Randomized field trial comparing the efficacy of florfenicol and oxytetracycline in a natural outbreak of calf pneumonia using lung reaeration as a cure criterion

BACKGROUND: Respiratory infections are the main indication for antimicrobial use in calves. Optimal treatment duration currently is unknown, but shorter duration would likely decrease selection for antimicrobial resistance. HYPOTHESIS/OBJECTIVES: Determine differences in cure rate and healing time between animals treated with florfenicol and oxytetracycline in a natural outbreak of respiratory disease using reaeration observed on thoracic ultrasound examination as healing criterion. ANIMALS: Commercial farm housing 130, 3 to 9 month old Belgian blue beef calves. METHODS: Randomized clinical trial during an outbreak of respiratory disease. Metaphylactic treatment was initiated, randomly treating animals with either florfenicol or oxytetracycline. Ultrasonographic follow‐up was done the first day and every other day for a 14‐day period. At the individual animal level, treatment was discontinued when reaeration of the lungs occurred. Differences in cure rate and healing time were determined. RESULTS: Of the 130 animals studied, 67.7% developed a lung consolidation ≥0.5 cm. The mean ultrasonographic healing time was 2.5 days in the florfenicol group compared to 3.1 days in the oxytetracycline group (P = .04). After single treatment, 80.6% and 60.3% had no consolidations in the florfenicol and oxytetracycline groups, respectively (P = .01). A Mycoplasma bovis strain was genetically and phenotypically determined to be susceptible to both antimicrobials. CONCLUSIONS AND CLINICAL IMPORTANCE: Ultrasonographic lung reaeration shows potential as a cure criterion to rationalize antimicrobial use for outbreaks of pneumonia. In our study, florfenicol resulted in a faster cure and higher reduction in antimicrobial usage than did oxytetracycline.