Generalised linear mixed models analysis of risk factors for contaminationof Danish broiler flocks with Salmonella typhimurium

We present a retrospective observational study of risk factors associated w ith the occurrence of Salmonella typhimurium (ST) in Danish broiler flocks. The study is based on recordings from 1994 in the ante-mortem database mai ntained by the Danish Poultry Council. The epidemiological units are the br oiler flocks (about 4000 flocks) which are clustered within producers. Broi ler flocks with ST-infected parent stocks show increased risk of salmonella infection, and also the hatchery affects the salmonella status significant ly. Among the rearing factors, only the use of medicine as well as the time of rearing, and the sampling method are significant. Epidemiological contr ol would seem most efficient on starting at the top levels of the productio n hierarchy from which a major part of the ST contamination is derived. A s econdary purpose of the study is to evaluate different statistical approach es and software for the analysis of a moderately-sized data set of veterina ry origin. We compare the results from five analyses of the generalised lin ear mixed model (GLMM) type. The first observation is that the results agre e reasonably well and lead to similar conclusions. A closer look reveals ce rtain patterns of bias and estimation accuracy that correspond well with th eoretical findings and practical experience reported in the statistical lit erature. (C) 1999 Elsevier Science B.V. All rights reserved.