The occurrence of human pathogenic Vibrio spp. and Salmonella in aquaculture

Summary This paper describes the occurrence of human pathogenic Vibrio spp. and Salmonella in aquaculture. Vibrio species are natural inhabitants of marine aquatic environments in both temperate and tropical regions, with most human infections acquired by exposure to such environments or to foods derived from them. There appears to be positive correlation between water temperature and the number of human pathogenic vibrios isolated as well as the number of reported infections. However, the occurrence of vibrios does not correlate with traditionally used bacterial indicator organisms of faecal pollution, and their presence in aquaculture is to be expected and cannot be controlled by water quality control measures such as wastewater treatment. Among more than 12 Vibrio spp. known to be associated with human disease, V. cholerae and V. parahaemolyticus are the most important causes of gastrointestinal diseases. Only V. cholerae serotypes O1 and O139, which are rarely found in the environment where aquaculture is practised, cause the disease cholera. Although fish and shellfish may be vehicles of cholera transmission, most episodes are identified as a single or a cluster of cases after consumption of raw seafood. Cholera cases caused by the consumption of commercial imported seafood products have to the author's knowledge not been reported. Seafood products should not contain strains of V. cholerae O1 and O139. However, non-O1 non-O139 strains, which are normally present in non-faecal polluted aquaculture environments, have a low pathogenic potential, and their presence in fish products seems only to constitute a minor health hazard. Accordingly, fresh and frozen fish products containing V. cholerae non-O1 non-O139 should not be rejected for human consumption. V. parahaemolyticus is an important cause of gastrointestinal disease, most often after the consumption of seafood. Several haemolysins are important virulence factors, with environmental strains often showing a lower virulence than clinical strains. V. vulnificus may, mainly among immunocompromised individuals, cause serious would infections and primary septicaemia after ingestion of raw seafood, especially oysters, or contact with seawater. However, confirmed human cases of septicaemic or gastrointestinal disease caused by V. vulnificus after consumption of fish have not been reported, and evidence of V. vulnificus as a cause of gastroenteritis has not been provided. Based on data mainly from temperate regions, Salmonella is generally not recognized as being part of the normal microflora in aquaculture environments, and their presence in seafood is therefore seen as a sign of poor standards of process hygiene and sanitation. Accordingly, most countries will not accept Salmonella in any seafood product. Although some studies suggest that Salmonella may be ubiquitous in tropical aquaculture environments, further studies are needed to confirm this. Modern gene-based typing techniques could provide important information about possible transmission of Salmonella strains from the aquaculture environment to processing facilities and the final products. Althhough Salmonella is one of the most important causes of human gastrointestinal disease world-wide, surveillances of food-borne outbreaks associated with the consumption of fish showed that Salmonella species are rarely implicated in gastrointestinal disease.