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

A retrospective longitudinal study was conducted to identify risk factors a ssociated with Salmonella enterica serovar typhimurium (S. typhimurium) inf ection in Danish broiler flocks. The data included all broiler flocks slaug htered in 1995, and the epidemiological unit was the individual broiler flo ck. 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 interact ion term were found significant by multivariate logistic regression analysi s. An increased risk for S, typhimurium infection was associated with two p arent flocks, one confirmed infected and one suspected of being infected wi th S. typhimurium, with two of the hatcheries, and with five houses on the farm. An interaction between season anal the previously mentioned hatcherie s, and a random effect at farm level was also found to be statistically sig nificant. Twelve variables were not found to be associated with S, typhimurium infect ion: medication, growth promoters, breed of the laying flock, animal densit y, size of the flock, area of the house, age of the house, geographical loc ation of the farm, observation of beetles, number of days between disinfect ion and replacement, visual appearance of the bedding, and age of the chick ens when they were tasted for Salmonella. Three variables (feed mill, slaug hterhouse,and Salmonella status of the preceding flock) were not evaluated in the multivariate analysis due to collinearity with other included variab les.