An epidemiological study of antimicrobial residues detected in Michigan cows' milk

Following widely publicized accounts of undetected antimicrobial residues in milk making it to market in the late 1980s, the Milk and Dairy Beef Quality Assurance Program (QAP) was developed. Though designed to prevent drug residues, the impact of the QAP on residue occurrence has yet to be determined. A commonly promoted and adopted preventative practice has been the unapproved use of residue detection assays to test for antimicrobial residues in milk from treated individual cows. The reliability of these assays in testing individual cow milk has yet to be established.; The epidemiological research presented here was achieved through two main studies that addressed six objectives. The first study was a retrospective study of Michigan dairy farms evaluating the Milk and Dairy Beef Quality Assurance Program (QAP) and its role in the prevention of violative antimicrobial residues in milk and the adoption of prudent drug management practices. The first and second objectives were to determine if QAP certification and specific management factors were associated with a reduced risk of having antimicrobial residues in milk. Certification in the QAP was associated with a tendency toward reduced risk (OR = 0.3 [0.07--1.32]) of having experienced a violative residue in bulk-tank milk. The risk of having had a residue was reduced on farms treating >10%, of their herd for metritis, and having their milk processor perform residue testing. However, on-farm residue testing and maintaining written identification records of treated cows was associated with an increased risk of having had a residue. In a separate set of analyses the associations between QAP certification and the use of prudent drug management practices were evaluated (Objective 3). Involuntary certification was associated with maintenance of good written treatment records and performance of on-farm residue testing. Voluntary certification was weakly associated with use of refrigerated drug storage. These results suggest that farms adopted specific management practices, irrespective of certification.; The second study was a longitudinal experimental study evaluating the reliability of 3 on-farm assays when used to test individual cow milk for antimicrobial residues following treatment for mild clinical mastitis. Methods were developed (Objective 4) to improve the high performance liquid chromatography (HPLC) analyses for detection of ampicillin and pirlimycin in milk. The reliability of the assays was expressed as sensitivity, specificity, and positive and negative predictive values (Objective 5). Ranging from 32.14 to 73.68%, the positive predictive values were poor for all three assays when using the assays' detection limits. Additional statistical analyses were used to determine whether somatic cell count, IgG1, bacterial isolates or specific antimicrobial treatments were associated with false-positive results (Objective 6). Milk IgG1 concentrations were positively associated with false positive results from the all 3 assays.; The tendency of the QAP to prevent violative residues provides encouraging information for the continued promotion and implementation of the Program. Dairy producers and veterinarians can use the findings to target their residue prevention efforts. Producers should reconsider their reliance on screening assays for testing individual cows' milk on-farm as a primary tool for residue prevention.