Variation in the faecal shedding of Salmonella and E-coli O157:H7 in lactating dairy cattle and examination of Salmonella genotypes using pulsed-field gel electrophoresis

Aims: To examine the variability in faecal shedding of Salmonella and Escherichia coli O15:H7 in healthy lactating dairy cattle and to evaluate the genetic relatedness of Salmonella isolates. Methods: Faecal samples were obtained from lactating Holstein dairy cattle on four commercial farms in the southwestern US. All farms were within an 8-km radius and were sampled in August 2001, January 2002 and August 2002 (60 cows per farm per sampling; n = 720 total samples). Samples were cultured for E. coli O157:H7 and Salmonella and a portion of the recovered Salmonella. isolates were examined for genetic relatedness using pulsed-field gel electrophoresis (PFGE). Results: Faecal shedding of E. coli O157:H7 and Salmonella varied considerably between farms and at the different sampling times. Large fluctuations in the percentage of positive animals were observed from summer to summer for both of these pathogens. Similarly, Salmonella serotype and serotype prevalence varied from farm to farm and within farm from one sampling time to another. Multiple Salmonella genotypes were detected for a number of serotypes and identical genotypes were found on different farms with one genotype of Salmonella Senftenberg identified on three of the four farms. Significance and Impact of the Study: This study demonstrated the wide variability in pathogen shedding within and among dairy farms all located in a small geographical region and highlights the complexity of pathogen control at the farm level.