Mannheimia haemolytica in feedlot cattle: associations with antimicrobial use, resistance and health outcomes.

Noyes, N.R., Benedict, K.M., Gow, S.P., Booker, C.W., Hannon, S., McAllister, T.A., and Morley, P.S. (2015). "Mannheimia haemolytica in feedlot cattle: associations with antimicrobial use, resistance and health outcomes.", Journal of Veterinary Internal Medicine, 29(2), pp. 705-713. doi : 10.1111/jvim.12547  Access to full text


Background: Mannheimia haemolytica is an important etiological agent in bovine respiratory disease. Objectives: Explore risk factors for recovery of susceptible and resistant M. haemolytica in feedlot cattle and explore associations with health outcomes. Animals: Cattle (n = 5,498) from 4 feedlots sampled at arrival and later in feeding period. Methods: Susceptibility of M. haemolytica isolates tested for 21 antimicrobials. Records of antimicrobial use and health events analyzed using multivariable regression. Results: M. haemolytica recovered from 29% of cattle (1,596/5,498), 13.1% at arrival (95% CI, 12.3–14.1%), and 19.8% at second sampling (95% CI, 18.7–20.9%). Nearly half of study cattle received antimicrobial drugs (AMDs) parenterally, mostly as metaphylactic treatment at arrival. Individual parenteral AMD exposures were associated with decreased recovery of M. haemolytica (OR, 0.2; 95% CI, 0.02–1.2), whereas exposure in penmates was associated with increased recovery (OR, 1.5; 95% CI, 1.05–2.2). Most isolates were pan-susceptible (87.8%; 95% CI, 87.0–89.4%). AMD exposures were not associated with resistance to any single drug. Multiply-resistant isolates were rare (5.9%; 95% CI, 5.1–6.9%), but AMD exposures in pen mates were associated with increased odds of recovering multiply-resistant M. haemolytica (OR, 23.9; 95% CI, 8.4–68.3). Cattle positive for M. haemolytica on arrival were more likely to become ill within 10 days (OR, 1.7; 95% CI, 1.1–2.4). Conclusions and Clinical Importance: Resistance generally was rare in M. haemolytica. Antimicrobial drug exposures in penmates increased the risk of isolating susceptible and multiply-resistant M. haemolytica, a finding that could be explained by contagious spread.

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