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Journal of Dairy Science Vol. 57 No. 8 944-950
© 1974 by American Dairy Science Association ®
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Use of Antibiotics in a Dairy Herd and Their Effect on Resistance Determinants in Enteric and Environmental Escherichia coli

L. D. Rollins, D. W. Pocurull, H. D. Mercer, R. P. Natzke1 and D. S. Postle2

Food and Drug Administration, Division of Veterinary Medical Research, Beltsville, Maryland 20705
1 Department of Animal Science
2 College of Veterinary Medicine, Cornell University, Ithaca, New York 14850

ABSTRACT

The frequency of antimicrobially resistant Escherichia coli in a dairy herd and its immediate environment was studied. The herd was given doses of 1,000,000 units of penicillin and 1 g of dihydrostreptomycin during a dry-cow mastitis treatment program. Over 1 yr, the incidence of E. coli resistant to dihydrostreptomycin (25 ug/ml) was determined for fecal and environmental samples. Antimicrobial susceptibility patterns and minimum inhibitory concentrations for dihydrostreptomycin were determined on E. coli isolates from these samples. Environmental samples were collected at 2-wk intervals from holding areas, and fecal samples were collected from cows before treatment and at 2 and 6 wk post-treatment. The entire herd was sampled initially and at the end of the test period.

Peaks of resistant E. coli of 10% from both cow and environmental sources had a marked pattern of similarity, indicating an enteric flora-environmental flora interaction. The therapeutic use of antibiotics, including their use in large doses in a dry-cow mastitis treatment program, resulted in minimum effects on resistance and minimum inhibitory concentrations of E coli.

The dairy herds represented the model situation where antibiotics are sued primarily parenterally for therapeutic purposes. Results contrast sharply as compared to drug resistance in the enteric flora of animal herds that conventionally have received antibiotics continuously in feed.






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Copyright © 1974 by the American Dairy Science Association ®.