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SUMMARY

Milk antimicrobial residues are a serious concern for the dairy industry. Residues of the tetracycline family of antimicrobials have been reported in market milk by investigators, using radioimmunoassay and microbial receptor technology (hereafter referred to as the Charm II test). In response to these reports, an investigation was conducted to determine the potential of 3 extra-label routes of oxytetracycline (otc) administration to cause milk residues above the Food and Drug Administration safe value of 30 parts per billion (ppb). Lactating Holstein cows were administered otc once by use of 1 of 3 routes: iv at 16.5 mg/kg of body weight (n = 6); im at 11 mg/kg (n = 6); and intrauterine (iu) at 2 g in 500 ml of saline solution/cow (n = 6). Duplicate milk samples were collected at the milking prior to drug administration and for the next 13 milkings at 12-hour intervals. Concentrations of otc in milk samples were analyzed by use of the Charm II test for tetracyclines (lmit of otc detection, approx 5 ppb) and were compared with concentrations determined by use of a high-performance lquid chromatography (hplc) method (lower lmit of otc quantitation, approx 2 ppb).

The potential for milk otc residues above the Food and Drug Administration safe value of 30 ppb after treatment was considerably greater for the iv and im routes, compared with the iu route. Mean peak otc concentrations in milk at the first milking after treatment for the hplc and Charm II tests were approximately 3,700 to 4,200 ppb for the iv route, 2,200 to 2,600 ppb for the im route, and 186 to 192 ppb for the iu route, respectively.

Pharmacokinetic analysis, based on milk otc concentrations, indicated that the area under the curve (auc) and milk maximal concentration (Cmax) differed significantly (P < 0.001) among routes of administration. The auc was similar for iv and im administrations; values for both were greater than the auc for iu administration. The Cmax was greatest for iv, intermediate for im, and least for iu administration. There were significant (P ≤ 0.01) differences in auc between assay methods (Charm II vs hplc) for the iv route. Concentrations of otc in milk determined by the Charm II test were often greater than those determined by hplc.

Administration of otc to lactating cows via these routes is extra-label drug use. Failure to withhold the product from early milkings of cows administered otc by the iv or im route should be considered a potential cause of otc residues in market milk. Milk from nearly all cows contained otc (< 30 ppb), the Food and Drug Administration safe level, by 120 hours after otc administration. Use of appropriate withholding times and antibiotic residue testing is indicated to avoid otc residues.

Free access
in American Journal of Veterinary Research

Abstract

Objective

A microbial receptor assay method (MRAM; Charm II test) for β-lactam antibiotics and a liquid chromatography (LC) method with a detection limit of 2 to 5 ppb were evaluated for detection of ampicillin or amoxicillin residues in milk samples from individuell cows.

Design

The MRAM was compared to the LC in 2 respects. Measured concentrations of drugs were compared, as well as the classification of samples relative to the FDA tolerance value of 10 ppb.

Animals

A total of 6 clinically normal lactating Holstein cows were used per drug.

Procedure

Ampicillin trihydrate or amoxicillin trihydrate was administered at an extra-label dosage of 22 mg/kg of body weight, IM, once to each of 6 cows/drug. Milk samples were collected at milkings prior to and for 156 hours after drug administration. Drug concentrations in milk samples from individual cows were determined by use of the MRAM and LC tests. Additionally, the classification of milk samples relative to the presence or absence of residues above the FDA tolerance value was determined. Pharmacokinetic analysis was performed on derived milk drug concentrations.

Results

Concentration of ampicillin in milk samples from all cows was < 10 ppb by the MRAM and LC methods by the fourth milking (48 hours) after treatment with ampicillin. Values were < 10 ppb by both methods for all cows treated with amoxicillin by the sixth milking (72 hours) after treatment. For individual milk samples, significant differences were found between test methods in the proportion of positive (failing) tests; the MRAM had a higher proportion of presumptive positives.

Conclusions

Even at an extra-label dosage of 22 mg/kg, IM, milk residues > 10 ppb (the FDA tolerance value) were not detected beyond the label milk withholding times for ampicillin (48 hours) and amoxicillin (96 hours). When used for testing milk of individual cows by the control point procedure, the MRAM had a tendency to give presumptive positive test results for milk samples containing < 10 ppb ampicillin or amoxicillin as determined by LC. (Am J Vet Res 1996;57:73-78)

Free access
in American Journal of Veterinary Research