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Mannheimia haemolytica  isolated from feedlot cattle were tested for tulathromycin resistance. Cattle were sampled over a 3-year period, starting 12 months after approval of tulathromycin for prevention and treatment of bovine respiratory disease. Nasopharyngeal samples from approximately 5,814 cattle were collected when cattle entered feedlots ( N  = 4) and again from the same cattle after ≥60 days on feed. The antimicrobial use history for each animal was recorded.  Mannheimia haemolytica  was isolated from 796 (13.7%) entry samples and 1,038 (20.6%) ≥ 60 days samples. Of the cattle positive for  M. haemolytica , 18.5, 2.9, and 2.4% were administered therapeutic concentrations of tulathromycin, tilmicosin, or tylosin tartrate, respectively. In addition, 13.2% were administered subtherapeutic concentrations of tylosin phosphate in feed. In years one and two, no tulathromycin-resistant  M. haemolytica  were detected, whereas five isolates (0.4%) were resistant in year three. These resistant isolates were collected from three cattle originating from a single pen, were all serotype 1, and were genetically related (≥89% similarity) according to pulsed-field gel electrophoreses patterns. The five tulathromycin-resistant isolates were multi-drug resistant also exhibiting resistance to oxytetracycline, tilmicosin, ampicillin, or penicillin. The macrolide resistance genes  erm (42),  erm (A),  erm (B),  erm (F),  erm (X) and  msr (E)- mph (E), were not detected in the tulathromycin-resistant  M. haemolytica . This study showed that tulathromycin resistance in  M. haemolytica  from a general population of feedlot cattle in western Canada was low and did not change over a 3-year period after tulathromycin was approved for use in cattle.

Susceptibility to tulathromycin in Mannheimia haemolytica isolated from feedlot cattle over a 3-year period