摘要：Objectives. Rainfall and runoff have been implicated in site-specific waterborne disease outbreaks. Because upward trends in heavy precipitation in the United States are projected to increase with climate change, this study sought to quantify the relationship between precipitation and disease outbreaks. Methods. The US Environmental Protection Agency waterborne disease database, totaling 548 reported outbreaks from 1948 through 1994, and precipitation data of the National Climatic Data Center were used to analyze the relationship between precipitation and waterborne diseases. Analyses were at the watershed level, stratified by groundwater and surface water contamination and controlled for effects due to season and hydrologic region. A Monte Carlo version of the Fisher exact test was used to test for statistical significance. Results. Fifty-one percent of waterborne disease outbreaks were preceded by precipitation events above the 90th percentile ( P = .002), and 68% by events above the 80th percentile ( P = .001). Outbreaks due to surface water contamination showed the strongest association with extreme precipitation during the month of the outbreak; a 2-month lag applied to groundwater contamination events. Conclusions. The statistically significant association found between rainfall and disease in the United States is important for water managers, public health officials, and risk assessors of future climate change. According to the US National Assessment on the Potential Consequences of Climate Variability and Change, 1 determining the role of weather in the incidence of waterborne disease outbreaks is a priority public health research issue for this country. Rainfall and runoff have been implicated in individual outbreaks in the United Kingdom and the United States. A waterborne disease outbreak of giardiasis in Montana was related to rainfall, 2 as was the largest reported waterborne disease outbreak ever documented, which occurred in Milwaukee, Wis, in 1993. There, an estimated 403 000 cases of intestinal illness and 54 deaths occurred, 3 and the outbreak was preceded by a period of heavy rainfall and runoff with a subsequent turbidity load that compromised the efficiency of the drinking water treatment plant. 4 , 5 Even outbreaks of Escherichia coli, generally considered a foodborne pathogen, have been linked to rainfall events. In fact, the largest reported outbreak of E coli O157:H7 occurred at a fairground in the state of New York in September 1999 and was linked to contaminated well water. Unusually heavy rainfall, which was preceded by a drought, coincided with this major outbreak. 1 Under conditions of high soil saturation, rapid transport of microbial organisms can be enhanced. Part of the rationale for this study, conducted through a US Environmental Protection Agency grant for studying the effects of global climate change on public health, comes from projections of more intense rainfall that may accompany global warming. In the past century, average daily temperatures in the conterminous United States increased by approximately 1°F. 6 Warmer air can hold more moisture, and changes in the hydrologic cycle in the United States have been evidenced by increases in cloud cover 7 and total precipitation. 8 Moreover, the type of precipitation has been changing in the United States, with increases in extreme precipitation events (those with an intensity of more than 2 inches per day). 9 , 6 , 10 These rainfall patterns are consistent with expectations of a more vigorous hydrologic cycle caused by anthropogenic greenhouse gas warming of the earth's surface. 11 – 13 The purpose of our study was to analyze the relationship between precipitation and waterborne diseases, using the complete database of all reported waterborne disease outbreaks in the United States from 1948 to 1994. Rainfall intensity is assumed to be a key determining factor in the fate and transport of pathogenic microorganisms, but the relationship has never been analyzed at the national level.
