Vibrio cholerae —An emerging pathogen in Austrian bathing waters?

**Background:** Vibrio cholerae is a natural inhabitant of aquatic ecosystems and a major waterborne pathogen. While toxigenic serogroups O1 and O139 cause epidemic cholera, non-toxigenic, non-O1/non-O139 strains (NTVC) are globally distributed in aquatic environments and can cause a range of infections, including gastrointestinal, ear, wound, and bloodstream infections. In Austria, NTVC infections have been linked to recreational activities in Eastern Austrian lakes and ponds, particularly Lake Neusiedler See. This review summarizes the current knowledge on NTVC in Austrian bathing waters, including epidemiology, environmental reservoirs, diagnostic methods, and public health implications.

**Methods:** This is a narrative review that synthesizes published data on NTVC in Austria, including epidemiological surveillance data from 2006 to 2022, environmental monitoring studies, and laboratory-based investigations. The review covers cultivation-based and cultivation-independent detection methods, as well as molecular characterization of isolates. Data on NTVC abundance in water and zooplankton, seasonal patterns, and genetic diversity are presented from previous studies conducted at Lake Neusiedler See and other Eastern Austrian bathing sites.

**Key Results:** Between 2006 and 2022, 38 NTVC infections were documented in Austria, of which 27 were likely or most likely acquired domestically. The majority of domestic cases were associated with Lake Neusiedler See. Ear infections were the most common type, followed by gastrointestinal infections. Two cases of necrotizing fasciitis occurred during the 2015 heat wave, one fatal. No clear increasing trend in cases was observed over the study period. Environmental monitoring at Lake Neusiedler See revealed NTVC concentrations up to 5.5 × 10^5 cells/L in water and up to 1.2 × 10^6 cells/L on zooplankton. In adjacent soda pools, concentrations reached 56 × 10^6 cells/L, among the highest reported worldwide. A survey of 36 bathing sites in Eastern Austria detected NTVC at 22 sites, with concentrations up to 3.9 × 10^5 CFU/L. All environmental isolates were negative for cholera toxin genes (ctx and tcp). Genetic analysis of 472 isolates from Lake Neusiedler See identified 94 different sequence types, with high diversity in reed habitats. Clinical isolates from 2008–2009 matched environmental strains from 2011–2012, suggesting long-term persistence of specific clones. Antimicrobial resistance profiling of Austrian NTVC isolates showed high resistance to sulfonamides (97.5%) and streptomycin (39%), but no resistance to first-line antibiotics (doxycycline, ciprofloxacin) or last-line options (4th generation cephalosporins, carbapenems).

**Clinical Implications:** NTVC infections in Austria remain rare and are primarily associated with recreational water exposure in Eastern Austria. Most infections are mild and self-limiting, but severe outcomes such as necrotizing fasciitis and septicemia can occur, particularly in immunocompromised individuals or those with chronic diseases. Clinicians should consider NTVC in patients presenting with ear, wound, or gastrointestinal infections after exposure to warm, low-salinity bathing waters, especially during summer months. Rapid antibiotic treatment with doxycycline, ciprofloxacin, or third-generation cephalosporins is recommended for severe cases. For travel-associated infections, an antibiogram is essential due to potential multidrug resistance. Public health awareness should focus on vulnerable populations, advising caution when recreating in waters with high NTVC concentrations. Despite rising water temperatures due to climate change, no clear increase in NTVC infections has been observed in Austria to date, but continued surveillance is warranted.