Antimicrobial Sensitivity Testing of Mycoplasma bovis Isolates Derived from Western Canadian Feedlot Cattle

Abstract

Mycoplasma bovis is particularly adept at evading the immune system, resulting in chronic infections of the lungs and joints of feedlot cattle. The chronicity of the lesions results in prolonged antimicrobial therapy, possibly exacerbating antimicrobial resistance. This cross-sectional study generated in vitro antimicrobial susceptibility testing (AST) data on 211 M. bovis isolates recovered from 159 healthy, diseased, and dead cattle, spanning the period of 2006-2018. Nine antimicrobials commonly administered to western Canadian feedlot cattle were assessed. The data were analyzed with non-parametric statistical tests with a level of significance of p < 0.05 (two-tailed). Minimum inhibitory concentration (MIC) values tended to increase between the isolates from healthy versus dead cattle and over time (2006-2018). Isolates from dead versus healthy cattle were more likely to be resistant to tulathromycin, gamithromycin, tylosin and enrofloxacin. There was no difference in the distributions of the MICs generated from the isolates recovered from the lungs and joints (p ≥ 0.124) and the lungs and deep nasal passages (p ≥ 0.157) of the same animals.

Keywords: Canada; Mycoplasma bovis; antimicrobial; beef; feedlot; resistance; susceptibility.

Figure 1

Minimum inhibitory concentration distribution for M. bovis isolates from healthy (n = 43), diseased (n = 19), and dead (n = 97) cattle (n = 159). Tulathromycin (TUL; 0.25–256 µg/mL), gamithromycin (GAM; 0.25–256 µg/mL), tilmicosin (TIL; 1–256 µg/mL), tildipirosin (TIP; 0.12–128 µg/mL), tylosin tartrate (TYL; 1–128 µg/mL), enrofloxacin (ENRO; 0.12–128 µg/mL), florfenicol (FFN; 0.25–256 µg/mL), oxytetracycline (OXY; 0.5–16 µg/mL), chlortetracycline (CTET; 1–256 µg/mL).