Antibiotic residues and prevalence of mastitis pathogen isolation in heifers during early lactation following prepartum antibiotic therapy.

The present study was conducted to determine if antibiotic treatment of heifer mammary glands earlier in the prepartum period reduced the occurrence of residues in milk without compromising efficacy in treatment of intramammary infections. Heifers were assigned randomly to two groups: 1. untreated negative control (n = 42); and 2. intramammary infusion of 200 mg cephapirin sodium (n = 40) 14 days prior to expected calving. Mammary secretions were collected before treatment and during early lactation, and were analyzed for residues by the Bacillus stearothermophilus disc assay. Only four of 127 samples (3.1%) obtained from cephapirin-treated mammary glands at 3 days after calving were positive for residues and most (three of four) were from a heifer that calved within 3 days of treatment. Mammary secretions were also collected before treatment, and at 3 and 30 days after calving for microbiological evaluation. For untreated control heifers, mastitis pathogens were isolated from 67.3% of samples obtained from mammary glands 14 days prior to expected calving, 55.6% obtained 3 days after calving and 36.4% of samples obtained 30 days postpartum. A similar percentage of samples (63.8%) was positive for mastitis pathogens at 14 days before expected calving prior to antibiotic treatment. However, only 15.1% of samples obtained at 3 days after calving and 7.9% of samples obtained 30 days postpartum from mammary glands of antibiotic-treated heifers contained mastitis pathogens. Coagulase-negative staphylococci were isolated most frequently. Intramammary infusion of antibiotics earlier in the prepartum period markedly reduced the occurrence of residues in milk during early lactation without affecting efficacy.