The recent rise of the West Nile virus in the U.S. was a profound indicator that global patterns of infectious disease are changing. Just what is driving the shifts is difficult to identify, although climate change has long been a suspect. Now two reports published online this week by the Proceedings of the National Academy of Sciences further implicate climate variability in the dynamics of one infectious disease: cholera.
Using monthly mortality data for the periods 1893 to 1940 and 1980 to 2001 in present-day Bangladesh, a team led by Xavier Rodo of the University of Barcelona compared cholera outbreaks to the occurrence of the climate phenomenon known as the El Ni¿o Southern Oscillation (ENSO). In the more recent years, the scientists found a link between ENSO and the incidence of cholera, with ENSO accounting for more than 70 percent of the disease variation. In the historical data set, however, that link was weak or absent. "What is new in this work is not showing that ENSO plays a role in the variability of cholera, but that the role of ENSO has intensified," notes study co-author Mercedes Pascual of the University of Michigan. Indeed, Jonathan A. Patz of the Johns Hopkins Bloomberg School of Public Health writes in an accompanying editorial that the study "likely represents the first piece of evidence that warming trends over the last century are affecting human disease."
In related work, Rita Colwell of the University of Maryland Biotechnology Institute and her colleagues studied the cholera bacterium, Vibrio cholerae, to investigate the effects of environmental changes on the disease-causing organism itself. The researchers analyzed 42 isolates of the bacterium from patients suffering from the disease and 20 samples from water sources used for a variety of purposes, including drinking. They determined that seasonal changes in the aquatic environment, which acts as a reservoir for V. cholerae, are affecting the composition of the bacterium and thus influencing the patterns of the disease.