Effect of feeding chlortetracycline or virginiamycin on shedding of salmonellae from experimentally-infected swine.

Swine from a herd routinely fed subtherapeutic levels of chlortetracycline (CTC) were fed a diet containing 55 mg of CTC/kg, a diet containing 55 mg of virginiamycin/kg, or a control diet. All animals were inoculated with Salmonella typhimurium that was susceptible to tetracycline. The quantity, duration and prevalence of shedding of S. typhimurium were determined. The infecting organism was first recovered from the animals fed CTC or the control diet on d 2, from animals fed virginiamycin on d 7 and from animals in a second control group on d 10. The infecting organism was recovered in fewer samples obtained during the initial 7 d postinfection than in those obtained during the last 24 d of the study. Little transfer of resistance to the infecting organism seemed to have occurred from the resident microflora because only two isolates (1%) had resistant patterns that differed from that of the infecting organism. Feeding CTC or virginiamycin to swine did not significantly increase or prolong shedding of an experimentally infected tetracycline-susceptible strain of S. typhimurium. Neither antibiotic affected the drug resistance of the infecting organism.