Antimicrobial use in feedlot calves: its association with culture rates and antimicrobial susceptibility.

Data were collected on anti-microbial usage and health problems, in beef feedlot calves. Although the association between them was not significant, many feedlot owners did not use recommended dosages or duration of treatment and this may have led to a reduced recovery rate.Injectable tetracyclines were the most frequent antimicrobial for primary treatment of sick calves; whereas chloramphenicol was selected most frequently when the primary treatment appeared to be ineffective. Treatment with anti-microbials reduced the likelihood of isolating both Pasteurella multocida and Haemophilus somnus. Chloramphenicol therapy reduced the likelihood of isolating Pasteurella haemolytica. Therapy with a particular antimicrobial, in the week prior to death, increased the level of resistance in P. haemolytica to that antimicrobial. Treatment with other antimicrobials also increased the level of resistance to that antimicrobial; although to a lesser degree. The lowest levels of resistance were observed in Pasteurella isolated from nontreated cattle. After adjustment for antimicrobial exposure, resistance to penicillin, tetracyclines and chloramphenicol occurred together more frequently than expected by chance alone.