Prevalence and spatial distribution of antibodies to Salmonella enterica serovar Typhimurium O antigens in bulk milk from Texas dairy herds.

The purpose of this study was to describe the herd antibody status to Salmonella Typhimurium as estimated from co-mingled milk samples and to describe the resulting geographical patterns found in Texas dairy herds. Bulk tank milk samples were collected from 852 Grade A dairies throughout Texas during the summer of 2001. An indirect enzyme-linked immunosorbent assay (ELISA) using S. Typhimurium lipopolysaccharide was performed with signal to noise ratios calculated for each sample. The ELISA ratio was used in fitting a theoretical variogram and kriging was used to develop a predicted surface for these ratios in Texas. A spatial process with areas of higher risk located in the panhandle and near Waller County was apparent. Lower risk areas included Atascosa, Cooke, Collin, Titus, Comanche and Cherokee Counties. Subsets representing large dairy sheds in northeast Texas, the Erath County area, and the Hopkins County area were also evaluated individually. Each result illustrated a spatial process with areas of low and high ELISA ratio predictions. Cluster analysis was performed for the entire state with cases defined as herds having milk ELISA ratios greater than or equal to 1.8. Using this cutoff, the prevalence of herds with positive bulk tank milk ELISAs was 4.3%. Significant clustering of cases was demonstrated by the Cuzick and Edward's test. The spatial scan statistic then identified the two most likely clusters located in and near the Texas Panhandle. This study demonstrated that the distribution of S. Typhimurium antibodies in bulk tank milk in Texas is describable by a spatial process. Knowledge of this process will help elucidate geospatial influences on the presence of S. Typhimurium in dairy herds and enhance our understanding of the epidemiology of salmonellosis.