A human disease indicator for the effects of recent global climate change.

C between weather and disease are well established, with many diseases occurring during certain seasons or erupting from unseasonable f lood or drought conditions. With new concerns about global warming, accompanied by greater climate variability, many recent studies have focused on disease fluctuations related to short-term or interannual climate oscillations (e.g., from weather extremes driven by El Niño). Yet, the nagging question remains as to whether or not there has been any documented change in human disease trends in response to long-term climate change, since warming has already occurred over the last century (1, 2). This trend analysis has been elusive because of the scarcity or inconsistent quality of health databases over long periods. Additionally, strong confounding factors especially complicate long-term trend analysis. Some of these include increasing trends in travel, trade and migration, erratic disease control efforts, emerging drug or pesticide resistance, human population growth, urban sprawl, agricultural development, and variable reporting biases. But Rodo et al. (3) have now succeeded in finding a robust relationship between progressively stronger El Niño events and cholera prevalence in Bangladesh, spanning a 70-year period; their use of a uniquely high quality extensive cholera database and innovative statistical methods were key. This study likely represents the first piece of evidence that warming trends over the last century are affecting human disease. The investigators used innovative statistical methods to conduct a time-series analysis of historical cholera data dating back to 1893 to examine the effect of nonstationary interannual variability possibly associated with climate change. In the last two decades, the El Niño Southern Oscillation (ENSO) differed from previous decades (3). Since the 1980’s, there has been a marked intensification of the ENSO beyond that expected from the known shift in the Pacific basin temperature regime that began in the mid 1970s. The authors found that the association of cholera incidence in the earlier half of the century (1893–1940) is weak and uncorrelated with ENSO, whereas late in the century (1980–2001), the relationship is strong and consistent with ENSO. Past climate change, therefore, may have already affected cholera trends in the region through intensified ENSO events.