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

Abstract

The primary objective of this randomized controlled challenge study was to investigate the effect of ampicillin on ultrasonographic (US) lung consolidation after experimental challenge with Pasteurella multocida in preweaned dairy calves. The secondary objectives were to determine whether ampicillin affected respiratory score, gross consolidation, or the detection of P. multocida in lung tissue at postmortem exam (PME). Holstein bull calves (n = 39) were transported to the University of Wisconsin-Madison School of Veterinary Medicine isolation facility at the mean (±SD) age of 52 ± 6 d. After a 7-d acclimation period, 30 calves were inoculated intratracheally with 10¹⁰ cfu of ampicillin-sensitive P. multocida. Lung US and respiratory scoring were performed 2, 6, 12, and 24 h post-challenge, then US once daily and respiratory scoring twice daily until d 14. Calves were randomized to receive ampicillin [n = 17, treatment (TX), 6.6 mg/kg i.m. once daily for 3 d] or placebo [n = 11, control (CON), saline, equal volume, i.m. once daily for 3 d] when ≥1 cm² of lung consolidation was observed and ≥6 h had elapsed since challenge. Lung lesions ≥1 cm² were considered positive for consolidation. Calves were respiratory score positive if ≥2 in 2 or more categories based on the Wisconsin respiratory health score chart. Area under the curve (AUC) was calculated for US score and respiratory score as a proxy for time with consolidation and clinical respiratory disease, respectively. Gross lung lesions and pathogens were quantified following PME. At the time of first treatment, consolidation had developed in 28/30 calves (TX, n = 17; CON, n = 11) and 6% (1 out of 17) of TX and 9% (1 out of 11) of CON calves had a positive respiratory score. The TX calves had a significantly lower median (interquartile range given in parentheses) AUC for US score [TX: 23 (20, 29), CON: 47 (33, 53)], whereas mean AUC for respiratory score was 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) of CON 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 for P. multocida in 77% (13 out of 17) of TX and 91% (10 out of 11) of CON calves. Lung health benefited from a 3-d ampicillin therapy, but benefits were short-lived. Treatment failures might be due to incomplete resolution of the initial lung infection. Future studies are needed to optimize TX strategies to improve long-term lung health.

Keywords: Pasteurella multocida; ampicillin; bovine respiratory disease; lung ultrasound.

Figure 1

Challenge study timeline. Triangles represent timing of deep nasopharyngeal swab collection. The first swabs were collected upon arrival to the livestock laboratory, 7 d before challenge. The second set of swabs was collected before challenge on d −1. The third set was collected at the time of the calf's first treatment, which occurred when at least 1 cm² of consolidation was observed and ≥6 h had elapsed since challenge (denoted by asterisk). The fourth set of swabs was collected following final examination on d 14, before euthanasia. US = lung ultrasound; CRS = clinical respiratory score.

Figure 2

Boxplots of ultrasound scores and total respiratory scores at each examination throughout the study period. The line within each box is the median ultrasound score; the lines forming the bottom and top of each box are the 25th and 75th percentiles, respectively (interquartile range). Whiskers represent the minimum and maximum data points that fall within the range of 1.5× the interquartile distance below the lower quartile and above the upper quartile. Outliers are represented by open circles. The black arrows indicate when experimental challenge using Pasteurella multocida occurred.

Figure 3

Proportion of calves with lobar pneumonia diagnosed by ultrasonographic score greater than 2 (USS; A) and positive clinical respiratory scores (CRS+; B) throughout the study period. Exams were performed on the day before challenge (−1 d), and 2, 6, and 12 h post-challenge, then once daily until d 14 of the study. Calves were randomized to receive ampicillin (TX; n = 17; 6.6 mg/kg i.m. once daily for 3 d) or placebo (CON; n = 11; saline, equal volume, i.m. once daily for 3 d) when ≥1 cm² of lung consolidation was observed and ≥6 h had elapsed since challenge. Calves were challenged intratracheally with 10¹⁰ cfu of Pasteurella multocida. Dark gray bars = CON; light gray bars = TX.

Figure 4

(A) Typical cranioventral distribution of consolidation in the left lung (top, white outline), and lobar consolidation of the right middle lung lobe (bottom, white outline). (B) In situ specimen. Carcass is in left lateral recumbency. The right 4th rib is being reflected dorsally, exposing a fibrous adhesion between the lung and thoracic body wall (arrow).