F08 - Effect of Tulathromycin Metaphylaxis on Mannheimia Haemolytica isolation and Health Outcomes in Stocker Heifers

Abstract: Background:
Bovine respiratory disease (BRD) is commonly controlled by metaphylaxis, but increasing prevalence of antimicrobial resistant (AMR) Mannheimia haemolytica (MH) may decrease efficacy.

Hypothesis/Objectives:
Determine the effect of macrolide metaphylaxis on: 1) morbidity and mortality in stocker cattle over a 10-week period and 2) nasopharyngeal isolation of MH, and MH genotypes.

Animals:
Commercial beef cross heifers (n=335, 232 ± 17.8 kg) purchased from regional auction markets for 4 trials from October 2019 to October 2021 (3 Fall and 1 Spring).

Methods:
Cattle were randomized to receive tulathromycin at 2.5 mg/kg SC (META, n=168) or not (NO META, n=167). Nasopharyngeal swabs (NPS) were obtained at arrival and 3 and 10 weeks later for culture, identification, and MH genotyping. Groups were separated with no contact; any calves requiring additional antimicrobial treatment were moved into separate pastures. After the 3-week sampling, calves were commingled.

Results:
Over all trials, total and BRD morbidity was significantly lower in META animals than NO META (Table 1, Chi-square, P=0.002); however, difference in BRD morbidity was due only to the Spring and Fall 2021 trials (Chi-square, P=0.002 & P=0.037, respectively). There was no difference in mortality, Week 3 and Week 10 MH isolation rate, or the genotypes isolated between treatment groups (Tables 1 & 2).

Conclusions:
BRD morbidity was reduced in calves receiving metaphylaxis, but the effect varied across trials. MH isolation was not decreased in META cattle, suggesting MH persisted despite metaphylaxis. Further investigation of the impact of AMR on MH in cattle receiving metaphylaxis is warranted.