Bovine respiratory disease (BRD) causes high morbidity and mortality in beef cattle worldwide. Antimicrobial resistance (AMR) monitoring of BRD pathogens is critical to promote antimicrobial stewardship in veterinary medicine for treatment and control of BRD. In this study, the antimicrobial susceptibility of BRD pathogens - Pasteurella multicoda (Pm), Mannheimia haemolytica (Mh) and Histophilus somni (Hs) - was determined to 18 antimicrobials including those commonly used in the treatment of cases in Australian feedlots including macrolides, tetracyclines and ceftiofur.
BRD pathogens were obtained from Veterinary Diagnostic Laboratories and direct submissions from large feedlots in two states from 2014-2018. A total of 55 Mh, 75 Pm and 35 Hs, were identified using MALDI-TOF MS. The BRD pathogens underwent minimum inhibitory concentration (MIC) testing to screen for antimicrobial resistance using the Sensititre system and MICs were interpreted using CLSI VET08 (CLSI, 2018). Resistant isolates were subtyped using Random Amplified Polymorphic DNA (RAPD-PCR) assay using arbitrary primers to resolve clonality.
Antimicrobial susceptibility testing confirmed that all Mh and Hs isolates from 2014-2018 were susceptible to all antimicrobials tested. All Pm isolates from 2014-2015 were susceptible to all tested antimicrobials. Resistance to at least one antimicrobial (ampicillin, oxytetracycline, penicillin, tilmicosin and tulathromycin) increased to 12.5% and 17.4% for the 2016-2017 and 2018 isolates, respectively. The nine resistant Pm isolates belonged to three different RAPD patterns.
An increased proportion of resistance was observed over the sampling period and tulathromycin resistance was observed for the first time in Pm from cattle in Australia. The two isolates that were resistant to tulathromycin belonged to the same clonal lineage, but were from QLD and SA, respectively. Tulathromycin is commonly used for treatment of BRD in Australia, hence the use of a regular antimicrobial resistance surveillance program for beef feedlots is necessary to detect changes in the susceptibility patterns for detection of emerging types of resistance and effective treatment of BRD.