind the ENSO-cholera connection in Bangladesh. But, she adds, much about that connection remains unresolved. “ENSO produces high amounts of rainfall in some areas in Asia and lower amounts in others,” Pascual explains. “Through our work with Ben Cash and Xavier Rodó (Barcelona) we're starting to get a better handle on what mediates the effect of ENSO on cholera in Bangladesh, but we're not in a position to make firm conclusions about the role of rainfall yet. We still need to know more about local climate drivers and how they mediate ENSO's influence. Whether we're better off predicting outbreaks on the basis of local or remote climate variables remains an open question that we're working on now.”
Yet another factor to consider, Pascual says, is the role of “inapparent” or asymptomatic cholera infections among the population and how they influence disease transmission. In a paper published in Nature in August 2008, University of Michigan's Aaron King, Pascual, and colleagues showed that the fraction of asymptomatic individuals can be greater than anticipated. These more mildly affected individuals have rapidly waning immunity, the scientists found, which could be crucial to interpreting patterns of disease outbreaks.
Ancient Romans experienced the impact of seasonal variation in temperatures on on malaria transmission. But Pascual's research is supplying quantitative evidence that climate change—a trend of increasing temperatures—is having an important effect on malaria.
In 2006, she published a paper in Proceedings of the National Academy of Sciences suggesting that even small temperature increases in the East African highlands could amplify mosquito populations and boost human exposure to the malaria parasite.