Prevalence and antimicrobial susceptibility of Mycoplasma bovis and Pasteurella multocida isolated from Albertan feedlot cattle

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Sultana, Razia
University of Lethbridge. Faculty of Arts and Science
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Lethbridge, Alta. : University of Lethbridge, Dept. of Biological Sciences
Bovine respiratory disease (BRD) is a significant health problem for the Canadian feedlot industry. While often polymicrobial in nature, Mycoplasma bovis and Pasteurella multocida are considered important respiratory pathogens for BRD. This study aimed to evaluate the prevalence and antimicrobial resistance of M. bovis and P. multocida isolated from Albertan feedlot cattle that were sampled 8 years apart. In the first study, nasopharyngeal swabs from cattle sampled at feedlot entry and after 60 days on feed were collected in 2008-2009 (Cohort 1). In a second study conducted in 2015-2016 (Cohort 2), nasopharyngeal swabs were collected from cattle diagnosed with BRD and matching healthy controls. Trans-tracheal samples were also collected from Cohort 2 cattle for M. bovis evaluation. For Cohort 1, the prevalence of M. bovis was lower in cattle at entry compared to when the same individuals were sampled ≥60 days later (P<0.05). For Cohort 2, the prevalence of M. bovis was greater in both nasopharyngeal and tracheal samples from cattle diagnosed with BRD, compared to controls (P<0.05). Similarly, P. multocida was more frequently isolated from the nasopharynx of BRD cases. Antimicrobial-resistant patterns changed broadly over the 8-year time period with resistance being lower (P<0.05) in Cohort 1 bacteria for florfenicol and tulathromycin. When evaluated for resistance genes by PCR, the majority (98%) of oxytetracycline-resistant isolates carried tet(H) while only 16 (15%) out of 106 tulathromycin-resistant isolates from Cohort 2 carried a known macrolide resistance gene. The genomes of nine tulathromycin-resistant isolates were sequenced, leading to the identification of a conserved gene cluster that was present in all isolates with tulathromycin-resistance but unknown macrolide resistance genes. One of the genes was a novel putative methylase, which doubled the minimum inhibitory concentration against tulathromycin when cloned into Escherichia coli. This study showed that macrolide resistance in M. bovis and P. multocida increased over an 8-year span, coinciding with the approval and adoption of tulathromycin to prevent BRD in Canadian cattle. Additionally, a novel putative macrolide resistance gene was identified and shown to be widespread in P. multocida from the feedlots enrolled in this study. The rapid development of resistance to a newly used antimicrobial indicates the need to reserve essential antimicrobials for the treatment of cattle, to maintain their efficacy.
bovine respiratory disease , Micoplasma bovis , Pasteurella multocida , feedlot cattle , antimicrobial resistance , macrolide resistance