Vibrio parahaemolyticus, V. cholerae, and V. vulnificus are the principal Vibrio species linked to seafood-borne infections. These bacteria have mainly been associated with temperate regions, but outbreaks caused by V. parahaemolyticus in local seafood have also recently been recognized in colder parts of the world like Alaska and Chile and there have been cases in Norway, and also in Denmark, where local seafood has been suspected as the source of infection. V. vulnificus wound infections caused by seawater or seafood handling have been documented in Sweden and Denmark.

Cholera serotypes.

The vibrio responsible for the seventh pandemic, now in progress, is known as V. cholerae O1, biotype El Tor. The current seventh pandemic began in 1961 when the vibrio first appeared as a cause of epidemic cholera in Celebes (Sulawesi), Indonesia. The disease then spread rapidly to other countries of eastern Asia and reached Bangladesh in 1963, India in 1964, and the USSR, Iran and Iraq in 1965-1966. Until 1992, only V. cholerae serogroup O1 caused epidemic cholera. Some other serogroups could cause sporadic cases of diarrhoea, but not epidemic cholera. Late that year, however, large outbreaks of cholera began in India and Bangladesh that were caused by a previously unrecognized serogroup of V. cholerae, designated O139, synonym Bengal. Isolation of this vibrio has now been reported from 11 countries in South-East Asia. It is still unclear whether V. cholerae O139 will extend to other regions, and careful epidemiological monitoring of the situation is being maintained.

Horizontal gene transfer can occur between O1 and non-O1 serogroups of V. cholerae, and existing toxigenic strains are thought to have originally evolved from nontoxigenic environmental strains. Horizontal gene transfer has also been demonstrated between O1 and non-O1 strains, and there is strong evidence that bacteriophages may be involved in transfer of virulence factors. In addition, a number of non-O1 and non-O139 serotypes can cause clinical disease despite the absence of the typical virulence (CT) genes. The significance of clinical disease caused by these non-O1 and non-O139 serotypes is currently unclear. However, the emergence in early 1993 of serotype O139 with epidemic potential suggests that other serotypes could also develop this potential. Serotype O139 apparently has enhanced environmental and antimicrobial resistance, a potentially alarming trend in the long history of cholera.

Cholera is one of three diseases requiring notification to WHO under the International Health Regulations. The notification of cases is published in the Weekly Epidemiological Record. The actual number of cases is considered to be much higher because of poor surveillance systems and frequent under-reporting, often motivated by fear of trade sanctions and lost tourism. WHO estimates that the officially reported cases represent around 5-10% of actual cases worldwide.