Effect of heifer-raising practices on E. coli antimicrobial resistance and Salmonella prevalence in heifer raisers

Antimicrobial resistance of Escherichia coli from dairy farms participating in an antimicrobial stewardship educational program for farm employees

Antimicrobial use in food-producing animals is under increasing scrutiny due to the potential effect on the selection of antimicrobial-resistant bacteria that may be transmitted to humans by direct contact, with the food chain, or the environment. Novel data monitoring commensal Escherichia coli from dairy farms is essential for understanding antimicrobial resistance (AMR) patterns and their association with herd health management practices. The objectives of this study were to: (1) compare the prevalence of antimicrobial resistance in the E. coli isolates from the hospital, fresh, and mid-lactation pens from 18 conventional dairy farms participating in an educational training program in antimicrobial stewardship practices in California and Ohio, and (2) to characterize the prevalence of antimicrobial resistance of commensal E. coli isolated from pooled fecal pat samples before and 3 mo after participating in the educational training program. Pooled fecal pat samples were collected from the hospital pen, the fresh pen (1 to 5 DIM), and the mid-lactation pens (90 to 150 DIM) on conventional dairies in CA (n = 9) and OH (n = 9). Fecal samples were collected as part of a larger study using a quasi-experimental design that assigned farms to the training intervention group (TG; 6 per state) or the control group (CG; 3 per state). For the TG, farm worker(s) identified as having the task of diagnosis and treatment of adult cows on the farm participated in a training program on antimicrobial stewardship practices. Pooled fecal samples (n = 7) were collected at enrollment and 3 mo after completing the intervention on each of the participating farms (n = 18), followed by culture for E. coli isolation and antimicrobial sensitivity testing using the broth microdilution methodology. Logistic regression models were used to evaluate the association between E. coli antimicrobial resistance patterns with the training intervention and farm-level factors. No effect was observed in the prevalence of resistant isolates between the control and intervention farms after the training was delivered. Isolates from the hospital pens were 2.48 (95% CI: 1.06-6.22, P = 0.03) and 5.61 (95% CI: 1.94-16.91, P < 0.001) times, more likely to be resistant to streptomycin and chloramphenicol, respectively, than isolates from the mid-lactation pens. Our findings indicate there was a higher prevalence of AMR in E. coli associated with the hospital pen within the farm, while the training program for 3 mo did not affect the prevalence of AMR in E. coli on the farms participating in the program. Further research efforts should be conducted to identify factors driving AMR at the pen level, as well as approaches that could be used to reduce the risk of disseminating AMR from sick pens to animals being housed and to other pens on the farm.

Impact of Clinical Salmonellosis in Veal Calves on the Recovery of Salmonella in Lymph Nodes at Harvest

The objective of this study was to determine the prevalence, serotypes, antimicrobial resistance phenotypes, and pulsed-field gel electrophoresis (PFGE) patterns of Salmonella recovered in feces and mesenteric and prefemoral lymph nodes (LNs) from cohorts of calves with and without a confirmed outbreak of salmonellosis. In a prospective cohort study, 160 calves from four farms without a reported outbreak (nonoutbreak farms) were sampled at farm and harvest. In addition, harvest samples from 80 calves of two farms with a confirmed outbreak (outbreak farms) were collected. A culture protocol for Salmonella isolation was applied for all samples and recovered isolates were further characterized by serotyping, antimicrobial susceptibility testing, and PFGE. Among nonoutbreak farms, Salmonella was recovered from 0% (0/160) farm fecal samples, 3.7% (6/160) harvest fecal swabs, 21.9% (35/160) mesenteric LNs, and 0.6% (1/160) prefemoral LNs. Serotypes identified in nonoutbreak herds included Salmonella Typhimurium, Cerro, Hartford, and Newport. Most isolates (64.3%, 27/42) exhibited a unique multidrug-resistant (MDR) phenotype, including resistance to extended-spectrum cephalosporins. Salmonella prevalence in harvest fecal samples and prefemoral LNs among calves from outbreak farms was numerically higher, but not significantly different than those without an outbreak. Serotypes recovered from outbreak farms included Salmonella Heidelberg and Typhimurium, and the monophasic Salmonella Typhimurium strains 4,5,12:i:- and 4,12:i:-, which have been also reported as highly pathogenic in humans. All isolates (33/33) exhibited an MDR phenotype. Salmonella strains recovered from ill calves in two outbreaks had indistinguishable PFGE patterns, suggesting between-farm transmission. In addition, the genotype of Salmonella Heidelberg causing an outbreak among calves was recovered from three prefemoral LNs of surviving members of the cohort at harvest. Implementation of preharvest biosecurity measures (limited personnel and visitor traffic, vehicle, footwear, and utensils disinfection) should be highly recommended to decrease the prevalence of Salmonella on farms and safeguard the food safety.