Effects of tilmicosin phosphate, administered prior to transport or at time of arrival, and feeding of chlortetracycline, after arrival in a feedlot,on Mannheimia haemolytica in nasal secretions of transported steers

Objective-To determine effects of time of administration of tilmicosin and feeding of chlortetracycline on colonization of the nasopharynx of transpor ted cattle by Mannheimia haemolytica (MH). Animals-454 steers (body weight, 200 kg). Procedure-3 studies included 4 truckloads of steers assembled and processed in the southeastern United States. for each truckload of steers, a third r eceived tilmicosin before transportation (PRIOR), then ail were transported to a feedlot in New Mexico (23 hours). Al arrival (day 0), another third r eceived tilmicosin (ARR). The remaining third did not receive tilmicosin (c ontrol steers [CTR]). Steers in studies 1 and 2 were housed in a feedlot, a nd steers in study 3 were housed on wheal pasture. One half of the steers f rom each group in studies 2 and 3 were fed chlortetracycline on days 5 to 9 . Steer with signs of respiratory tract disease were treated. Nasal swab sp ecimens were examined for MH to determine colonization. Results-PRIOR and ARR steers had a lower incidence of respiratory tract dis ease and MH colonization than CTR steers, but PRIOR and ARR steers did not differ. Feeding chlortetracycline did not have an effect. Conclusions and Clinical Relevance-Tilmicosin can inhibit MH from colonizin g the nasopharynx of cattle. Because tilmicosin inhibits the growth of MH i n the respiratory tract, medication with tilmicosin prior to transport shou ld reduce the incidence of acute respiratory tract disease during the first week at the feedlot when calves are most susceptible to infectious organis ms.