Performing Department
Population Health & Reproduction
Non Technical Summary
California has the largest number of milking dairy cattle in the U.S. Our study will increase the understanding of drugs at risk of antimicrobial resistance in Salmonella. The laboratory methods used will allow the culture of Salmonella from the feces of cull dairy cows during two years on 6 California dairies, and the testing of these samples for antimicrobial drug resistance. We will also collect data from farm management to analyze potential factors that could be linked with a higher occurrence of Salmonella or drug resistance. Our research findings are expected to help identify the risk factors and patterns of antimicrobial resistance in Salmonella and hence promote food safety.
Animal Health Component
100%
Research Effort Categories
Basic
(N/A)
Applied
100%
Developmental
(N/A)
Goals / Objectives
1) To estimate the crude and seasonal prevalence of antimicrobial drug resistant Salmonella spp. in cull dairy cows on 6 California dairies over two years, and the prevalence stratified by reason for culling.2) To identify management factors associated with the selection of antimicrobial resistance in Salmonella spp. in cull dairy cows on 6 California dairies.
Project Methods
Fecal Sampling: We plan to collect fecal samples from an additional 240 cull dairy cows from 6 convenience sample dairies in the San Joaquin Valley of California (in addition to 249 samples already collected). For each dairy, 10 cows will be randomly selected on culling day for fecal sampling quarterly over the 12 month period to evaluate seasonal effects. Assuming a 10% prevalence for Salmonella spp., a sampling fraction of 25% for risk factors (questionnaire below), alpha=5%, this sample size will generate approximately 90% power to detect factors that elevate the odds ≥ 2.0 for the occurrence of Salmonella spp. Individual fecal samples will be collected rectally using clean palpation sleeves in 2-oz polypropylene tubes and transported on ice to the VMTRC. Fecal sampling will be performed with the approval of UC Davis Institutional Animal Care and Use Committee (IACUC) AUP# 18019, 18941). Microbiology: One gram of feces will be enriched in selenite broth for 24h at 37oC. The broth will then be sub-plated onto a non-selective XLD plate and a selective XLT-4 plate and incubated for 24h at 37oC. Plates with colonies that show a positive H2S reaction will have three colonies swabbed, separately, onto a Blood Agar-MacConkey split plate. One colony from the Blood Agar-MacConkey plate will be used for biochemical testing. The split plates and biochemical tubes will be inoculated for 24h at 37oC. Any samples with biochemical tests indicating the presence of Salmonella will be further tested to identify the Salmonella serogroup and serotype. PCR testing will be performed to confirm culture results at the Tulare branch of the California Animal Health and Food Safety (CAHFS) laboratory.Antimicrobial resistance testing: All Salmonella isolates will be sent to the Veterinary Medical Teaching Hospital (VMTH) Clinical Laboratories at UC Davis (Davis, Ca) for antimicrobial susceptibility testing using a commercial microbroth dilution method (Sensititre, Trek Diagnostic Systems, Cleveland, OH). Antimicrobial susceptibility testing will be conducted using a National Antimicrobial Resistance Monitoring System (NARMS) panel for 15 antimicrobial drugs of the beta lactam, tetracycline, aminoglycide, fluroquinolone and macrolide classes. Susceptibility of the isolates to antimicrobial drugs will be categorized as susceptible, intermediate, or resistant by using breakpoints established by Clinical and Laboratory Standards Institute (CLSI) guidelines10. Antimicrobial testing will be performed at the Veterinary Medicine Teaching Hospital bacteriology laboratory.Data collection and analysis: For each sampling event a standardized questionnaire will be filled out regarding therapeutic and vaccination history, reason for culling, manure management of cull pen, and on-farm training for antibiotic therapy and withdrawal times. The crude prevalence of antimicrobial resistant Salmonella spp. in cull dairy cows will be estimated. The stratum-specific prevalence will be estimated for the type of drug, herd, season and reason for culling. Prevalence of multidrug resistant Salmonella spp. will be estimated similarly as a crude estimate and stratified over the same strata. Mixed effects logistic regression will be used to estimate the association between risk factors and resistant Salmonella in the fecal samples for each of the 15 antimicrobials, with season, dairy, lactation, therapeutic antimicrobial use, days in milk and reason for culling evaluated as potential predictors.