We studied cases of Vibrio and Shewanella infections during 2010–2018 in the summer months in Denmark (June to August); in the decade spanning 2010–2020, 2018 was the warmest registered summer in the country. We obtained information about the cases from the Danish Microbiology Database, a national database containing all clinical microbiology reports from Denmark (8). We extracted identification results, confirmed by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry, on Vibrio and Shewanella spp. cultured from blood, wound swabs, deep soft tissue, ear, trachea, urine, and feces as well as information about date of sampling. We also extracted the person identification number of each patient studied from Denmark’s Central Person Registry (CPR) (9). Clinical patient information was not available, but sample types were used as a proxy for type of infection. We registered cases by month per patient (Appendix 1). We counted the number of cases by calendar year and stratified them by the genus of isolated bacterial pathogen. Using the CPR number, we eliminated duplicate positive results. By linking to data from the CPR register, we retrieved information on address of residence for each case at the time of sampling. We performed geomapping and geocoding in QGIS 1.8.0 Lisboa (https://www.qgis.org) for the spatial analysis of Shewanella and Vibrio cases and plotting of number of infections per municipality, which we further interpreted based on seawater salinity in the mapped areas. We obtained sea surface temperatures of the coastal waters of Denmark during summer from the Danish Meteorological Institute (Appendix 1; Appendix 2). We performed the Pearson correlation test in R version 4.2.1 (The R Foundation for Statistical Computing, https://www.r-project.org) to determine correlation between annual summer seawater temperatures and number of Vibrio and Shewanella cases.

Figure 1

Figure 1. Relationship between Shewanella and Vibrio spp. infections and seawater temperature in Denmark, by year, 2010–2018. A, B) Pearson correlation between annual numbers of diagnosed ...

We found a positive correlation between average summer seawater temperatures (15°C–22°C) and the number of cases of Vibrio (29–172) and Shewanella (18–134) infections diagnosed in Denmark during 2010–2018 (p<0.0001; Figure 1, panels A, B). Results showed a higher number of infections during warmer summers compared with colder summers. Ear infections (n = 595) and wound infections (n = 424) were the most frequent clinical manifestations (Table); V. alginolyticus and S. algae were predominant in ear infections. V. parahaemolyticus was the most frequently isolated from wounds (n = 103, 24%), V. vulnificus (n = 14, 36%) and S. putrefaciens (n = 10, 26%) were predominant in septicemia cases, and S. putrefaciens was the species most associated with deep soft tissue infections (Table). Clinical manifestations varied by bacterial species. More than one third of V. vulnificus infections manifested as septicemia, supporting evidence of the high virulence of this species (10–12).

Vibrio and Shewanella infections increased during every summer in the study period. The summers of 2014 and 2018 were characterized by particularly high sea surface temperatures and showed an association to higher incidence in infections (Figure 1, panels A, B). In all studied years, the frequency of Vibrio and Shewanella spp. infections increased beginning in week 23, reaching a peak in the warmest months (July and August), followed by a tail of decreasing number in subsequent months (Appendix 1 Figure, panel A). A recent study on 2018 data alone reported that most human vibriosis cases reported in the Nordic region were likely linked to exposure to the warm seawater that year (7). We found that infections were more prevalent in men and boys 10–19 years of age and in elderly persons, 60–80 years of age (Appendix 1 Figure, panel B). We suspect that those results are likely because active adolescents may have scratches or wounds while performing recreational water activities (e.g., swimming, rowing, windsurfing, or fishing) and because of the vulnerability of elderly persons in general.

Figure 2

Figure 2. Number of cases of Shewanella and Vibrio spp. infection (n = 98), by municipality, Denmark, 2018.

We found a marked geographic distribution in results obtained from 2018, when most cases were in persons who lived near coastal areas with brackish waters characterized by low saline levels (<30 parts per thousand; Figure 2) (13). In contrast, along the west coast of Jutland, where the salinity of the North Sea is high and the water colder, the frequency of infections was lower. This difference suggests that increased temperature of low-salinity water favors the growth of Vibrio and Shewanella bacteria. It is important to consider that the association between place of residence and number of cases is challenged because geographic distances are short in Denmark and multiple exposures at different geographic sites during a summer season are to be expected. The lack of information on prior seawater exposure and information on international travel for each case is a limitation for the correlation between number of Vibrio and Shewanella infections and seawater exposure. Nevertheless, the observed geographic distribution of cases and the presented correlation between the number of cases in cold and warm summers strongly supports a relationship between higher-temperature/low-saline seawater exposure and risk of infection.

In this nationwide study, we show a correlation between number of Vibrio and Shewanella human infections and coastal summer water temperature in Denmark during 2010–2018. In addition, we were able to map residency of most cases to geographic areas with coastlines of low salinity. A combination of climate change effects (i.e., increasing coastal sea surface temperature at higher latitudes during summer) and a more elderly population indicates the need for increased awareness of the risk of these emerging infections and their public health impact. Rising temperatures will lead to an increase in burden of disease for these marine infections in an expanding area of the northern hemisphere (14). We propose that persons in Denmark who are exposed to seawater in summer should consider covering open wounds with a waterproof bandage, particularly the elderly and immunocompromised. We also recommend that persons thoroughly wash new cuts exposed to seawater and inform healthcare professionals of recent seawater exposure when seeking medical attention. Persons with defected eardrums should use earplugs. Our study lends support to categorizing all Vibrio and Shewanella infections in humans as mandatory notifiable diseases in Denmark and other countries in Europe that have seawater borders to monitor the incidence of these infections.