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 c ows allocated to two control groups (sampling only or placebo) or two thera py groups was used to screen for possible factors associated with penicilli n-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 infecte d 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 havin g penicillin-resistant strains of Staph. aureus in any quarter at both or o ne of these two samples after the dry period. Using logistic regression, fo ur 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 lea st once during the previous lactation, the weighted SCC of all cows' milk b y daily milk yield within the herd, and therapy in the lactation formula co mpared with the two control groups. Our finding that the use of lactation f ormula increases the risk of resistance development is contradictory to pre sent arguments underlying Norwegian dry cow therapy strategy.