Possible risk factors associated with penicillin-resistant strains of Staphylococcus aureus from bovine subclinical mastitis in early lactation.

A randomized controlled field study of selective dry cow therapy with 686 cows allocated to two control groups (sampling only or placebo) or two therapy groups was used to screen for possible factors associated with penicillin-resistant strains of Staphylococcus aureus after the dry period. Therapy was given either as a total dose of 400,000 IU of penicillin and 100 mg of neomycin per infected quarter as dry cow preparation or as a total dose of 1.2 million IU of penicillin and 1200 mg of dihydrostreptomycin per infected quarter as a lactation formula. Success cows had all quarters identified as being free of penicillin-resistant strains of Staphylococcus aureus both at calving and at 30 +/- 17 d after calving. Failure cows were those having penicillin-resistant strains of Staph. aureus in any quarter at both or one of these two samples after the dry period. Using logistic regression, four variables were found to be associated with penicillin-resistant strains of Staph. aureus after the dry period. These included the identification of penicillin-resistant strains of Staph. aureus either at 45 +/- 32 d before drying off and at drying off, treatment for acute clinical mastitis at least once during the previous lactation, the weighted SCC of all cows' milk by daily milk yield within the herd, and therapy in the lactation formula compared with the two control groups. Our finding that the use of lactation formula increases the risk of resistance development is contradictory to present arguments underlying Norwegian dry cow therapy strategy.