Association between antimicrobial drug class for treatment and retreatment of bovine respiratory disease (BRD) and frequency of resistant BRD pathogen isolation from veterinary diagnostic laboratory samples.

Although 90% of BRD relapses are reported to receive retreatment with a different class of antimicrobial, studies examining the impact of antimicrobial selection (i.e. bactericidal or bacteriostatic) on retreatment outcomes and the emergence of antimicrobial resistance (AMR) are deficient in the published literature. This survey was conducted to determine the association between antimicrobial class selection for treatment and retreatment of BRD relapses on antimicrobial susceptibility of Mannheimia haemolytica, Pasteurella multocida, and Histophilus somni. Pathogens were isolated from samples submitted to the Iowa State University Veterinary Diagnostic Laboratory from January 2013 to December 2015. A total of 781 isolates with corresponding animal case histories, including treatment protocols, were included in the analysis. Original susceptibility testing of these isolates for ceftiofur, danofloxacin, enrofloxacin, florfenicol, oxytetracycline, spectinomycin, tilmicosin, and tulathromycin was performed using Clinical and Laboratory Standards Institute guidelines. Data were analyzed using a Bayesian approach to evaluate whether retreatment with antimicrobials of different mechanistic classes (bactericidal or bacteriostatic) increased the probability of resistant BRD pathogen isolation in calves. The posterior distribution we calculated suggests that an increased number of treatments is associated with a greater probability of isolates resistant to at least one antimicrobial. Furthermore, the frequency of resistant BRD bacterial isolates was greater with retreatment using antimicrobials of different mechanistic classes than retreatment with the same class. Specifically, treatment protocols using a bacteriostatic drug first followed by retreatment with a bactericidal drug were associated with a higher frequency of resistant BRD pathogen isolation. In particular, first treatment with tulathromycin (bacteriostatic) followed by ceftiofur (bactericidal) was associated with the highest probability of resistant M. haemolytica among all antimicrobial combinations. These observations suggest that consideration should be given to antimicrobial pharmacodynamics when selecting drugs for retreatment of BRD. However, prospective studies are needed to determine the clinical relevance to antimicrobial stewardship programs in livestock production systems.

Effect of meloxicam and lidocaine administered alone or in combination on indicators of pain and distress during and after knife castration in weaned beef calves.

To assess the effect of meloxicam and lidocaine on indicators of pain associated with castration, forty-eight Angus crossbred beef calves (304 ± 40.5 kg of BW, 7-8 months of age) were used in a 28 day experiment. The experiment consisted of a 2 × 2 factorial design where main factors included provision of analgesia and local anaesthesia. Analgesia consisted of: no-meloxicam (N; n = 24) single s.c. administration of lactated ringer's solution and meloxicam (M; n = 24) single dose of 0.5 mg/kg of s.c. meloxicam. Local anesthesia consisted of: no-lidocaine (R; n = 24) ring block administration of lactated ringer's solution or lidociane (L; n = 24) ring block administration of lidocaine. To yield the following treatments: no meloxicam + no lidocaine (N-R; n = 12), no meloxicam + lidocaine (N-L; n = 12), meloxicam + no lidocaine (M-R; n = 12) and meloxicam + lidocaine (M-L; n = 12). Salivary cortisol concentrations were lower (lidocaine × time effect; P < 0.01) in L calves than R calves 0.5 and 1 hours after castration, while concentrations were lower (meloxicam × time effect; P = 0.02) in M calves than N calves at 2, 4 and 48 hours. The serum amyloid-A concentrations were greater (lidocaine × time effect; P < 0.01) in R calves than L calves on days 1, 3, 21 and 28 after castration. Haptoglobin concentrations were greater (meloxicam × time effect; P = 0.01) in N calves than M calves 24 and 48 hours after castration. Lower (lidocaine effect; P < 0.01) visual analog scale (VAS) scores, leg movement frequencies and head movement distance were observed in L calves than R calves at the time of castration. Escape behaviour during castration was lower (lidocaine effect; P < 0.05) in L calves than R calves based on data captured with accelerometer and head gate devices. Scrotal circumference had a triple interaction (lidocaine × meloxicam × time; P = 0.03), where M-R calves had greater scrotal circumference than M-L calves 28 d after castration, but no differences were observed between both groups and N-R and N-L calves. No differences (P > 0.05) were observed for average daily gain (ADG), weights or feeding behaviour. Overall, both lidocaine and meloxicam reduced physiological and behavioural indicators of pain. Although there was only one meloxicam × lidocaine interaction, lidocaine and meloxicam reduced physiological and behavioural parameters at different time points, which could be more effective at mitigating pain than either drug on its own.