Influence of subtherapeutic levels of a combination of neomycin and oxytetracycline on Salmonella typhimurium in swine, calves, and chickens.

Subtherapeutic levels of oxytetracycline plus neomycin in animal feeds did not bring about increases in the quantity, prevalence, or shedding of Salmonella typhimurium in swine, calves, or chickens. In fact, the medication generally reduced the proportion of animals carrying S. typhimurium. The medicated groups were fed rations containing oxytetracycline plus neomycin commencing 5 days prior to oral inoculation with S. typhimurium and continuing through a 28-day postinoculation period. Colonization of S. typhimurium occurred in all three animal species, as evidenced by clinical signs of infection and/or colony counts in feces. Only from swine and on only one occasion was a single resistant colony isolated. It is concluded that no evidence has been obtained which would implicate the continuous low-level feeding of oxytetracycline and neomycin for a 4-week period to a potential increased incidence of disease in animals or as a hazard to humans.