Effect of antibiotic treatment in preweaned Holstein calves after experimental bacterial challenge with Pasteurella multocida

摘要

The primary objective of this randomized controlledchallenge study was to investigate the effect of ampicillinon ultrasonographic (US) lung consolidation afterexperimental challenge with Pasteurella multocida inpreweaned dairy calves. The secondary objectiveswere to determine whether ampicillin affected respiratoryscore, gross consolidation, or the detection of P.multocida in lung tissue at postmortem exam (PME).Holstein bull calves (n = 39) were transported to theUniversity of Wisconsin-Madison School of VeterinaryMedicine isolation facility at the mean (±SD) age of 52± 6 d. After a 7-d acclimation period, 30 calves wereinoculated intratracheally with 10~(10) cfu of ampicillinsensitiveP. multocida. Lung US and respiratory scoringwere performed 2, 6, 12, and 24 h post-challenge, thenUS once daily and respiratory scoring twice daily untild 14. Calves were randomized to receive ampicillin [n= 17, treatment (TX), 6.6 mg/kg i.m. once daily for3 d] or placebo [n = 11, control (CON), saline, equalvolume, i.m. once daily for 3 d] when ≥1 cm~2 of lungconsolidation was observed and ≥6 h had elapsed sincechallenge. Lung lesions ≥1 cm~2 were considered positivefor consolidation. Calves were respiratory score positiveif ≥2 in 2 or more categories based on the Wisconsinrespiratory health score chart. Area under the curve(AUC) was calculated for US score and respiratoryscore as a proxy for time with consolidation and clinicalrespiratory disease, respectively. Gross lung lesionsand pathogens were quantified following PME. At thetime of first treatment, consolidation had developed in28/30 calves (TX, n = 17; CON, n = 11) and 6% (1out of 17) of TX and 9% (1 out of 11) of CON calveshad a positive respiratory score. The TX calves had asignificantly lower median (interquartile range given inparentheses) AUC for US score [TX: 23 (20, 29), CON:47 (33, 53)], whereas mean AUC for respiratory scorewas not different between groups (TX: 93 ± 28, CON:96 ± 17). On d 14, 70% (12 out of 17) of TX and 100%(11 out of 11) of CON calves had lung consolidation,and 24% (4 out of 17) of TX and 27% (3 out of 11) ofCON calves had clinical respiratory disease. On PME,median consolidation was 10% (6, 15) for TX and 10%(2, 28) for CON calves. Lung cultures were positive forP. multocida in 77% (13 out of 17) of TX and 91% (10out of 11) of CON calves. Lung health benefited froma 3-d ampicillin therapy, but benefits were short-lived.Treatment failures might be due to incomplete resolutionof the initial lung infection. Future studies areneeded to optimize TX strategies to improve long-termlung health.