Brent C. Credille

The objective of this blinded, cluster randomized, complete block trial was to evaluate the impact of metaphylaxis on health, performance, antimicrobial resistance, and contextual antimicrobial use (AMU) in high-risk beef stocker calves. Calves (n=155) were randomly assigned to receive either saline or tulathromycin at the time of arrival processing. Deep nasopharyngeal swabs (DNP) were collected from each calf at arrival and 14 days later. Calves were monitored for bovine respiratory disease (BRD) for 42 days. Body weights were obtained at arrival, day 14, 28, and 42. Contextual antimicrobial use (AMU) was calculated using dose and mass-based metrics. Calves given tulathromycin had greater average daily gain (ADG, 0.96 ± 0.07 kg vs 0.82 ± 0.07 kg; P = 0.034) and lower prevalence of BRD than controls (17% vs 40%; P = 0.008]. Proportions of calves with BRD pathogens identified at arrival were similar between treatment groups [17%; P = 0.94]. Proportions of calves with BRD pathogens identified at day 14 were lower for calves receiving tulathromycin compared to controls (15% vs 60%, P < 0.001). Overall, 81% of P. multocida isolates and 47% of M. haemolytica isolates were pansusceptible. When measured as regimens per head in, AMU in calves receiving tulathromycin was higher than calves receiving saline (P = 0.01). Under the conditions of this study, metaphylaxis had positive impacts on health and performance of high-risk beef stocker calves, did not contribute to the selection of resistant bacterial isolates in the nasopharynx of treated cattle, and increased AMU.

OBJECTIVE: Evaluate agreement among the antimicrobial susceptibility profiles of Mannheimia haemolytica or Pasteurella multocida obtained by transtracheal wash, nasal swab, nasopharyngeal swab, and bronchoalveolar lavage. ANIMALS: 100 Holstein and Holstein-cross bull calves with bovine respiratory disease. METHODS: Calves > 30 days old with naturally occurring bovine respiratory disease were sampled sequentially by nasal swab, nasopharyngeal swab, transtracheal wash, and then bronchoalveolar lavage. Samples were cultured, and for each antimicrobial, the MIC of 50% and 90% of isolates was calculated, and isolates were categorized as susceptible or not. Categorical discrepancies were recorded. Percent positive agreement and kappa values were calculated between isolates for each of the sampling methods. RESULTS: Antimicrobial susceptibility varied by pathogen and resistance to enrofloxacin, florfenicol, tilmicosin, and spectinomycin was detected. Minor discrepancies were seen in up to 29% of classifications, with enrofloxacin, penicillin, and florfenicol more frequently represented than other drugs. Very major and major discrepancies were seen when comparing florfenicol (1.9%) and tulathromycin (3.8 to 4.9%) across sampling methods. Some variability was seen in agreement for enrofloxacin for several comparisons (8.3 to 18.4%). CLINICAL RELEVANCE: Susceptibility testing of isolates from 1 location of the respiratory tract can reliably represent susceptibility in other locations. Nevertheless, the potential for imperfect agreement between sampling methods does exist. The level of restraint available, the skill level of the person performing the sampling, the age and size of the animal, disease status, and treatment history all must be factored into which test is most appropriate for a given situation.

OBJECTIVE: To identify growth and reproductive measurements that can be used to select heifers with the potential to be more reproductively efficient. SAMPLE: A total of 2,843 heifers consigned to the Georgia Heifer Evaluation and Reproductive Development program between 2012 and 2021 with a mean (min, max) age of heifers at delivery of 347 days (275, 404). PROCEDURES: Reproductive tract maturity score (RTMS), weight at delivery as a percentage of target breeding weight, hip height 3 to 4 weeks after delivery, and average daily gain during the first 3 to 4 weeks after delivery were evaluated as potential predictors of the variables of interest. RESULTS: The model-adjusted odds of pregnancy were 1.40 to 1.67 times higher for heifers with an RTMS of 3, 4, or 5 when compared to heifers with an RTMS of 1 or 2. For every 2.5-cm increase in hip height and every 1-month increase in age at the beginning of the breeding period the model-adjusted odds of pregnancy were 1.10 and 1.16 times higher, respectively. The model-adjusted pregnancy hazard rate for heifers with an RTMS of 3, 4, or 5 was 1.19 to 1.25 times higher than that of heifers with an RTMS of 1 or 2. For every 2.5-cm increase in hip height, the model-adjusted hazard rate for pregnancy was 1.04 times greater. CLINICAL RELEVANCE: Physical traits related to animal maturity and attainment of early puberty can be used to select heifers that are more likely to become pregnant early in their first breeding season.

OBJECTIVE: To describe the causes and distribution of lameness in beef and dairy cattle presenting to either a primary care ambulatory or in-house clinical service at a veterinary teaching hospital. SAMPLE: This retrospective clinical case study was conducted by reviewing hospital admissions of beef and dairy cattle seen by in-house or ambulatory clinicians at a veterinary teaching hospital from 2001 to 2021. PROCEDURES: Final diagnosis was recorded and, when available, the affected limb, whether a lesion was localized to the digit, and the affected claw were also recorded. Data were stratified by location of initial evaluation (ambulatory vs in hospital), period of presentation, production class, age, sex, and whether the final diagnosis was infectious or noninfectious. RESULTS: Overall, 2,220 animals met criteria for inclusion in the study. The most common diagnoses were noninfectious in nature and affected a hind limb more often than a forelimb. More than 82% of all cattle had lameness localized to the digit, with the lateral claw being affected more than 80% of the time. CLINICAL RELEVANCE: Lameness is an important concern from both a humane and economic standpoint. The results of this study will help with the diagnosis and prevention of lameness in beef and dairy cattle of different ages and production classes.

