Antimicrobial susceptibility and factors affecting the shedding of E. coli O157:H7 and Salmonella in dairy cattle.

A.C. F ITZGERALD, T .S. E DRINGTON, M .L. L OOPER, T .R. C ALLAWAY, K .J. G ENOVESE, K .M. B ISCHOFF, J . L . M C R E Y N O L D S , J . D . T H O M A S , R . C . A N D E R S O N A N D D . J . N I S B E T . 2003. Aims: To examine factors affecting faecal shedding of the foodborne pathogens Escherichia coli O157:H7 and Salmonella in dairy cattle and evaluate antimicrobial susceptibility of these isolates. Methods: Faecal samples were obtained in replicate from lactating (LAC; n ¼ 60) and non-lactating (NLAC; n ¼ 60) Holstein cattle to determine influence of heat stress, parity, lactation status (LAC vs NLAC) and stage of lactation [£60 or >60 days in milk (DIM)] and cultured for E. coli O157:H7 and Salmonella. A portion of the recovered isolates were examined for antimicrobial susceptibility using the broth microdilution technique. Results: No effects of heat stress were observed. Lactating cows shed more (P 0 AE10) bacterial shedding in NLAC cows. Cows £60 DIM shed more (P 60 DIM. Seventeen Salmonella serotypes were identified with the most prevalent being Senftenberg (18%), Newport (17%) and Anatum (15%). Seventy-nine of the Salmonella isolates were resistant to at least one of the seven antibiotics. Escherichia coli O157:H7 isolates were resistant to 11 different antibiotics with multiple resistance to nine or more antibiotics observed in five isolates. Significance and Impact of the Study: This study demonstrated differences in the shedding patterns of foodborne pathogens due to the stage of the milk production cycle and may help identify times when on-farm pathogen control would be the most effective.