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Travellers' diarrhoea: prospects for successful immunoprophylaxis.

Enterotoxigenic Escherichia coli (ETEC) are the major aetiological agent of travellers' diarrhoea, usually accounting for 30-60% of cases. Thus a safe and effective vaccine against ETEC could play an important role in prevention of this infection. A successful vaccine must somehow protect against a heterogenous array of ETEC pathogens comprising many O:H serotypes and three enterotoxin phenotypes (LT+/ST-, LT-/ST+, and LT+/ST+). Three major approaches to vaccine development are currently in progress including: 1) LT and ST toxoids that stimulate antitoxic immunity; 2) purified pili vaccines that induce anti-adhesion immunity; 3) attenuated strains (prepared by recombinant DNA techniques) that mimic natural infection by stimulating antibacterial and anti-adhesive immunity as well as antitoxic immunity. Since Shigella represents the second most common cause of travellers' diarrhoea, immunoprophylaxis against this pathogen is also desirable. A recently described attenuated Salmonella typhi strain (Ty21a) expressing Shigella sonnei O antigen on its surface represents an attractive Shigella vaccine prototype.