Efficacy of multiple and single intramammary infusions of
benzathine cloxacillin (500 mg/quarter) were compared for treatment of
mastitis during the dry period. Treatments were three infusions, one each at 0, 7, and 14 d into the dry period; one infusion at drying off; and no infusions. Cows were grouped (30/treatment) based on previous
mastitis history, mean monthly SCC, mature equivalent milk production, and pathogen detected pretreatment. Infected quarters/cow at parturition, posttreatment were 1.26, 1.6, and 2.35 for multiple, single, and no infusion groups, respectively. Milk production and somatic cell count posttreatment were not affected by treatment. Across all genera of microorganism, 75.5, 73.6, and 52.8% of
infections were eliminated in the multiple, single, and no infusion groups, respectively. Primary effect of multiple infusions with
cloxacillin in the dry period was to prevent new
streptococcal infections. Sensitivity tests on staphylococcal isolates indicated that
cloxacillin was still an effective dry cow treatment after 7 yr of use in the herd. Resistance to
cloxacillin in approximately half the posttreatment staphylococcal isolates appeared to be nonenzyme (
penicillinase) dependent. Multiple dry cow treatments with
cloxacillin do not offer any advantage over a single treatment.