Lessons Learned from Gastroenteritis Outbreaks Associated to Wild Swimming in the German–Dutch Vecht Watershed During Hot Summers: What Measures Can Be Taken?

Given the increasing frequency of heat waves, it is likely that swimming in surface water not officially designated as swimming water (wild swimming) will become more popular. The goal of this exploratory case study was to determine the extent of wild swimming in the Vecht river basin in Germany and the Netherlands and to identify and minimize biological risks. Through several years of field observations, supplemented by data from key informants and online sources, we identified the number of visitors, their level of exposure to water, and the total number of water-contact-associated outbreaks. During the hot summers of 2018 to 2020, between 29,000 and 37,000 people a year sought cooling in the streams, rivers and canals of this watershed, into which 52 sewage treatment plants discharge. As a result, 85% of the wild swimmers in the area swam in surface waters that do not comply with the European Bathing Water Directive. Between 2016 and 2020, at least eight outbreaks of gastroenteritis potentially linked to wild swimming occurred in the region. Most outbreaks have been associated with waters containing the highest concentrations of sewage effluent. A total of 1201 people participated in activities linked to the outbreaks. Of those, at least 107 (11%), primarily children who had engaged in intensive water-based activities, became infected. Potential prevention strategies were assessed. Targeted awareness raising, promoting safe alternatives for water recreation, outbreak surveillance, and adaptation of prevention manuals, are expected to be relatively easy to apply, effective, socially acceptable and not very costly.