Longitudinal survey of the occurrence of Salmonella in pigs and the environment of nucleus breeder and multiplier pig herds in England.

Eight pig breeding units previously associated with Salmonella Typhimurium were visited during a period of up to seven years. Samples from voided faeces, surfaces, fomites and wildlife were cultured. Certain serovars (Derby, Stanley, Give, Bredeney, Mbandaka and Manhattan) were isolated repeatedly on certain units, while others (Agona, Ajiobo, Heidelberg, Meleagridis, Muenchen, Montevideo, Rissen and Senftenberg) were detected only once or intermittently. Serovars Kedougou, Newport and Typhimurium were isolated consistently on some units but only intermittently on others. There was an association between the Salmonella serovar in pens and in the immediate environment of the pens. Pens holding breeding stock destined for production herds were frequently positive for Salmonella. Herds under common ownership showed similar serovar combinations. Serovars from wildlife were typical of the associated premises. Cleaning and disinfection was frequently ineffective. On one unit, a low level of Salmonella was attributed to a small herd size, good cleaning and disinfection, and good rodent control. Breeding herds are therefore susceptible to endemic infections with multiple Salmonella serovars, and cleaning, disinfection and vector control may be inadequate in many cases. The prevalence of S Typhimurium was greater in youngstock, which may have important implications for public health.