Cows (n=2,053) from 6 seasonally calving dairy herds were enrolled in a trial to compare the efficacy of 2 dry cow treatments. Cows received either a combination dry cow
therapy of 600 mg of
cloxacillin (CL) followed by an internal teat sealant (ITS) containing 2.6 g of
bismuth subnitrate in all 4 quarters immediately following their final milking for the season, or only an intramammary infusion of 600 mg of CL. All cases of clinical
mastitis were recorded and cultured during the first 150 d of lactation in each herd, and cow somatic cell count (SCC) was measured between 7 and 50 d postcalving. A large difference was found between treatment groups in the rate at which cows were diagnosed with clinical
mastitis over the first 21 d of lactation, after which time the rate at which cows were diagnosed with clinical
mastitis was similar between treatment groups. Analysis of the relative proportions of cows with clinical
mastitis was performed at both the gland and cow levels. The relative risk (RR) of clinical
mastitis diagnosed within 21, 30, and 100 d of calving in a gland treated with the ITS-CL combination was, respectively, 0.30 [95% confidence interval (CI)=0.21-0.44], 0.39 (0.28-0.53), and 0.58 (0.46-0.75) that of the CL group. An interaction between treatment and previous SCC was found when clinical
mastitis was analyzed at the cow level. In a subset of cows that had low SCC in their previous lactation, the RR of
mastitis in cows with the ITS-CL combination within 21, 30, and 100 d of calving was, respectively, 0.54 (95% CI=0.33-0.87), 0.57 (0.37-0.88), and 0.69 (0.50-0.99) that of cows that received only CL at drying off. In the subset of cows that had at least 1 high SCC in the previous lactation, the RR of
mastitis in the ITS-CL combination group within 21, 30, and 100 d of calving was, respectively, 0.26 (95% CI=0.16-0.44), 0.37 (0.24-0.57), and 0.72 (0.55-0.96) that of the CL-only group. The ITS-CL combination of dry cow treatments was associated with a reduction in subclinical
mastitis [SCC ≥250,000 cells/mL; RR=0.80 (95% CI=0.65-0.98)] when compared with treatment with CL alone. The use of an ITS in combination with CL dry cow treatment was associated with significantly lower clinical and subclinical
mastitis in the following lactation, with a greater difference found in cows that had a history of subclinical
mastitis in the previous lactation.