Abstract: Background: Bovine respiratory disease (BRD) is commonly controlled by metaphylaxis; increasing prevalence of antimicrobial resistant (AMR) Mannheimia haemolytica (MH) may decrease efficacy.
Hypothesis/Objectives: Determine effect of tulathromycin metaphylaxis on: 1) AMR of MH isolated, and 2) health of high-risk stocker calves at 3 (WK3) and 10 (WK10) weeks.
Animals: Auction-derived beef cross heifers (n=335, 232 ± 17.8 kg) purchased for 4 trials from 2019 to 2021
Methods: Cattle were randomized and administered tulathromycin at 2.5 mg/kg SC (META, n=168) or not (NO-META, n=167). Nasopharyngeal swabs were obtained at arrival, WK3, and WK10 for culture, susceptibility testing, whole genome sequencing (WGS), and AMR gene identification. Isolates were multidrug resistant (MDR) if they were not susceptible to \geq\ 3 antimicrobial classes.
Results: BRD morbidity was significantly lower in META (14.9%) animals than NO-META (29.3%) (P=0.002) but there was no difference in WK3 or WK 10 MH isolation between groups. Odds of MDR MH recovery was significantly higher in META at WK3 and WK10 (WK3:OR=12.62, 95%CI=5.42-29.40, P< 0.0001; WK10:OR=5.95, 95%CI=1.35-26.27, P=0.019) compared to NO-META, and treated animals at WK3 (OR=6.00, 95%CI=2.36-15.26, P=0.0002) compared to healthy. AMR genes were identified in more META (100%, n=75) compared to NO-META (52%, n=31/60) isolates in WK3. Metagenomic assessment of swabs is underway.
Conclusions: Tulathromycin metaphylaxis was associated with increased MDR MH isolation in high-risk heifers at WK3 and WK10. Isolation of MH was not decreased in META cattle. Metaphylaxis reduced morbidity, possibly through non-antimicrobial mechanisms; work investigating such mechanisms could guide approaches that decrease BRD without increasing AMR.
Learning Objectives:
Upon completion, participants will be able to describe the difference in prevalence of multidrug antimicrobial resistant Mannheimia haemolytica isolated from stocker cattle that receive tulathromycin metaphylaxis and cattle that do not.
Upon completion, participants will be able to describe the fact that multidrug antimicrobial resistance is linked to expression of integrative conjugative elements (ICE) in Mannheimia haemolytica.
Upon completion, participants will be able to describe the fact that stocker cattle treated with tulathroycin metaphylaxis have lower prevalence of clinical respiratory disease than cattle that do not receive metaphylaxis.
