The dangers of swimming in the Baltic. Potential Vibrio spp. infections

A review of Collin B, Rehnstam-Holm AS. 2011. Occurrence and potential pathogens of Vibrio cholera, Vibrio parahaemolyticus and Vibrio vulnificus on the South coast of Sweden. FEMS Microbiology Ecology. 78:306-313.

Vibrio spp. Is a widely spread aquatic bacteria and whilst many are harmless to humans, some strains are potential pathogens to humans. Three of these that this study focuses on are Vibrio cholerae, known to cause epidemic and pandemic cholera, Vibrio vulnificus, which may cause fatal wound infections as well as food related infections and Vibrio parahaemolyticus, which predominantly causes seafood-bounre bacterial gastroenteritis, but may also on rare occasions cause wound infection. However it is unknown whether V. cholerae has the ability to cause these wound infections.

Summer 2006 saw several infections of wounds, three of which were fatal, to people that were exposed to bathing waters in the Baltic sea. Despite this however there have been relatively few investigations to the presence of Vibrio spp. in this area, and those that have been done focus on the total counts of Vibrio spp. with no discrimination between potential pathogenic and non-pathogenic strains. This is the first paper to explicitly investigate pathogenic strain in the Sound, between Denmark and Sweden.

The aim of this study was to firstly identify if Vibrio strains that can potentially be pathogenic to humans were present, to separate and identify specific gene codes that are responsible for causing virulence (i.e. ctx which produces cholera toxins in V. cholerae) and finally to examine the killing index of the three strains.

Sampling took place during June through September, not only because Vibrio spp. is temperature regulated, but as this is when people are most exposed to seawater making it the most relevant time. Mytilus edulis are known to efficiently ingested bacteria from the surrounding water and accumulate these within their tissues. Samples of M. edulis were collected and the contents of each separate mussel was homogenised. From this homogenate, PCR DNA analysis was performed to identify species-specific virulence genes for each of the three Vibrio strains. Cultures of each strain were also grown. From the grown cultures, and clinical batches cell toxicity tests were performed for each strain to determine the killing index on Chinese hamster ovary cells.

The results of this study show that all three strains were present and are common on the Sound. Every sample, besides two which were under 17^oC, had all three strains present. Each strain tested positive for at least one gene associated with virulence factors; V. cholerae positive for toxR in 53% of samples, V. vulnificus positive for vvh and/or viuB in63% and V. parahaemolyticus positive for virulence genes tdh and/or trh in 53%. The killing index of V. cholerae was 81% in field strains and 76% in clinical strains, V. vulnificus was 85% and V. parahaemolyticus was 79%.

The results here demonstrate the potential problem of Vibrio spp. infection in this area. It is clear that three of the strains that are pathogenic to humans occur very frequently in these waters during the summer months when people are bathing. As virulence genes specific to V. vulnificus and V. parahaemolyticus were present in around half of the samples, the potential risk for infection is high which is reinforced by the high killing indices for the strains, especially that the field killing index was higher than clinical strains.

What this paper demonstrates clearly is the need for further research into this area. As the presence of these bacterium have the potential to take lives it is extremely important for proper screening to occur in this region. In recent years this has been neglected with the detriment to human health. Only with fast regular screening is there the possibility to reduce these problems.