Concentrations of methicillin in blood, normal milk and mastitic milk of cows after intramuscular injection of methicillin and tamethicillin

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Abstract
Equal doses of MET and TAM were administered i.m. in a crossover trial involving 4 dairy cows. Acute mastitis was induced in each cow by infusing 2 quarters of the udder with Escherichia coli endotoxin 3 h before antibiotic adminstration. Peak serum MET concentrations after MET injection were significantly (P < 0.001) higher than peak serum drug concentrations after TAM injection. The half-life of MET in serum after MET and TAM treatments were 18 min and 2 h, respectively. Normal milk MET concentrations during the first 8 h after TAM administration were significantly (P < 0.05) higher than after MET treatment. Mastitic milk MET concentrations during the period 2-6 h after MET injection were significantly (P < 0.01) higher than after TAM administration. MET concentrations which were equal to or higher than the minimal inhibitory concentrations for penicillin G-resistant staphylococci were maintained in the mastitic milk for 8 h after treatment with MET and TAM.

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