Detection of milk antibiotic residues by use of screening tests and liquid chromatography after intramammary administration of amoxicillin or penicillin G in cows with clinical mastitis

Abstract

Objective - To compare results of 6 commercially available milk antimicrobial screening tests with results of liquid chromatography (LC) when testing milk samples from individual cows treated for mild clinical mastitis by intramammary (IMM) infusion with amoxicillin or penicillin G. Animals - 6 cows with noninduced clinical mastitis: 3 treated by IMM infusion with amoxicillin and 3 treated by IMM infusion with penicillin G. Procedure - Composite milk samples were collected before, during, and after treatment. Samples were assayed by use of the screening tests and their results and those of LC were compared. The LC results were assumed to represent the true result. Results - Results of screening tests compared well with results of LC, with agreement of 94%. Positive screening test results for samples containing drug values below the established tolerance or safe level, as evaluated by LC, were obtained from 2 cows in which abnormal milk, as well as marked increases in composite milk somatic cell count, were observed. With the exception of 1 test in 1 cow, all screening tests had negative results at the end of the labeled milk-withholding time. Conclusions and Clinical Implications - On the basis of results of the limited sample reported, the screening tests appeared to provide good agreement overall, compared with LC results, when testing milk of individual cows treated by IMM infusion with amoxicillin or penicillin G. Positive screening test results for milk samples containing amoxicillin or penicillin G at values below the established tolerance or safe level, as evaluated by LC, may occasionally be obtained.