Bovine respiratory disease (BRD) is the most common cause of morbidity and mortality in North American beef cattle. In recent years, isolation of strains of Mannheimia haemolytica that are resistant to multiple different classes of antimicrobials has become commonplace. New research would suggest that the routine use of antimicrobials by some cattle operations might be driving emerging resistance patterns, with the majority of the spread observed due to propagation of strains of M. haemolytica that have acquired integrative conjugative elements. To date, there is little information evaluating the impact of antimicrobial resistance on clinical outcome in cattle with BRD.

Background Extensive drug resistance (XDR) is an emerging concern with Mannheimia haemolytica, and a variety of testing methods are available for characterizing in vitro antimicrobial susceptibility. Objectives To compare the concordance among disk diffusion, broth microdilution, and whole genome sequencing (WGS) for susceptibility testing of M. haemolytica before and after mass treatment using tulathromycin. Animals Forty‐eight M. haemolytica isolates collected from high‐risk beef stocker calves before and after mass treatment (metaphylaxis) using tulathromycin (Draxxin, Zoetis, Parsippany, NJ) given at the label dosage of 2.5 mg/kg body weight SC in the neck. Methods In vitro antimicrobial susceptibility was determined for all 48 isolates using disk diffusion, broth microdilution, and WGS. Concordance was calculated between pairs of susceptibility testing methods as follows: number of isolates classified identically by the 2 testing methods for each timepoint, divided by the number of isolates tested at that timepoint. Discordance was calculated as follows: number of isolates classified differently by the 2 testing methods for each timepoint, divided by the number of isolates tested at that timepoint. Results Concordance between testing methods ranged from 42.3% to 100%, depending on antimicrobial evaluated, timing of sample collection, and testing method used. Very major errors were identified in up to 7.7% of classifications whereas minor errors were seen in up to 50% of classifications depending on antimicrobial evaluated, timing of sample collection, and testing method used. Conclusions and Clinical Importance Our results show that discrepancies in the results of different susceptibility testing methods occur and suggest a need for greater harmonization of susceptibility testing methods.

The primary objective of this study was to evaluate the association between serum metabolic parameters and the risk of bovine respiratory disease (BRD) in high-risk beef stocker calves. Jugular venous blood samples were collected from mixed-breed beef bull, steer, and heifer calves (n=468) at the time of arrival processing at a stocker facility in northeast Georgia. Serum samples were then submitted for determination of serum creatinine, total calcium, phosphorus, magnesium, albumin, serum urea nitrogen, glucose, cholesterol, beta-hydroxybutyrate (BHBA), non-esterified fatty acid (NEFA), sodium, potassium, and chloride concentrations, as well as sodium:potassium ratio and NEFA:cholesterol ratio. Calves were monitored for the development of signs consistent with BRD for 45 days following arrival. A multi-variable logistic regression model was created to evaluate the association between serum variables and subsequent risk of BRD. In this analysis, BRD was associated with higher serum potassium, lower serum urea nitrogen, and lower BHBA concentrations at arrival processing. The area under the receiver operating characteristic curve for the ability of the model to predict morbidity was 0.645. These data suggest that hydration status, nutrient balance, and degree of rumen development may play a role in the development of BRD in high-risk beef stocker calves.

Bovine respiratory disease (BRD) is the most common cause of morbidity and mortality in North American beef cattle. Mannheimia haemolytica (Mh) is the bacterial pathogen most often isolated from cattle with BRD and the prevalence of antimicrobial resistance (AMR) in this organism has increased in recent years. Antimicrobials are commonly used to prevent BRD in cattle at high-risk of developing BRD, however, recent work would suggest that this practice might be one factor contributing to the increased prevalence of AMR in Mh. We hypothesized that the administration of the short-acting fluoroquinolone, enrofloxacin, would be just as effective as the long-acting triamilide, tulathromycin, in preventing BRD but would be less likely to select for AMR Mh in stocker calves at high-risk of developing BRD. Three hundred forty-one stocker calves were enrolled in the study with 172 calves in 4 pens being randomly assigned to treatment with enrofloxacin and 169 calves in 4 pens randomly assigned to treatment with tulathromycin. Calves within each treatment group were allocated to one of 4 replicate pens based on the week of enrollment. Of calves receiving enrofloxacin, 33.7% required treatment for BRD at least once within 45 days after arrival, compared to 18.3% of calves receiving tulathromycin (P = 0.040). The percentages of calves that required more than one treatment for BRD within 45 days after arrival did not differ statistically for those receiving enrofloxacin compared to those receiving tulathromycin (10.5% and 4.7%, respectively; P = 0.107) Likewise, the percentages of calves that died during the 45 day follow-up period did not differ for those receiving enrofloxacin compared to those receiving tulathromycin (12.2% and 10.1%, respectively; P = 0.592). Mannheimia haemolytica was cultured from 11% of calves sampled at arrival, and from 50% of calves sampled at revaccination 14-17 days later. There was a significant effect of sampling time on the proportion of calves carrying multi-drug resistant (MDR) isolates, with calves having a higher prevalence of MDR isolates at revaccination than arrival (100% vs. 13%; P < 0.001). Future research evaluating the impact of MDR on response to antimicrobial therapy is necessary.