Scientific Opinion on monitoring and assessment of the public health risk of “Salmonella Typhimurium-like” strains

Monophasic variants of Salmonella Typhimurium-like strains, lacking the fljB-encoded second phase H antigen – ie, with the antigenic structure 1,4,[5],12:i:- appear to be of increasing importance in many Member States. Such variants are referred to as ‘monophasic S. Typhimurium’. Strains lacking expression of the phase one or both flagellar antigens are also possible, but uncommon. Current standard methods are considered suitable for isolation of such monophasic Salmonella Typhimurium strains. For identification of the monophasic 1,4,[5],12:i:- variant, it is advisable to proceed with serotyping until a first negative result of agglutination after flagellar phase inversion, and then apply a PCR protocol in order to confirm the lack of the second phase antigen. To ensure complete consistency of reporting, all isolates of putative Salmonella should be fully serotyped and the full antigenic formula reported. If the full antigenic formula is not available but a phage type that is consistent with S. Typhimurium lacking phase two flagellar antigens has been confirmed, and the lack of the second phase flagellar antigen has been verified by PCR, then the term ‘monophasic S. Typhimurium’ is recommended for reporting purposes in the current situation. On the basis of genetic similarity and ability to obtain a recognised Salmonella Typhimurium phage type, these emerging epidemic monophasic strains with formula 1,4,[5],12:i:- are regarded as variants deriving from S. Typhimurium. Moreover, monophasic S. Typhimurium strains have been shown to have similar virulence and antimicrobial resistance characteristics to other strains of S. Typhimurium. Similar to what was observed in the past for epidemic clones of S. Typhimurium, recent studies in numerous countries worldwide confirm the rapid emergence and dissemination of such strains in food animals, companion animals and humans. The public health risk posed by these emerging monophasic S. Typhimurium strains is therefore considered comparable to that of other epidemic S. Typhimurium strains