Characterisation of Salmonella enterica serotype Typhimurium isolates from wild birds in northern England from 2005-2006

Background: Several studies have shown that a number of serovars of Salmonella enterica may be isolated from wild birds, and it has been suggested that wild birds may play a role in the epidemiology of human and livestock salmonellosis. However, little is known about the relationship between wild bird S. enterica strains and human- and livestock-associated strains in the United Kingdom. Given the zoonotic potential of salmonellosis, the main aim of this study was to investigate the molecular epidemiology of S. enterica infections in wild birds in the north of England and, in particular, to determine if wild bird isolates were similar to those associated with disease in livestock or humans. Results: Thirty two Salmonella enterica isolates were collected from wild birds in northern England between February 2005 and October 2006, of which 29 were S. enterica serovar Typhimurium ( S. Typhimurium); one S. Newport, one S. Senftenberg, and one isolate could not be classified by serotyping. Further analysis through phage typing and macro-restriction pulsed-field gel electrophoresis indicated that wild passerine deaths associated with salmonellosis were caused by closely-related S. Typhimurium isolates, some of which were clonal. These isolates were susceptible to all antimicrobials tested, capable of invading and persisting within avian macrophage-like HD11 cells in vitro, and contained a range of virulence factors associated with both systemic and enteric infections of birds and mammals. However, all the isolates lacked the sopE gene associated with some human and livestock disease outbreaks caused by S. Typhimurium. Conclusion: The wild bird isolates of S. enterica characterised in this investigation may not represent a large zoonotic risk. Molecular characterisation of isolates suggested that S. Typhimurium infection in wild passerines is maintained within wild bird populations and the causative strains may be host-adapted.