Determinants of success or failure in the elimination of major mastitis pathogens in selective dry cow therapy.

Discriminant factors for the elimination of major pathogens (mainly Staphylococcus aureus and Streptococcus dysgalactiae) were identified from a randomized, double-blind, field study on dry cow therapy. From an original study of 686 cows, 269 fulfilled the criteria for this analysis: 93 from the control group, 83 from group C (treated with Benestermycin vet. "LEO") and 93 from group D (treated with Leocillin with Dihydrostreptomycin vet. "LEO"). A "success" cow was defined as a cow that had all quarters healthy in two tests after dry period. Isolation of a major pathogen (mainly Staph. aureus or Strep. dysgalactiae) in any quarter in both samples after the dry period was defined as "failure." Better elimination was associated with short-acting preparations (therapy D) rather than with long-acting therapy C (odds ratio = 0.32), as was a smaller mean value of the last three composite milk somatic cell counts (CMSCC) before dry cow therapy. Cows with a major pathogen in the right hind quarter at drying off or at least one case of acute clinical mastitis during the previous lactation were more likely to have a major pathogen in the next lactation (odds ratio = 4.1 and 3.6, respectively). Young cows without major pathogens at drying off should not be recommended for dry cow therapy, and cows with high CMSCC should be considered for culling if their profiles also show previous acute clinical mastitis in combination with generally high CMSCC and low cure rates in the herd.