The purpose of this study was to determine antimicrobial residues in milk from dairy cows treated for clinical mastitis. Milk samples were collected from udder quarters from 97 cows treated with intramammary drugs and simultaneously with other drugs via the systemic route. Data on cow health and used drugs were obtained from the farm veterinarians. Milk samples from 66 affected udder quarters were taken for bacterial identification before treatment. In many of the treated cows withhold period (WP) of 14 milkings was prescribed due to the Slovenian National Legislation for off-label use of drugs. The milk samples from treated udder quarters, taken before and after the prescribed WP, were analysed for antimicrobial residues. Additionally, milk samples from 48 cows were taken from untreated healthy udder quarters to check if any crossover of drugs had occurred. Three screening tests were used for antimicrobial detection. The positive samples were analysed with the appropriate confirmatory method. In 22 (22.7%) cows the milk samples from the treated infected quarters contained antimicrobial residues above the maximum residue limit (MRL) after the WP of 14th milking. Thirteen milk samples had tetracycline residues above the MRL, 6 amoxicillin, 4 cefquinome and 1 neomycin. Antimicrobial residues were not detected in milk samples from the untreated quarters. Out of 12 cows treated intramammarily with dihydrostreptomycin instead of systemic as it is intended, 7 (58.3%) milk samples had residues above the MRL after the 14th milking. The treatment interval differed significantly (p ⡤ 0.05) between the positive and the negative milk samples treated with tetracycline, cefquinome and amoxicillin. In our study cows treated at 12 hour interval had higher chances of having residues. The mastitis caused by E. coli was statistically significant (p ⡤ 0.05%) between the positive milk samples and the negative milk samples. No significant differences were found between the positive and negative cows regarding the combination antimicrobial therapy.
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