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 (less than or equal to 30 ng/ml), using the Charm II (by control point determination) and HPLC methods. There was a significant (P less than or equal to 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 (less than or equal to 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. However, the tendency for the test to yield presumptive-violative test results at OTC concentrations lower than the FDA safe concentration (as evaluated by HPLC), suggests that caution should be exercised in using the test as the sole basis on which a decision is made to reject milk.

Eight Holstein cows, 4 inoculated intracisternally in 1 quarter of the mammary gland with Escherichia coli and 4 noninfected controls, were administered ceftiofur sodium (3 mg/kg of body weight, IV, q 12 hours) for 24 hours, beginning at 14 hours after inoculation of infected cows. All challenge-exposed cows became infected, with mean +/-SEM peak log10 bacterial concentration in milk of 5.03 +/-0.69 colony-forming units/ml. The infection resulted in systemic signs (mean peak rectal temperature, 41.5 +/- 0.3 C; anorexia; signs of depression) and local inflammation (mean peak albumin concentration in milk, 7.89 +/- 1.71 mg/ml). Ceftiofur was detectable in milk from all challenge-exposed cows, compared with only 1 of 4 noninfected cows, and the mean period after inoculation that ceftiofur was detectable in milk was longer (P < 0.05) in infected (147.7 +/- 27.5 hours) than noninfected cows (1.3 +/- 1.3 hours). However, maximal ceftiofur concentration attained in milk for all cows was 0.28 micrograms/ml, and was 0.20 micrograms/ml or less for all but 2 milk samples collected for 10 days after challenge exposure. Mean serum concentration of ceftiofur peaked at 1.0 +/- 0.3 micrograms/ml and 0.7 +/- 0.1 micrograms/ml for infected and noninfected COWS, respectively. After each ceftiofur dose, mean peak and trough concentrations of ceftiofur in serum did not differ between groups; however, concentration of ceftiofur in serum was higher at 7 hours after each dose in noninfected cows, suggesting more rapid clearance of the drug in infected cows. Ceftiofur was not detected in serum (< 0.05 micrograms/ml) of any cow at or after 120 hours following inoculation of infected cows Storage of serum samples at -20 C for 3 weeks resulted in a 98.8% decrease in ceftiofur activity, compared with that in fresh serum samples. Eighty-seven percent of this loss occurred 30 minutes after mixing serum and ceftiofur; thus, about 13% of the original activity was lost in storage.

Tissue specimens from muscle, liver, kidney, and injection sites were collected, and serum was obtained from 3 calves euthanatized on each of posttreatment days 5 and 22. Calves were treated with 6.7, 13.4, or 20 mg of oxytetracycline (OTC)/kg of body weight, IM, once daily for 3 days; these dosages are 1, 2, and 3 times the label dose, respectively. One control calf was euthanatized on each of posttreatment days 5 and 22. In treated male calves killed 2 days after the last injection, OTC residues were detected in all tissues and serum, using high-performance liquid chromatography. Tissues from all injection sites also were considered positive for antimicrobial residues, using swab test on premises (STOP), microbial inhibition test (MIT), and thin-layer chromatography-biautography (TLCB) test. Kidney tissues from a calf given 13.4 mg of OTC/kg and kidney and liver tissues from a calf given 20 mg of OTC/kg also were considered positive, using the MIT and TLCB. Results of the STOP only were considered positive for the liver and kidney of a calf given 20 mg of OTC/kg, but substitution of Saskatoon antibiotic medium-3 for the original medium (antibiotic medium-5) allowed the STOP to detect residues in these tissues from all treated calves. In female calves killed 19 days after the last injection, the STOP, MIT, and TLCB procedures revealed positive results for tissues from some injection sites, but revealed negative results for other tissues. High-performance liquid chromatographic analyses detected OTC in tissues from injection sites from all treated calves, in muscle and liver from a calf given 20 mg of OTC/kg, and in kidneys from calves given 13.4 or 20 mg of OTC/kg. The STOP, MIT, and TLCB procedures lacked the sensitivity of high-performance liquid chromatography for detection of OTC residues.

During the fiscal year 1988, USDA-FSIS detected 3,095 antimicrobial violations in bob veal calves, using the calf antibiotic and sulfonamide test. Of the 3,095 carcass submissions involved, 945 were tested further to identify the causative agents. The results of tests on the available kidney, liver, and muscle specimens are reported. Kidney specimens yielded a specific agent most often (71.2%), with neomycin (42.6%) being cited most among agents found in kidneys. Neomycin was found less frequently in liver (4.5%) and muscle (0.2%). Among all tissues, unidentified microbial inhibitors were either the largest or second largest category found (kidney, 10.5%; liver, 27.1%; muscle, 7.8%), and no other agent exceeded 7.0% (streptomycin in kidney). The proportion of liver and muscle specimens that had unidentified microbial inhibitors is particularly important because the next most common classes were streptomycin in liver at 5.5% and sulfamethazine in muscle at 2%. The frequency of unidentified microbial inhibitors may justify the addition of tests to the FSIS battery for identification of agents. Not all tissues were tested for sulfonamides, hence these agents are likely to have been underreported. Less than 10% of the muscle specimens evaluated yielded an agent, suggesting most calf antibiotic and sulfonamide test-positive carcasses may have been safe with regard to residues in meat, although organs might have been adulterated. Specimens for verification were not selected completely randomly from the population of all calf antibiotic and sulfonamide test-positive animals and calves selected for testing were not chosen strictly by random sampling; therefore, extrapolation of the contents of this report to the bob veal calf industry must be done with caution.

The 102 farms received a positive result of the milk drug residue test were selected to investigate the reasons of drug residues in bulk milk. The most frequent causes of drug residues were milker or producer mistakes (28.4%), failure to observe withdrawal time (21.5%), and withholding milk from treated quarters only (19.6%). Milker or producer mistakes occurred high at the farms having a parlor system (4 cases out of 11 farms), and related to the inadequate records and marking of treated cows. The lack of knowledge on the absorption of antibiotic from treated quarters and its excretion from untreated quarters caused mainly withholding milk from treated quarters only. Among the 91 farms identified the cause of drug residues, most of the route of drug administration was intramammary infusion (81.3%), and mostly drug used for the treatment of cows was beta-lactam antibiotics (57.1%).