A comparative randomized field trial on intramammary and intramuscular dry cow antibiotic treatment of subclinical Staphylococcus aureus mastitis in dairy cows.

The efficacy of two dry cow treatment (DCT) regimens for subclinical Staphylococcus aureus mastitis was evaluated in naturally infected dairy cows. At dry-off, cows were assigned to two treatment groups by randomized blocks on the basis of parity and somatic cell count (SCC). Two antibiotic DCT regimens were used, namely: (1) a single intramammary infusion containing sodium nafcillin, procaine benzylpenicillin and dihydrostreptomycin; and (2) systemic cefquinome administered intramuscularly, twice at a 24-h interval. In the intramammary (IMM) treatment group, the S. aureus intramammary infection (IMI) rate was reduced from 40% (56/140 quarters) before dry-off to 20% (28/140) after calving. Seventy per cent (39/56) of the S. aureus-positive quarters were negative after calving, and 13% (11/84) of the negative quarters were positive after calving. In the systemic treatment group, the S. aureus IMI rate increased from 39% (29/74 quarters) before dry-off to 55% (41/74) after calving. Twenty-eight per cent (8/29) of the S. aureus-positive quarters were negative after calving and 45% (20/45) of the negative quarters were positive after calving. The odds ratio of an S. aureus-positive quarter being negative after calving in the IMM group relative to the systemic therapy group was 44.6 (95% confidence interval = 2.1-909.1, P < 0.01). Parity, quarter, milk SCC and N-acetyl-beta-D-glucosaminidase were tested in the model, and were found to have no significant effect on S. aureus cure rates or new IMI rates. The IMM treatment resulted in a higher cure rate compared with that observed in previous studies. The very low cure rate after systemic cefquinome treatment was comparable to the spontaneous cure rate observed in untreated controls in previous studies. The unfavourable results of the cefquinome systemic DCT might reflect inadequate pharmacokinetic properties of the drug regarding poor udder penetration in subclinical mastitis and short antimicrobial effect compared with the IMM treatment.