We observed a statistically significant association between rainfall and pediatric ED visits for AGI, with an estimated 11% increase in visits 4 days after rainfall. The etiology of AGI in children is due largely to viral agents (mainly rotavirus, norovirus, and other enteric viruses such as enterovirus, calicivirus, and adenovirus) which have typical incubation times from 1 to 7 days (American Academy of Pediatrics 2006
; Elliott 2007
). Bacterial causes of AGI (such as Campylobacter sp.
, Salmonella sp.
, and Escherichia coli sp.
), although reportable and much less common, also have similar incubation times on the order of one to several days. Although protozoan causes such as Giardia
can have longer incubation times than 7 days, these organisms were specifically screened per water treatment protocol. Thus, we felt that given the suspected timing of exposure from rainfall and incubation of the likely infectious agents, a lag window of 1–7 days from rainfall would be appropriate for investigation.
Further, one previous study showed the risk of treatment failure (defined as the presence of fecal coliforms) of drinking water supplies was linearly associated with rainfall on the prior day (Richardson et al. 2009
); another found a lag of 1–2 days between increased water turbidity and self-reported gastrointestinal illness (Egorov et al. 2003
). Increased water turbidity levels in the drinking water of Philadelphia, Pennsylvania, have been associated with increased pediatric AGI visits to the ED 4 days later as well (Schwartz et al. 1997
). An association between raw water turbidity and ED visits for gastrointestinal illness in Atlanta, Georgia, has also been shown (Tinker et al. 2008
). Data from an ongoing study of the epidemiology of AGI in our ED show that patients have symptoms for an average of 2–3 days prior to the visit (Gorelick M, unpublished data). Thus, the observed 4-day time lag in our study is consistent with the expected timing of exposure.
Our results add to the current knowledge that AGI outbreaks reported to health officials are often associated with rainfall by demonstrating that unreported, endemic AGI is associated with rainfall as well. A review of 548 disease outbreaks reported to the U.S. Environmental Protection Agency between 1948 and 1994 found a significant association between rainfall and illness, with 68% of the events preceded by precipitation events above the 80th percentile (Curriero et al. 2001
). Investigators in Canada found that in the last quarter of the 20th century, rainfall events above the 93rd percentile increased the risk of a waterborne disease outbreak by a factor of 2.3 (Thomas et al. 2006
). Disease outbreaks after heavy rainfall have been attributed to a variety of pathogens, including Cryptosporidium
(Atherholt et al. 1998
; MacKenzie et al. 1994
(Atherholt et al. 1998
; Weniger et al. 1983
), and enterovirus (Jean et al. 2006
). What is novel about our findings is that the observed increase in ED visits for AGI occurred in the absence of any outbreaks reported to public health authorities in our region, suggesting that rainfall-associated illness may be underestimated. We were unable to demonstrate any additional effect of extreme rainfall. Most likely this is attributable to the relatively small number of extreme precipitation events during the study period.
The association between rainfall and pediatric ED visits for AGI in our population is plausible, given the presence of pathogens that are detectable in surface and groundwater at baseline levels that increase after rainfall (Signor et al. 2005
). Corsi et al. demonstrated that adenovirus, enterovirus, norovirus, hepatitis A virus, and rotavirus have all been detected in over half of water samples from local waterways in the Milwaukee River watershed and significantly increase in concentration after storm runoff events (Corsi S, Hughes P, Borchardt M, Spencer S, Baldwin A, unpublished data). Several surveillance studies of groundwater systems across the United States have demonstrated infiltration of viral pathogens (Abbaszadegan et al. 2003
; Borchardt et al. 2003
; Fout et al. 2003
), which suggests that these drinking water sources are also vulnerable to the effects of AGI pathogen introduction from rainfall and surface water contributions.
Drinking water may become contaminated by multiple routes, including ineffective treatment of source water affected by sewage discharges or through breeches in the distribution system that allow contaminated water to enter. Many cities around the Great Lakes, including Milwaukee, are served by combined sewer systems that can become inundated with rainwater and release untreated sewage. The vast majority of patients in our region live in households served by municipal water utilities that follow state and federal treatment recommendations and whose water meets all current standards (Milwaukee Water Works 2008
; U.S. EPA 2002
). Disease transmission due to upstream contamination of the water sources should therefore be prevented or mitigated by treatment. However, municipal water systems may be overwhelmed during heavy rainfall events (Curriero et al. 2001
). Moreover, although procedures for wastewater treatment are effective in reducing concentrations of enteric pathogens and quality is tested by monitoring for fecal coliform indicator bacteria, Giardia
, and chemical contaminants, the effectiveness of treatment in removing viral pathogens is unclear (Reynolds et al. 2008
), and viral testing is not routinely done. Failure of such indicator bacteria standards to reflect the occurrence of enteric viruses has been described previously (Borchardt et al. 2004
; Gerba et al. 1979
). It is also possible that contamination of drinking water may occur after leaving water treatment facilities because of infiltration and inflow in areas of aging infrastructure where clean water and sewage pipes run in close proximity to each other and are made of porous material (Hunter et al. 2001
; LeChevallier et al. 2003
; Tinker et al. 2009
). The observed association of rainfall and illness was independent of combined sewer overflows that occurred during excessive rainfall. Indeed, only one of the sewer overflow events during this study period was independently associated with increased AGI visits, suggesting that contamination of source water by sewage release is not the most likely mechanism for exposure and that more complex pathways are involved. Interestingly, this was the only sewage overflow event that took place during winter. Although the significance of this is unclear, it is possible that contaminated runoff is greater when the ground is frozen or that the rapid freeze/thaw cycle that occurred would have compromised distribution system integrity in a more severe manner than usual. This is speculative but plausible, and consistent with a suggested mechanism of post-treatment facility contamination via an aging distribution infrastructure.
The importance of this association between rainfall and pediatric AGI is underscored by the fact that global climate change is expected to increase the intensity and frequency of extreme precipitation events (Patz et al. 2000
). With further study, the health impact of such changes could be mitigated by changes in monitoring or interventions such as changes in water treatment and delivery systems practices or issuing boil alerts around periods of heavy rain.
Our study is limited by the lack of individual data regarding disease etiology, clinical course, drinking water source and habits, and recreational water exposures. We also used only daily precipitation totals from a single weather station and did not have data on precipitation intensity, potentially masking localized effects. In addition, because of the relatively small number of visits from ZIP codes served by groundwater, we did not attempt to analyze the data by this water source, which may be an important factor in linking AGI risk to specific types of municipal water systems.
Our data include ED visits only, which could skew the data based on severity of illness. However, if the latter is true, our results would then likely reflect an underestimation of the true incidence of disease. Random-digit dialing or cohort follow-up studies may be more effective methods in assessing community-wide incidence of disease. The use of ED data gives consistency in referencing one data set for the community, and as mentioned above, ours is the only pediatric ED in the region. Furthermore, ED data have been used as a uniform sentinel for community-wide events of gastrointestinal illness in similar contexts before (Schwartz et al. 1997
; Tinker et al. 2009