Comparison of Therapeutic Versus Subtherapeutic Levels of Chlortetracycline in the Diet for Selection of Resistant Salmonella in Experimentally Challenged Chickens

Abstract

Sixteen broiler chicks per group were fed chlortetracycline1 (CTC) at 0 (control) and 55, 110, and 220 ppm (subtherapeutic levels) continuously for 44 days or 550 ppm (therapeutic level) for three 5-day periods from 1 to 19 days of age. All birds were challenged at 4 days of age with a 10:1 mixture of CTC-sensitive and resistant (CTCr) Salmonella typhimurium. Chicks were sampled periodically through postchallenge day (PCD) 41, half in each pen by cloacal swabbing and the remainder by collection of droppings. Escherichia coli was monitored at PCD 6 and 34.Salmonella recovery from cloacal swabs indicated increased (P< .05) prevalence of CTCr salmonella-positive birds in the 550 ppm treatment at PCD 6 and 13 compared to all other treatments, and at PCD 27 compared to 0, 55, and 110 ppm CTC. Mean recovery scores followed a similar pattern. Area under the curve analysis of CTCr salmonella scores from cloacal swabs for PCD 3 to 41 confirmed increased (P<.05) selection for CTCr salmonella by 550 ppm CTC. Isolations from droppings showed increased (P<.05) CTCr salmonella prevalence at PCD 20 for the 100 ppm treatment group compared to control, and at PCD 34 for the 110 ppm groups compared to all other groups and for 220 and 550 ppm groups compared to 0 and 55 ppm CTC birds. The CTCr salmonella counts in droppings were higher (P<.05) at PCD 34 for the 110, 220, and 550 ppm groups compared to the control. The CTCrE. coli counts were higher (P<.05) in all medicated groups compared to control at PCD 6, but were not different among groups at PCD 34. Overall, short-term feeding of a high level of CTC to chickens selected as much or more CTC salmonella than did continuous feeding with subtherapeutic levels.