How Cities Affect the Stomach Flu

A study in Bangladesh highlights markedly different disease dynamics in the urban core versus the more rural periphery.
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People crowd onto a train in Dhaka, Bangladesh. (Photo: Allison Joyce/Getty Images)

People crowd onto a train in Dhaka, Bangladesh. (Photo: Allison Joyce/Getty Images)

Few things are as dreadful as the stomach flu. At best, it's a lost weekend. At worst, it can kill. Rotavirus, one of the leading causes of the stomach flu, takes the lives of 20 to 60 kids under age five each year in the United States, according to statistics from the Centers for Disease Control and Prevention. Worldwide, that figure is more like 450,000, so it's obvious why public-health officials would want to find new ways to combat the disease. Well, here's a novel approach: Look to the physical geography of cities—in particular, the differences between a city's urban core and its suburban and rural periphery.

Why would that distinction matter? In a word, climate. Consider Dhaka, Bangladesh, a city of around seven million people with a population density on par with New York City. A 2011 study found that cholera outbreaks in Dhaka were sensitive to changes in climate, such as the El Niño phenomenon, but only in the densely populated core of the city. That core, the study concluded, "acts to propagate risk to the rest of the city." The conclusion was that understanding climate variability within cities could be important for predicting and containing future cholera outbreaks.

The incidence rate of rotavirus was about three times higher in central Dhaka compared with the periphery.

Now, some of the same researchers, this time led by Pamela Martinez, a graduate student at the University of Chicago, and ecology professor Mercedes Pascual, are taking a look at how climate variability within Dhaka might affect rotavirus. In particular, they focused on flooding caused by El Niño Southern Oscillation, as the effect is formally known, and whether the relationship between flooding and rotavirus infections varied over time and across different parts of Dhaka.

Analyzing 22 years of data, the team found the incidence rate of rotavirus was about three times higher in central Dhaka compared with the periphery, but, more importantly, the core and periphery had different seasonal patterns. In the core, rotavirus infections peak twice during the year, once in the winter and again during monsoon season—similar, the authors point out, to the pattern seen in tropical countries. (Dhaka is practically on top of the Tropic of Cancer, the northern edge of the tropics.) In Dhaka's periphery, on the other hand, rotavirus peaks only once, in the winter, similar to other, truly rural areas of Bangladesh.

To better understand those results, the team fit their data to a standard disease transmission model, but one in which flooding could affect rotavirus transmission rates. That revealed rotavirus outbreaks were roughly 10 times more sensitive to flooding in the core compared with the periphery, perhaps because the urban center is so much more crowded.

"[O]ur results underscore the importance of considering the spatial heterogeneity of large urban environments" when studying disease, the authors write in Proceedings of the National Academy of Sciences. "Demographic and socioeconomic conditions, such as crowded living spaces can exacerbate the effects of unusual climactic events and seasonal changes, especially in the populated cities of the developing world and in the future under climate change."

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