Prevalence and temporal trends in antimicrobial resistance of bovine respiratory disease pathogen isolates submitted to the Wisconsin Veterinary Diagnostic Laboratory: 2008-2017.

The objective of this study was to describe the prevalence and trends in antimicrobial resistance for bacterial pathogens associated with bovine respiratory disease (BRD) isolated from samples submitted to the Wisconsin Veterinary Diagnostic Laboratory (WVDL). Data were retrospectively collected from bovine respiratory isolates including Pasteurella multocida, Mannheimia haemolytica, Histophilus somni, and Bibersteinia trehalosi identified at the WVDL between January 2008 and December 2017. Antimicrobial susceptibility testing data were queried from antimicrobial resistance databases at the WVDL. A total of 4,261 isolates were identified. Pasteurella multocida was most frequently identified, accounting for 2,094 isolates (49% of total) over the study period. Mannheimia haemolytica was the second most frequently isolated bacterial respiratory pathogen (n = 1,267, 30%) followed by H. somni (n = 749, 18%) and B. trehalosi (n = 151, 4%). Over the 10-yr period, B. trehalosi had the highest median percentage of isolates that were resistant to at least one antibiotic at 33% (interquartile range: 24, 47) followed by M. haemolytica (13%; 8, 23). For P. multocida, 10% (4, 26) of isolates were classified as resistant to at least one antibiotic, whereas H. somni had the fewest resistant isolates (9%; 3, 15). When comparing 2013-2017 to 2008-2012, the overall percentage of resistant isolates for P. multocida and B. trehalosi decreased, whereas the percentage of resistant isolates for M. haemolytica and H. somni increased. Increased resistance against florfenicol, fluoroquinolones, gentamicin, tilmicosin, and tulathromycin was observed for M. haemolytica. These data show that antimicrobial susceptibility for BRD bacterial pathogens has changed in the population served by the WVDL over this 10-yr period. For P. multocida, resistance is relatively low and has either improved or at least remained constant for the majority of drugs labeled for treatment of respiratory disease in dairy cattle. Veterinarians and producers should be aware of the bacterial pathogens most commonly associated with BRD and work toward early disease detection, proper antibiotic administration, and monitoring lung lesions to ensure that their treatment protocols improve lung health.

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

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.