Effects of different antibiotic treatments of subclinical mastitis in lactating cows

101 cows with 197 udder quarters with subclinical mastitis from 23 dairy farms were selected for different antibiotic treatments under field conditions. Group 1 consisting of 27 animals and 50 infected udder quarters was treated twice intramammaryly with 250 mg Cefacetril. Group 2 (26 animals/ 50 quarters) was treated twice intramuscular with 10 Mio IU Penethamathydrojodid and with 5 Mio IU respectively. Group 3 was treated twice with the combination of the intramammary and the intramuscular therapies in the above mentioned groups. Group 4 served as control. 23 animals with 36 subclinical infected udder quarters were treated twice intramuscular with 2.0 ml of sterile isotonic sodium-solution. Therapeutic success was controlled with bacteriological and cytological examinations of quarter milk probes one week, two weeks and four weeks after the end of treatments. Concerning bacteriological healing both the intramammary and the combined therapy had an even success with a 72.9% elimination rate of pathogenic bacteria whereas the intramuscular therapy led to bacteriological healing in 36.7%. In comparison cytological healing rates (<100 x 10(3) cell counts per ml milk) with 29.8% in group 1, 32.3% in group 3 and 8.2% in group 2 were not satisfying. S. aureus was the predominating isolated bacteria. With respect to bacterial species found in the subclinically infected milk probes, elimination rates in Streptococcaceae and in Enterococcaceae was evident higher than in Staphylococcaceae. The results are discussed.