Influence of Chlortetracycline in Swine Feed on Reproductive Performance and on Incidence and Persistence of Antibiotic Resistant Enteric Bacteria

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Abstract
During a 3 1/2 year period, comparisons were made of the reproductive performance and of the incidence and persistence of antibiotic resistant enteric bacteria between a swine herd fed 55 mg chlortetracycline (CTC)/kg of diet continuously (Coldstream herd) and a herd not fed antibiotics (Princeton herd). Reproductive data involving 612 litters were collected on the University of Kentucky's specific-pathogen-free Yorkshire herd located at Princeton, Kentucky from 1963 to 1975. Antibiotics were used routinely but not continuously in the feed and for treatment purposes as needed until May 1972, after which their use for all purposes was discontinued. From May 1972 through December 1975, CTC was included in all diets of the Coldstream herd located at Lexington, Kentucky. Following antibiotic withdrawal in the Princeton herd, litter size at birth and at weaning declined linearly (P<.01) with time. Conception rate (P<.05), litter size at birth and at 3-week weaning (P<.001), pig survival (P<.001) and average pig weaning weights (P<.01) were significantly reduced in the Princeton herd after antibiotics were discontinued compared with means for the 9-year period prior to antibiotic withdrawal. Post-weaning gains of pigs also were significantly (P<.001) reduced during the period that antibiotics were not fed. Litter size at birth and at weaning was greater (P<.01) in the Coldstream herd fed CTC continuously, compared with that of the Princeton herd not fed antibiotics. Total coliform (P<.05) and CTC-resistant coliform (P<.01) counts were higher in fecal samples from pigs in the Coldstream herd than in the Princeton herd. Total coliform (P<.01) and CTC-resistant coliform (P<.08) counts decreased with length of time pigs in the Princeton herd were fed the antibiotic-free diet, while CTC-resistant coliform counts (P<.05) increased in the Coldstream herd. The percentage of the total coliform population resistant to 25 μg CTC/ml, using the pour plate enumeration method, was 33% in the Princeton herd at the end of the study, while remaining in excess of 80% in the Coldstream herd (P<.001). Using the disc sensitivity method, resistance level to 30 μg tetracycline of enteric bacteria isolated from swine not fed antibiotics for over 42 months exceeded 50%, compared with a level exceeding 90% for swine fed CTC continuously. More than 50% of the fecal isolates from the Coldstream herd were resistant to four or more antimicrobials compared with 21% of the isolates from the Princeton herd. The data indicate that once a resistant level is established, a long time period is required before an appreciable change occurs in the level of resistance. Resistant patterns of most fecal isolates contained tetracycline, penicillin and sulfa-methizole, with many also containing streptomycin, neomycin, kanamycin and ampicillin. Less than 1% of the isolates were resistant to chloramphenicol or gentamicin. Isolates from soil, feed and water were more resistant to ampicillin and cephalothin and less resistant to tetracycline, sulfarnethizole and streptomycin than fecal isolates. Copyright © 1978. American Society of Animal Science. Copyright 1978 by American Society of Animal Science.

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