Gastrointestinal Illness among Triathletes Swimming in Non-Polluted versus Polluted Seawater Affected by Heavy Rainfall, Denmark, 2010-2011

Abstract

Recent years have seen an increase in the frequency of extreme rainfall and subsequent flooding across the world. Climate change models predict that such flooding will become more common, triggering sewer overflows, potentially with increased risks to human health. In August 2010, a triathlon sports competition was held in Copenhagen, Denmark, shortly after an extreme rainfall. The authors took advantage of this event to investigate disease risks in two comparable cohorts of physically fit, long distance swimmers competing in the sea next to a large urban area. An established model of bacterial concentration in the water was used to examine the level of pollution in a spatio-temporal manner. Symptoms and exposures among athletes were examined with a questionnaire using a retrospective cohort design and the questionnaire investigation was repeated after a triathlon competition held in non-polluted seawater in 2011. Diagnostic information was collected from microbiological laboratories. The results showed that the 3.8 kilometer open water swimming competition coincided with the peak of post-flooding bacterial contamination in 2010, with average concentrations of 1.5x104 E. coli per 100 ml water. The attack rate of disease among 838 swimmers in 2010 was 42% compared to 8% among 931 swimmers in the 2011 competition (relative risk (RR) 5.0; 95% CI: 4.0-6.39). In 2010, illness was associated with having unintentionally swallowed contaminated water (RR 2.5; 95% CI: 1.8-3.4); and the risk increased with the number of mouthfuls of water swallowed. Confirmed aetiologies of infection included Campylobacter, Giardia lamblia and diarrhoeagenic E. coli. The study demonstrated a considerable risk of illness from water intake when swimming in contaminated seawater in 2010, and a small but measureable risk from non-polluted water in 2011. This suggests a significant risk of disease in people ingesting small amounts of flood water following extreme rainfall in urban areas.

Risk factors for gastroenteritis associated with canal swimming in two cities in the Netherlands during the summer of 2015: A prospective study

“…This conclusion is furtherly supported by the trend we found between the number of mouthfuls of water swallowed and the risk to develop AGI. This finding is consistent with other studies where ingestion of water while swimming was associated with gastrointestinal complaints [2, 5, 7, 14]. …”

“…Previous outbreak studies have largely focused on the effects of swimming on AGI in official recreational water [1, 5], inland beaches [4, 7, 8], lakes [3] or marine water [14, 15]. This study has increased our understanding of the health risks of swimming in water in urban areas.…”

“…Outbreaks of gastrointestinal and other health complaints after swimming events in untreated recreational water have been described recently [3, 14]. As measurements of faecal indicator levels are not fully predictive for gastrointestinal outbreaks [5], more investigation is needed to gain a better understanding of the health risks associated with swimming in urban canal water.…”

Quantitative Impact Assessment of Sewer Condition on Health Risk

“…During the past years, there has been an increased interest in microbial impacts through pluvial flooding, see, e.g., [1][2][3][4][5][6][7][8][9] and combined sewer overflow (CSO) spills [10], as well as economic, social, and psychological impacts of urban flooding [11]. The latter are expressed as stressors on communities due to repeated flood events.…”

“…According to De Wit et al [15] and Mead et al [16], these pathogens account for the majority of gastrointestinal illnesses in The Netherlands and the US. Harder-Lauridsen et al [10] …”