The effect of antimicrobial feed additives on the colonization of the alimentary tract of chickens by Salmonella typhimurium

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Abstract
Groups of 33 chickens were fed continuously on commercial diets containing feed additives and were infected orally when 4 days old with a nalidixic acid-resistant mutant of S. typhimurium. Concentration of S. typhimurium from their feces and cecal contents was estimated. Avoparcin and lincomycin, like nitrovin and tylosin, favored alimentary tract colonization by S. typhimurium in infected chickens compared to chickens fed on non-medicated diets. Amprolium, monensin, dimetridazole, arsenilic acid and nitro-hydroxyphenylarsonate had no obvious effect on the salmonella excretion pattern. When 5 chickens in each group were experimentally infected to study the effect of feed additives on infectious acquired by contact, avoparcin, lincomycin, nitrovin and tylosin again favored colonization of the alimentary tract with S. typhimurium and so did dimetridazole. Arsenilic acid hindered infection development. Amprolium, monensin and nitro-hydroxyphenylarsonate were without obvious effect. Many chickens fed on diets favoring S. typhimurium colonization were still excreting the microorganisms in their feces when killed at 56 days, the age at which broiler chickens kept under commercial conditions are usually slaughtered.