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Antimicrobial susceptibility testing (AST) is an important component of antimicrobial stewardship. This article discusses how AST methods and breakpoints are developed, describes when AST may or may not be useful in clinical practice, and discusses how to interpret AST results from bacterial isolates from cattle, sheep, and goats. Discussion of when AST is not appropriate or when veterinarians should have low confidence in AST results is also included. Applicability of genomic testing for antimicrobial susceptibility is briefly addressed., (Copyright © 2022 Elsevier Inc. All rights reserved.)

Cephalosporin antimicrobials can be utilized for the treatment of sepsis in neonatal foals, particularly when an aminoglycoside is contraindicated. Some cephalosporins, however, are not utilized because of cost, sporadic availability, or uncertainty about efficacy. The plasma disposition of ceftazidime, a third-generation cephalosporin with a broad spectrum of activity against a wide variety of gram-negative bacteria and minimal renal side effects has not been reported in neonatal foals. In this study, the plasma disposition of single intravenous (IV) and intramuscular (IM) doses of ceftazidime in neonatal foals was determined. Six healthy one to two-day-old foals were given 25 mg/kg of ceftazidime by IV and IM routes in a cross-over design, with a 48-h washout period between doses. Non-compartmental analysis was used to estimate plasma pharmacokinetic parameters. Median t 1/2 was 2 h and median AUC 0-last was 364 µg h/ml for both IV and IM administration. Median C max after IM administration was 101 µg/ml, with a median T max of 0.7 h. Relative bioavailability of IM injection was 90%. There were no statistically significant differences between estimated IV and IM pharmacokinetic parameters. Plasma concentrations remained above the human CLSI susceptible breakpoint for Enterobacteriaceae for over 8 h following IV and IM administration., (© 2021 John Wiley & Sons Ltd.)

Macrolides are important antimicrobials frequently used in human and veterinary medicine in the treatment of pregnant women and pregnant livestock. They may be useful for the control of infectious ovine abortion, which has economic, animal health, and human health impacts. In this study, catheters were surgically placed in the fetal vasculature and amnion of pregnant ewes at 115 (±2) days of gestation. Ewes were given a single dose of 2.5 mg/kg tulathromycin subcutaneously, and drug concentrations were determined in fetal plasma, maternal plasma, and amniotic fluid at 4, 8, 12, 24, 36, 48, 72, 144, and 288 hr after drug administration. Pharmacokinetic parameters in maternal plasma were estimated using noncompartmental analysis and were similar to those previously reported in nonpregnant ewes. Tulathromycin was present in fetal plasma and amniotic fluid, indicating therapeutic potential for use against organisms in these compartments, though concentrations were lower than those in maternal plasma. Time-course of drug concentrations in the fetus was quite different than that in the ewe, with plasma concentrations reaching a plateau at 4 hr and remaining at this concentration for the remainder of the sampling period (288 hr), raising questions about how tulathromycin may be transported into or metabolized and eliminated by the fetus., (© 2019 John Wiley & Sons Ltd.)

Objective: To determine oxytetracycline concentrations in plasma and in fluid from Corynebacterium pseudotuberculosis (CPT)-inoculated tissue chambers (used as experimental abscess models) and uninoculated (control) tissue chambers in sheep after IM or local administration of the drug and to investigate whether CPT growth was reduced or eliminated by these treatments., Animals: 10 clinically normal female sheep., Procedures: Sterile tissue chambers were surgically implanted in both paralumbar fossae of each sheep; ≥ 2 weeks later (day -6), 1 randomly selected chamber was inoculated with CPT, and the opposite chamber was injected with sterile growth medium. Sheep received oxytetracycline IM (n = 5) or by percutaneous injection into CPT-inoculated (4) or uninoculated (1) chambers on day 0. Tissue fluid from each chamber and venous blood samples for plasma collection were obtained at predetermined times over 6 days for bacterial counts (tissue chambers) and analysis of oxytetracycline concentrations (tissue chambers and plasma). Sheep were euthanized on day 6. Regional lymph nodes were collected bilaterally from each sheep for culture., Results: Measurable concentrations of oxytetracycline were present in each chamber throughout the study, regardless of administration route or presence of CPT. No CPT growth was detected after the 48-hour time point in inoculated chambers injected with oxytetracycline; however, CPT was isolated from all inoculated chambers throughout the study after IM drug administration. One regional lymph node (ipsilateral to a CPT-inoculated, oxytetracycline-injected chamber with no CPT growth after 48 hours) was culture positive for CPT., Conclusions and Clinical Relevance: Intralesional administration of oxytetracycline may eliminate growth of CPT locally, but complete elimination of the organism remains difficult.

Objective: To compare apparent prevalence and patterns of antimicrobial resistance in Campylobacter spp in feces collected from pigs reared with antimicrobial-free versus conventional production methods in 8 states in the Midwestern United States., Design: Cross-sectional study., Sample Population: 95 swine farms that used antimicrobial-free (n = 35) or conventional (60) production methods., Procedures: Fecal samples from 15 pigs/farm were collected. Biochemical and multiplex-PCR analyses were used to identify Campylobacter spp. The minimal inhibitory concentrations of erythromycin, azithromycin, ciprofloxacin, nalidixic acid, gentamicin, and tetracycline for these organisms were determined by use of a commercially available antimicrobial gradient strip. The data were analyzed by use of population-averaged statistical models., Results: Campylobacter spp were isolated from 512 of 1,422 pigs. A subset (n = 464) of the 512 isolates was available for antimicrobial susceptibility testing. The apparent prevalence of Campylobacter spp isolates from pigs on conventional farms (35.8%) and antimicrobial-free farms (36.4%) did not differ significantly. Resistances to azithromycin, erythromycin, and tetracycline were significantly higher on conventional farms (70.0%, 68.3%, and 74.5%, respectively) than antimicrobial-free farms (20.1%, 21.3%, and 48.8%, respectively). Resistances to azithromycin, erythromycin, and tetracycline declined as the number of years that a farm was antimicrobial-free increased., Conclusions and Clinical Relevance: Production method did not affect the apparent prevalence of Campylobacter spp on swine farms. However, antimicrobial-free farms had a significantly lower prevalence of antimicrobial resistance. Although cessation of antimicrobial drug use will lower resistance over time, investigation of other interventions designed to reduce resistance levels is warranted.