Effect of Chlortetracycline Feeding on the Salmonella Reservoir in Chickens

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Abstract
An experimental model was developed to study the influence of antibiotic feeding on the salmonella reservoir in chickens. This model had the features of being very sensitive, allowing for the recovery of very low numbers of infecting salmonella, and, as was the case in the first experiment, closely simulated natural conditions in permitting the spread of salmonella from non‐medicated carrier birds to their penmates. The data from both experiments demonstrate clearly that feeding a sub‐therapeutic level of chlortetracycline did not increase the numbers of salmonella being shed into the environment. In fact, the salmonella pool was significantly reduced in chickens fed chlortetracycline at 200 g/ton. In the first experiment, conducted under more natural conditions, not only did the continuous use of chlortetracycline markedly reduce the spread of salmonella to a non‐inoculated population of broiler chickens but it failed to cause the emergence of chlortetracycline‐resistant salmonella. In the second experiment, where medicated feed was administered 6 days before the birds were individually infected with large numbers of salmonella, resistant organisms did appear; however these conditions did not lead to increased numbers of salmonella being shed.