Survival analysis of factors affecting incidence risk of Salmonella Dublin in Danish dairy herds during a 7-year surveillance period

A national surveillance programme for Salmonella Dublin, based on regular bulk-tank milk antibody screening and movements of cattle, was initiated in Denmark in 2002. From 2002 to end of 2009 the prevalence of test-positive dairy herds was reduced from 26% to 10%. However, new infections and spread of S. Dublin between herds continued to occur. The objective of this study was to investigate factors affecting incidence risk of S. Dublin infection in Danish dairy herds between 2003 and 2009. Herds were considered at risk when they had been test-negative for at least four consecutive year-quarters (YQs), either at the start of the study period or after recovery from infection. Survival analysis was performed on a dataset including 6931 dairy herds with 118,969 YQs at risk, in which 1523 failures (new infection events) occurred. Predictors obtained from register data were tested in a multivariable, proportional hazard model allowing for recurrence within herds. During October to December the hazard of failures was higher (hazard ratio HR=3.4, P=0.0005) than the rest of the year. Accounting for the delay in bulk-tank milk antibody responses to S. Dublin infection, this indicates that introduction of bacteria was most frequent between July and October. Purchase from test-positive cattle herds within the previous 6months was associated with higher hazard of failures (HR=2.5, P