The prolonged heatwave has increased the risk of contracting an infection caused by Vibrio bacteria from seawater in coastal areas of Finland. People with weakened immune systems should now consider avoiding swimming in the Baltic Sea - especially if they have broken skin or other sores.
Vibrio’s are normally present in some saline brackish water such as the Baltic Sea, but when the surface water remains warm for a longer period of time, their numbers can increase to very high levels. They do not occur in fresh water, such as inland lakes and rivers. Vibriosis transmitted by swimming water is not cholera.
Four serious cases of vibriosis, in which the bacteria were found in blood cultures, were reported to the Infectious Diseases Register in July. The cases were caused by the bacterial species V. cholerae and V. vulnificus. The infections originated from the coastal areas of southern and southwestern Finland. In all four cases, the infection was probably acquired through a skin break while swimming.
Serious cases of vibriosis have also occurred more frequently in previous hot summers in 2014, 2018 and 2021.
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Infection is usually acquired during swimming through a skin wound or other skin break. Swimming is then followed by local skin inflammation. Local skin inflammation may require antibiotic treatment. If the immune system is reduced, the inflammation can progress rapidly and the bacteria can end up in the bloodstream, causing a severe, even life-threatening, systemic infection.
Swimmers can also develop ear infections caused by vibrios or varying degrees of stomach illness if they accidentally swallow seawater rich in bacteria. Symptoms of stomach illness caused by vibrios can last from hours to several days.
The risk of vibriosis is increased as long as the surface temperature of seawater remains high.
The European Centre for Disease Prevention and Control (ECDC) reports on the vibriosis situation weekly and uses satellite data to study surface water conditions in the Baltic Sea, which can be used to model the vibriosis risk area. The Finnish Institute for Health and Welfare monitors the disease situation and risk assessment in coastal areas of Finland.
Currently, the risk of vibriosis is increased on the coast, especially in the areas of Northern Ostrobothnia, Southwest Finland, Åland, Eastern Uusimaa, Helsinki and Kymenlaakso. In other coastal areas of Finland, the risk is medium.
All Vibrio cholerae strains detected in humans and other vibrios isolated from blood are sent to the National Institute for Health and Welfare for further investigation. If necessary, the municipal environmental health service can also examine the concentration of vibrio bacteria in bathing waters.