Statens Serum Institut has warned swimmers to avoid warm seawater if they have open wounds or compromised immune systems, as elevated sea temperatures create ideal conditions for the bacteria Vibrio and Shewanella, which thrive when water exceeds 20 degrees Celsius.
The bacteria, found primarily in the Baltic Sea and coastal areas around the islands of Funen, Zealand, and Bornholm, can cause wound infections, ear inflammation, and in rare cases, blood poisoning. Infections are most common among bathers along the Danish straits and the South Funen Archipelago, with people aged 65 to 79 accounting for the highest incidence at just over 50 cases annually.
Denmark recorded 934 marine bacteria infections between 2021 and 2025. An unusually warm summer in 2018 produced a significant spike in cases. In 2026, the institute established a laboratory-based surveillance system to monitor the bacteria, which favor the lower salinity of inner Danish waters.