GE-I-7: Health hazards from vibrios – case study

Higher water temperatures resulting from climate change drive biological processes in water bodies, favouring, for instance, the growth and reproduction of infectious vibrio species belonging to the bacteria genus of vibrio. The findings from water samples taken in the Baltic Sea demonstrate the strong influence of maximum water temperature on the bacteria concentration of Vibrio vulnificus.

Vibrios on the coast – health risk to bathers

Local residents are not the only people who appreciate the German coast for its high leisure and recreation value: Coastal areas are among the most popular regions for holidays and rehabilitation purposes in Germany. Especially in the summer months when air and water temperatures reach their maximum levels, numerous visitors and patients frequent many rehabilitation centres. Air temperatures rising as a result of climate change leads to the North Sea and the Baltic Sea getting warmer (cf. Indicator KM-I-1), thus making the coast even more attractive to bathers (cf. Indicator TOU-I-1).

On the other hand, a higher water temperature also favours the growth and the increase of specific aquatic microorganisms. Among the beneficiaries are species of the bacteria genus Vibrio. The warmer, moderately salty, brackish and seawater areas on the North Sea coast and especially on the Baltic Sea coast provide ideal habitats for bacteria, known colloquially as vibrios. In high concentrations, vibrios can pose health risks: Given certain conditions, some of more than a hundred vibrio species can cause infections in humans. As far as the Baltic Sea is concerned, these species include Vibrio fluvialis, Vibrio parahaemolyticus, Vibrio alginolyticus and Vibrio vulnificus²⁹.

It is mostly through open wounds that vibrios enter the human body. They are less frequently absorbed through inadvertent swallowing of infested water or the ingestion of contaminated shellfish or fish. Subject to the type of pathogen, an infection can present in different ways. Apart from causing diarrhoea and stomach aches, the infection can damage a person’s health, for instance through body tissue dying (necroses) and in severe cases to an infection of the blood flow (sepsis), sometimes even leading to a person’s death. The infection risk increases as a function of the increase in vibrio concentrations in the water. However, the absorption of vibrios does not always trigger an infection. The underlying health condition of a person also influences the risk and severity of the infection. The chronically sick, immune-deficient and elderly people are particularly at risk. In 2018 and 2019 there were eight mortalities among these groups of people, that were associated with vibrios³⁰.

In order to keep an eye on the infection risk, the authority with competence for health and social matters (Landesamt für Gesundheit und Soziales) in Mecklenburg-Western Pomerania has been collecting data since 2008 on the summer concentrations of various vibrio species occurring in the Baltic Sea. Sampling is conducted on a random basis in most bathing locations along the coastline. However, the vibrio concentration measured is just an approximate value compared to the actual conditions prevailing in the Baltic Sea. The sample contains a small amount of water, a small proportion of which is pipetted and examined for pathogens. Given that the bacteria are not distributed evenly throughout the water body – occurring instead in clusters – a precise analysis of the bacteria concentration is not possible. Nevertheless, the findings permit the categorisation of the infection risk faced by bathers. In cases where a heightened infection risk exists, warnings are issued in due course.

The connection between rising water temperatures and the development and reproduction activity or the bacteria concentration is particularly evident in respect of Vibrio vulnificus. This species of bacteria generally tolerates a broad spectrum of water temperatures. Evidence typically becomes available above a water temperature of 10 °C minimum. The growth of the organisms is usually limited at these temperatures. It is not until the water becomes warmer, typically from about 18 °C, that the bacteria begin to reproduce more vigorously. The optimum water temperatures for growth are between 20 °C and roughly 30 °C. If the water temperature reverts to a level below this range, the vibrios often remain active for some considerable time. Once the water temperature drops to less than 10 °C, the bacteria die or – as presumed by scientists – they partly survive in an inactive state in the sediment layer of the water body³¹. As with other Vibrio species, an infection caused by Vibrio vulnificus can be life-threatening.

If temperatures rise fast early in the year or they become particularly high in the course of the year, the bacteria can be accounted for earlier and in higher concentrations. In 2020, for example – owing to a summer heatwave with an extremely high number of sunshine hours on the coast of Mecklenburg-Western Pomerania – roughly 77 % of all samples provided evidence for the existence of Vibrio vulnificus in the Baltic Sea. Moreover, the bacteria reached concentrations of between 1 million and just under 10 million colony-forming units per litre (KbE/l). The term colony-forming unit denotes individual or agglomerated microorganisms which – after reproduction – form a colony in a nutrient medium. The size of this agglomeration indicates the number or concentration of bacteria. The proportion of positive samples was higher only in 2014 which was another extremely warm year. The highest concentration detected so far – amounting to more than 100 million KbE/l – was recorded in the record heat year of 2018 when a stable high pressure bridge extending from the Azores to the Barents Sea maintained high water temperatures from May until well into August.

The development and reproduction of Vibrio vulnificus is not bound up with the water temperature alone. Other influential factors entail, for example, that – just like in the years of 2008 and 2009 – there can be numerous individual pieces of evidence showing the occurrence of increased bacteria concentrations even in less warm water. The salt content of water plays a controlling part in these scenarios. The influence of salt content is not yet fully understood. However, according to current findings, the salinity of the Baltic Sea amounting to between 0.5 to 2.5 % is in line with the tolerance spectrum of Vibrio vulnificus. In the years of 2008, 2009 and 2014 the salt content was slightly higher at the point of sampling than in the other years of the time series, which means it was possibly closer to the ideal value for Vibrio vulnificus. Distinctly higher content values up to 3 % and higher have been recorded in only a few cases in the Skagerrak. The salt content in the North Sea is typically even higher, which is why bacteria are accounted for less frequently and in lower concentrations than in the Baltic Sea.

^{29 - Baker-Austin C., Oliver J.D., Alam M., Afsar A., Matthew K. W., Firdausi Q., Martinez-Urtaza J. 2018: Vibrio spp. infections. Nature Reviews Disease Primers, 4, 1-19.}

^{30 - Brehm T.T., Berneking L., Sena Martins M., Dupke S., Jacob D., Drechsel O., Bohnert J., Becker K., Kramer A., Christner M., Aepfelbacher M., Schmiedel S., Rohde H. (German Vibrio Study Group) 2021: Heatwave-associated Vibrio infections in Germany, 2018 and 2019. Euro Surveill, 26(41): 2002041. doi: 10.2807/1560-7917.ES.2021.26.41.200204}

^{31 - Strom M., Paranjpye R. 2000: Epidemiology and pathogenesis of Vibrio vulnificus. Microbes and Infection, Band 2, Heft 2, 177-188}

GE-I-7: Health hazards from vibrios – case study | Umweltbundesamt