Between October 1999 and February 2001 the salmonella status of 449 dairy farms in England and Wales was determined by environmental sampling on up to four occasions. Data were collected through interview-based questionnaires, and multivariable analyses were used to identify risk factors associated with the farms either being Salmonella positive (prevalence data) or becoming Salmonella positive (incidence data). Region, herd size, month of visit and the lack of a clean visitor parking area were significantly associated with the prevalence of Salmonella species, and there was a significant trend towards an increased risk in late summer and autumn. The introduction of six- to 24-month-old cattle into a herd was associated with a reduced prevalence, but the introduction of adult cattle only, or calves with other cattle, was associated with an increased (but not significant) risk of farms being Salmonella positive. Month of visit, the lack of a clean visitor parking area, the use of part-time workers and not feeding calves whole milk, but not region or herd size, were associated with an increased incidence of salmonella.
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