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Abstract

The fluoroquinolone antimicrobial agents, enrofloxacin and marbofloxacin, were US Food and Drug Administration (FDA) approved in the United States for use in dogs in 1988 and 1999, respectively. There have been many advances since then concerning the pharmacokinetic-pharmacodynamic (PK-PD) evaluation of fluoroquinolones, and there are data available on the susceptibility of targeted pathogens since the original approval. Using this information, the Clinical and Laboratory Standards Institute (CLSI) Veterinary Antimicrobial Susceptibility Testing Subcommittee (VAST) revised its antimicrobial susceptibility testing breakpoints. The previous breakpoints (used in older editions of CLSI standards) for enrofloxacin in dogs were susceptible (S), ≤ 0.5 µg/mL, intermediate (I) 1–2 µg/mL, and resistant (R) ≥ 4 µg/mL. The new breakpoints are S ≤ 0.06 µg/mL for a dose of 5 mg/kg, 0.12 µg/mL for a dose of 10 mg/kg, 0.25 µg/mL for a high dose of 20 mg/kg, and R ≥ 0.5 µg/mL. The breakpoints of 0.12 and 0.25 µg/mL represent a new susceptible-dose dependent (SDD) category. For marbofloxacin, previous breakpoints were S, ≤ 1 µg/mL, I 2 µg/mL, and R ≥ 4 µg/mL. The new breakpoints are S ≤ 0.12 µg/mL for a dose of 2.8 mg/kg, 0.25 µg/mL for a dose of 5.5 mg/kg (SDD), and R ≥ 0.5 µg/mL. The new breakpoints will be published in the next edition of CLSI-Vet01(S) and deviate considerably from the prior breakpoints. Laboratories are encouraged to revise their testing standards. These changes will likely reduce the unnecessary use of these fluoroquinolones in dogs.

Abstract

Objective—To evaluate the impact of oxytetracycline exposure on horizontal transfer of an antimicrobial resistance plasmid.

Sample—Populations of Salmonella enterica subsp enterica serovar Typhimurium and Escherichia coli.

Procedures—Mixed populations of plasmid donor (Salmonella Typhimurium) and recipient (E coli) bacteria were assigned to 1 of 2 simulated oxytetracycline dosing regimens (high peak concentration-short elimination half-life [HC-SHL] or low peak concentration—long elimination half-life [LC-LHL]) or served as untreated control replicates. Donor, recipient, and transconjugant (E coli that acquired the plasmid) bacteria populations were quantified at 12, 24, and 36 hours after oxytetracycline administration by use of culture on selective bacterial growth media.

Results—The ratio of transconjugant to donor bacteria was significantly reduced in the oxytetracycline-exposed replicates, compared with the ratio for the control replicates, at 12 hours. At 24 and 36 hours, results for the HC-SHL regimens were not significantly different from results for the respective control replicates, and results for the LC-LHL regimens also were not significantly different from results for the respective control replicates. The oxytetracycline concentration at these time points (12 hours in the HC-SHL regimen and all 3 time points in the LC-LHL regimen) were in excess of the minimum inhibitory concentration of the recipient bacteria.

Conclusions and Clinical Relevance—Transfer of antimicrobial resistance plasmids may be suppressed in vitro by oxytetracycline exposure at concentrations greater than the minimum inhibitory concentration of the recipient bacteria.

Abstract

Objective—To evaluate plasma concentrations of substance P (SP) and cortisol in calves after castration or simulated castration.

Animals—10 Angus-crossbred calves.

Procedures—Calves were acclimated for 5 days, assigned to a block on the basis of scrotal circumference, and randomly assigned to a castrated or simulated-castrated (control) group. Blood samples were collected twice before, at the time of (0 hours), and at several times points after castration or simulated castration. Vocalization and attitude scores were determined at time of castration or simulated castration. Plasma concentrations of SP and cortisol were determined by use of competitive and chemiluminescent enzyme immunoassays, respectively. Data were analyzed by use of repeated-measures analysis with a mixed model.

Results—Mean ± SEM cortisol concentration in castrated calves (78.88 ± 10.07 nmol/L) was similar to that in uncastrated control calves (73.01 ± 10.07 nmol/L). However, mean SP concentration in castrated calves (506.43 ± 38.11 pg/mL) was significantly higher than the concentration in control calves (386.42 ± 40.09 pg/mL). Mean cortisol concentration in calves with vocalization scores of 0 was not significantly different from the concentration in calves with vocalization scores of 3. However, calves with vocalization scores of 3 had significantly higher SP concentrations, compared with SP concentrations for calves with vocalization scores of 0.

