More than half of the waterborne disease outbreaks in the United States in the past 50 years were preceded by heavy rainfall, according to a study conducted at the Bloomberg School of Public Health. Such rainfall and its subsequent runoff have been assumed to be a key factor in the transport of pathogenic microorganisms, but this study represents the first quantitative analysis of the relationship between extreme precipitation and waterborne disease outbreaks at the national level and over an extended period. The results are published in the August issue of the American Journal of Public Health.

"The significance of the association between precipitation and disease is amplified when you consider the effects of global climate change, which predict an increase in precipitation in parts of the United States," says Jonathan Patz, assistant professor of environmental health sciences and principal investigator of the study. "As the temperature rises, climatologists expect more intense rainfall events and, as our study suggests, a potential increased risk of waterborne disease outbreaks as well."

To analyze the relationship between precipitation and waterborne diseases, the researchers used data from the United States Environmental Protection Agency of the 548 reported waterborne disease outbreaks in the United States from 1948 to 1994. A waterborne disease outbreak is defined as an incident where a drinking water source causes two or more persons to become ill at similar times. The most common type of disease was acute gastrointestinal illness. The data included the infectious agent, the community and state where the outbreak occurred, and the month and year of each outbreak. The outbreak source was designated as either surface water or groundwater contamination.

This data was combined with precipitation data from the National Climatic Data Center. Total monthly precipitation readings from weather stations across the United States from 1948 to 1994 were recorded and assigned to the 2,105 watersheds in the United States. A watershed acts as the drinking water source for the encompassing area. The city or county where each outbreak occurred was also recoded to its corresponding watershed.

The researchers found that 51 percent of the outbreaks were preceded by extremely high levels of precipitation, levels that were in the top 10 percent of accumulated rainfall over the period studied. Sixty-eight percent of the outbreaks were preceded by precipitation levels in the top 20 percent of accumulated rainfall. After controlling for variations across regions and seasons, this association remained significant.

Outbreaks due to surface water contamination, which accounted for approximately 24 percent of all outbreaks, were more associated with extreme precipitation occurring during the month of the outbreak and one month prior, while outbreaks due to groundwater contamination, which accounted for approximately 36 percent of all outbreaks, were more associated with extreme precipitation occurring within a three-month lag preceding the outbreaks. The water contamination source for the remaining 40 percent of the outbreaks were undetermined.

Frank Curriero, an assistant scientist in biostatistics and lead author on the study, says, "Further research incorporating other site-specific parameters such as land-use patterns and treatment-facility specifications is needed to allow for the development of more localized predictive models. Such research," he says, "will be vital to water managers and public health planners."

This study was supported by a STAR grant from the United States Environmental Protection Agency, Office of Research and Development.