Epidemiological and microbiological investigation of the large increase of vibriosis in northern Europe in 2018

Northern European countries and countries bordering the Baltic Sea have witnessed an increase of vibriosis cases during recent heatwaves. Here, we described the epidemiology of vibriosis cases and the genetic diversity of Vibrio isolates from Norway, Sweden, Denmark, Finland, Poland, Estonia, and Latvia in 2018, a year with an exceptionally warm summer. We conducted a retrospective study and analysed demographics, geographic distribution, seasonality, causative species, and severity of non-travel related vibriosis cases in 2018. Data sources included surveillance systems, national laboratory notification databases and/or nationwide surveys to public health microbiology laboratories. Moreover, we performed whole genome sequencing and multilocus sequence typing of available isolates from 2014-2018 to map their genetic diversity. In 2018, we identified 445 non-travel related vibriosis cases in the study countries, which was considerably higher than the median of 126 cases between 2014-2017 (range: 87-272). The main reported mode of transmission was exposure to seawater. We observed a species-specific geographic disparity of vibriosis cases across the Nordic-Baltic region. Developing severe vibriosis was associated with infections caused by Vibrio vulnificus (adjOR: 17.2; 95% CI: 3.3-90.5) or Vibrio parahaemolyticus (adjOR: 2.1; 95% CI: 1.0-4.5), being ≥65 years of age (65-79 years, adjOR: 3.9; 95% CI: 1.7-8.7; 80+ years, adjOR: 15.5; 95% CI: 4.4-54.3) or acquiring infections during summer (adjOR: 5.1; 95% CI: 2.4-10.9). Although phylogenetic analysis revealed diversity between Vibrio isolates, two V. vulnificus clusters (