Study of factors influencing the effectiveness of two treatments, penicillin-streptomycin and rifamycin, against experimentally induced staphylococcal mastitis in lactating cows.

The effectiveness of two antibiotic preparations, a combination of Penicillin-Streptomycin and Rifamycin SV, was measured on persistent subclinical infections produced experimentally by two strains of Staphylococus aureus in the bovine mammary gland. Effects of different parameters upon the efficiency of these treatments were analysed. The combination Penicillin-Streptomycin cured 23 quarters of 46 treated (50 %) against 14 out of 55 for the Rifamycin SV (25 %). This difference was significant (P less than 0.02). Whatever the antibiotic preparation used the highest cure rate was observed for the cows in first lactation (60 % against 24 %) and beyond the second month of lactation (43 % against 10 %). The factors dependent on the cow were related to these two parameters. Front quarters were cured in higher percentage (43 %) than rear quarters (29 %), although the difference was not statistically significant. The influence of other parameters on the cure rate, such as previous cleared infections and number of CFU and somatic cells in the quarters at the time of treatment, depended on the treatment used. The bacteriological results showed that cures could be correctly evaluated only after a minimum interval of three weeks post treatment. From the data a standardised model is proposed for the evaluation of the effectiveness of treatments.