HERD SIZE AND SERO-PREVALENCE OF SALMONELLA-ENTERICA IN DANISH SWINE HERDS - A RANDOM-EFFECTS MODEL FOR REGISTER DATA

The association between herd size and sero-prevalence of Salmonella wa s assessed in a random-effects model with herd size, county and date o f slaughter as fixed effects. A total of 510915 meat-juice samples fro m 14593 herds located in 13 counties in Denmark was included in the st udy. A random-effects model was developed from separate models for sma ller strata of data from herds with approximately equal sizes. The com bined model was analysed and the results reported. Herd size was posit ively associated with the sere-prevalence of Salmonella enterica, but the size of the association was biologically of little importance, bec ause the within-herd and the between-herd variations were relatively l arge in comparison. The relative magnitudes of the variance components indicated that factors associated with both the herd level and the pi g level could be important in the prediction of seroprevalence of S. e nterica. (C) 1998 Elsevier Science B.V.