The prophylactic administration of
injectable tilmicosin for
pneumonia in weaned beef calves was investigated in 1,806 animals. Comparisons were made among calves receiving an "on-arrival" injection of
tilmicosin, calves receiving a single injection of long-acting
oxytetracycline, and calves receiving no prophylaxis. Morbidity and mortality attributable to
pneumonia, morbidity and mortality attributable to all causes, and case fatality were significantly lower in the group of calves that received
tilmicosin, compared with calves that received long-acting
oxytetracycline and calves that received no prophylactic
antibiotic. Mean time to initial
pneumonia treatment was significantly extended in calves that received prophylaxis, compared with those that received no
antibiotic on arrival at the feedlot. Calves that received
tilmicosin gained significantly more weight than calves that received
oxytetracycline. Calves that were not treated for
pneumonia during the trial period gained significantly more weight than did those calves that were treated for
pneumonia regardless of experimental group. The majority of mortalities were attributable to fibrinous
pneumonia (31/34). Important bacterial isolates (Pasteurella spp, Haemophilus somnus, Actinomyces pyogenes) obtained at necropsy did not have resistance to
tilmicosin in association with administration of
tilmicosin as prophylaxis for
pneumonia. However, bacterial resistance to
trimethoprim/
sulfonamide and to
oxytetracycline were commonly found in these postmortem isolates.