EFFECT OF DRY-COW THERAPY ON SUBCLINICAL MASTITIS - AN EVALUATION OF LONG-ACTING AND SHORT-ACTING INTRAMAMMARIA

This field study was conducted to evaluate the effect of selective dry -cow therapy with long-acting and short-acting antibiotics, respective ly, and also in comparison to control groups without antibiotic treatm ent. A total of 684 cows from 288 different herds in three Norwegian r egions fulfilled the criteria of the study design. There were 104 cows in control group A (sampling only), 115 cows in control group B (plac ebo), 221 cows treated with long-acting intramammaria Benestermycin ve t. 'Leo' for 1 day at drying off in group C, and 244 cows treated with four short-acting intramammaria Leocillin with Dihydrostreptomycin ve t. 'Leo' every second day before drying off in group D. The overall ef fect, measured as the cow being healthy after therapy, was 14.2% in co ntrol groups and 33.7% in therapy groups 30 +/- 17 days into the next lactation. Of quarters infected with S. aureus both in lace lactation (45 +/- 32 days before drying off) and at drying off, 38.4% in the con trol group were bacteriologically negative 30 +/- 17 days into the nex t lactation, compared with 49.5% in the long-acting group and 68.6% in the short-acting group. Of quarters infected with Str, dysgalactiae b oth in late lactation (45 +/- 32 days before drying off) and at drying off, 10 out of 27 were still infected with Str. dysgalactiae in the c ontrol group 30 +/- 17 days into next lactation, compared with 0 out o f 31 in the therapy groups. Dry-cow therapy in coagulase negative Stap hylacoccus spp. (CNS)-infected quarters led to a 5.2 odds ratio of bei ng healthy quarters 30 +/- 17 days into the next lactation, compared w ith control groups. Despite this, the overall frequency of CNS in the material was unchanged after therapy compared with controls.Short-acti ng compared to long-acting preparations had a significantly better eff ect in preventing new infection with S. aureus or Str. dysgalactiae in untreated healthy quarters in cows with fewer than three infected qua rters. This difference in preventive effect was greater in cows with o ne infected quarter during previous lactation (the new infection rates being 0.078 for short-acting and 0.149 for long-acting) than in those with two infected quarters (the new infection rates being 0.042 and 0 .063, respectively).