Warm Baltic waters raise risk of bacterial infections

People with open wounds or weakened immune systems should exercise caution when swimming in the Baltic Sea, particularly around Funen, Zealand, Lolland, Falster, and Bornholm, where lower salinity and rising water temperatures create ideal conditions for harmful bacteria, according to Danish broadcaster DR.

Water temperatures above 20°C, recorded in recent days, promote the growth of Vibrio and Shewanella bacteria, which can enter the body through cuts or scrapes, potentially causing wound infections, ear infections, or, in rare cases, septicaemia, said Steen Ethelberg, professor and head of section at the Statens Serum Institut.

Ethelberg emphasised that serious infections remain uncommon, with most cases occurring in individuals with pre-existing wounds, particularly the elderly or those with chronic liver disease. Between 2021 and 2025, Denmark recorded 934 infections, averaging 157 annually, with the highest incidence among those aged 65–79.

Symptoms such as sudden redness around a wound or fever following a swim warrant immediate medical attention, as untreated infections can escalate rapidly, he warned.

The bacteria thrive in warm, low-salinity waters like those of the Baltic, with minimal presence along Jutland’s west coast or in freshwater lakes. Rising sea temperatures—up 1.5–2°C since the 1980s—are expected to extend the bacteria’s growth period, potentially doubling infection rates during exceptionally warm summers, as seen in 2018.