Ampicillin and amoxicillin residue detection in milk, using microbial receptor assay (Charm II) and liquid chromatography methods, after extra-label administration of the drugs to lactating cows.

OBJECTIVE: A microbial receptor assay method (MRAM; Charm II test) for beta-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 individual 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.

Subclinical mastitis and milk production in primiparous Simmental cows.

The prevalence of subclinical mastitis in beef cows was investigated and its relationship with milk production and 205-d adjusted calf weaning weights (ADJWW) determined. Primiparous Simmental cows (n = 25) were milked six times at approximately 30-d intervals. Before each milking, milk samples were aseptically collected from each quarter for bacteriological analyses. After recording milk weights, samples were obtained for determination of somatic cell counts (SCC), butterfat (BF), and protein (PRO). Somatic cell counts were transformed to Dairy Herd Improvement Association linear scores for statistical analyses. Cows were retrospectively assigned to one of two groups based on their SCC linear score being either less than (LOSCC) or more than (HISCC) 4.5, which corresponded to a SCC of 292 x 10(3) cells/mL. Milk samples from 32% (8/25) of the cows and 18% (18/100) of the quarters were culture-positive for bacteria (Staphylococcus aureus, Streptococcus dysgalactiae, Streptococcus species, Actinomyces pyogenes, and[or] coagulase-negative Staphylococcus) at one or more times. Milk production was higher in LOSCC than in HISCC cows (6.3 +/- .2 vs 5.4 +/- .3 kg/24 h; P = .004). Mean SCC linear score tended to be negatively correlated with mean milk production over the entire study (r = -.36; P = .08). Adjusted weaning weights were similar between calves suckling LOSCC and HISCC cows (243 +/- 5 vs 236 +/- 10 kg; P = .57). In summary, milk SCC concentrations in primiparous Simmental cows were highly variable. Higher SCC concentrations were associated with lower milk production in cows.

Comparison of a radioimmunoassay (Charm II) test with high-performance liquid chromatography for detection of oxytetracycline residues in milk samples from lactating cattle.

A radioimmunoassay test for tetracyclines (Charm II) was compared with high-pressure liquid chromatography (HPLC) for detection of oxytetracycline (OTC) residues in milk samples from individual lactating cows. Oxytetracycline was administered by 1 of 3 routes (IV, IM, or intrauterine) to 21 lactating dairy cows. A total of 292 duplicate milk samples were collected from milkings before and through 156 hours after OTC administration. Concentration of OTC in these samples was determined by use of the Charm II test and an HPLC method with a lower limit of quantitation, approximately 2 ng of OTC/ml. Samples were also classified with respect to presence of OTC residues relative to the FDA safe concentration (< or = 30 ng/ml), using the Charm II (by control point determination) and HPLC methods. There was a significant (P < or = 0.05) difference between test methods in classification of milk samples with respect to presence or absence of OTC at the FDA safe concentration. A total of 48 of the 292 test results (16.4%) did not agree. Using the HPLC test results as the standard with which Charm II test results were compared, 47 false presumptive-violative test results and 1 false presumptive-nonviolative Charm II test result (a sample containing 31 ng of OTC/ml, as evaluated by HPLC) were obtained. The samples with false presumptive-violative Charm II results contained < 30 ng of OTC/ml, as evaluated by HPLC. In some respects, the Charm II test performed appropriately as a screening test to detect OTC residues in milk samples from individual cows.(ABSTRACT TRUNCATED AT 250 WORDS)

Potential for oxytetracycline administration by three routes to cause milk residues in lactating cows, as detected by radioimmunoassay (Charm II) and high-performance liquid chromatography test methods.

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 tes for tetracyclines (limit of OTC detection, approx 5 ppb) and were compared with concentrations determined by use of a high-performance liquid chromatography (HPLC) method (lower limit 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.(ABSTRACT TRUNCATED AT 250 WORDS)