COMPARISON OF SUCCESS OF ANTIBIOTIC-THERAPY DURING LACTATION AND RESULTS OF ANTIMICROBIAL SUSCEPTIBILITY TESTS FOR BOVINE MASTITIS

Antimicrobial susceptibility testing was conducted on a variety of mas titis pathogens. The infected quarters were subsequently treated durin g lactation with a commercially available product containing penicilli n and novobiocin that was designed for lactating cows. Cows were treat ed as per the recommendations of the product manufacturer, and cures w ere determined by the absence of bacteria in both sets of duplicate qu arter milk samples that were collected at 28 d posttreatment. Comparis ons were made between the susceptibility of the bacteria and the thera peutic success or failure. All isolates tested were considered to be s usceptible to the penicillin and novobiocin combination. Bacteriologic cure rates for newly acquired Staphylococcus aureus intramammary infe ction (IMI) (<2 wk in duration) at 28 d posttreatment were 70%. Cure r ates for chronic Staph. aureus IMI (>4 wk duration) were much lower (3 5%), reaffirming previous reports of the intractable nature of chronic Staph. aureus IMI. Cure rates for subclinical IMI caused by other org anisms were 90% for Streptococcus agalactiae, 91% for Streptococcus ub eris, 90% for Streptococcus dysgalactiae, 77% for other Streptococcus spp., and 71% for Staphylococcus spp. other than Staph, aureus. In vit ro testing was considered to be a predictor of therapy outcome for IMI caused by Staphylococcus spp., newly acquired Staph. aureus, Strep. u beris, Strep. dysgalactiae, and Strep. agalactiae, but was not conside red to be a useful predictor of efficacy for chronic IMI caused by Sta ph. aureus.