Conclusions and Clinical Relevance—Similar cortisol concentrations were measured in castrated and control calves. A significant increase in plasma concentrations of SP after castration suggested a likely association with nociception. These results may affect assessment of animal well-being in livestock production systems.

Abstract

Objective—To optimize methods for the use of computed tomography (CT) to assess pathologic changes in the lungs of calves and to determine the effect of treatment on lung consolidation.

Animals—10 male Holstein calves.

Procedures—Calves were anesthetized to facilitate CT imaging of the thorax. After initial images were obtained, pneumonia was induced in the calves by inoculation through a bronchoscope. Two calves were used in a preliminary study to refine the inoculation dose and optimize CT images. Four calves were administered florfenicol and 4 calves were untreated control animals. Serial images were obtained 24, 48, and 72 hours after inoculation. After final images were obtained, calves were euthanized, and lung consolidation was estimated by use of lung surface area scoring and water displacement. These estimates were compared with estimated lung consolidation obtained by use of CT.

Results—Calves had rapid disease progression. Percentage of lung consolidation was not significantly different between treatment groups for any of the estimation methods. Results of an ANOVA of the 3 assessment methods indicated significant differences among methods. Estimates of the percentage of lung consolidation obtained by use of surface area scoring and CT correlated well, whereas water displacement estimates correlated poorly with other methods of consolidation estimation.

Conclusions and Clinical Relevance—Because of the correlation with other methods for estimation of lung consolidation, CT has the potential to be used to monitor disease progression in calves with experimentally induced respiratory tract disease.

Abstract

OBJECTIVE To compare predictive values, extent of agreement, and gamithromycin susceptibility between bacterial culture results of nasopharyngeal swab (NPS) and bronchoalveolar lavage fluid (BALF) samples obtained from calves with bovine respiratory disease (BRD).

ANIMALS 28 beef calves with clinical BRD.

PROCEDURES Pooled bilateral NPS samples and BALF samples were obtained for bacterial culture from calves immediately before and at various times during the 5 days after gamithromycin (6 mg/kg, SC, once) administration. For each culture-positive sample, up to 12 Mannheimia haemolytica, 6 Pasteurella multocida, and 6 Histophilus somni colonies underwent gamithromycin susceptibility testing. Whole-genome sequencing was performed on all M haemolytica isolates. For paired NPS and BALF samples collected 5 days after gamithromycin administration, the positive and negative predictive values for culture results of NPS samples relative to those of BALF samples and the extent of agreement between the sampling methods were determined.

RESULTS Positive and negative predictive values of NPS samples were 67% and 100% for M haemolytica, 75% and 100% for P multocida, and 100% and 96% for H somni. Extent of agreement between results for NPS and BALF samples was substantial for M haemolytica (κ, 0.71) and H somni (κ, 0.78) and almost perfect for P multocida (κ, 0.81). Gamithromycin susceptibility varied within the same sample and between paired NPS and BALF samples.

CONCLUSIONS AND CLINICAL RELEVANCE Results indicated culture results of NPS and BALF samples from calves with BRD should be interpreted cautiously considering disease prevalence within the population, sample collection relative to antimicrobial administration, and limitations of diagnostic testing methods.

Abstract

Antibiograms are important tools for antimicrobial stewardship that are often underutilized in veterinary medicine. Antibiograms summarize cumulative antimicrobial susceptibility testing (AST) data for specific pathogens over a defined time period; in veterinary medicine, they are often stratified by host species and site of infection. They can aid practitioners with empiric therapy choices and assessment of antimicrobial resistance trends within a population in support of one-health goals for antimicrobial stewardship. For optimal application, consideration must be given to the number of isolates used, the timeframe of sample collection, laboratory analytical methodology, and the patient population contributing to the data (eg, treatment history, geographic region, and production type). There are several limitations to veterinary antibiograms, including a lack of breakpoint availability for bacterial species, a lack of standardization of laboratory methodology and technology for culture and AST, and a lack of funding to staff veterinary diagnostic laboratories at a level that supports antibiogram development and education. It is vital that veterinarians who use antibiograms understand how to apply them in practice and receive relevant information pertaining to the data to utilize the most appropriate antibiogram for their patients. This paper explores the benefits and challenges of developing and using veterinary antibiograms and proposes strategies to enhance their applicability and accuracy. Further detail regarding the application of veterinary antibiograms by privately practicing clinicians is addressed in the companion Currents in One Health article by Lorenz et al (JAVMA, September 2023).