Risk factors associated with Salmonella enterica serovar typhimurium infection in Danish broiler flocks

A retrospective longitudinal study was conducted to identify risk factors associated with Salmonella enterica serovar typhimurium (S. typhimurium) infection in Danish broiler flocks. The data included all broiler flocks slaughtered in 1995, and the epidemiological unit was the individual broiler flock. The S. typhimurium status was determined by microbiological examination of 60 fresh fecal samples. This procedure should detect an infected flock with a probability above 95%, if the prevalence is above 5%, and given that the sensitivity of the test is 100%. Nineteen variables were selected for analysis. Five factors and an interaction term were found significant by multivariate logistic regression analysis. An increased risk for S. typhimurium infection was associated with two parent flocks, one confirmed infected and one suspected of being infected with S. typhimurium, with two of the hatcheries, and with five houses on the farm. An interaction between season and the previously mentioned hatcheries, and a random effect at farm level was also found to be statistically significant. Twelve variables were not found to be associated with S. typhimurium infection: medication, growth promoters, breed of the laying flock, animal density, size of the flock, area of the house, age of the house, geographical location of the farm, observation of beetles, number of days between disinfection and replacement, visual appearance of the bedding, and age of the chickens when they were tested for Salmonella. Three variables (feed mill, slaughterhouse, and Salmonella status of the preceding flock) were not evaluated in the multivariate analysis due to collinearity with other included variables.