Treatment of Streptococcus agalactiae mastitis in dairy cows: comparative efficacies of two antibiotic preparations and factors associated with successful treatment.

A commercially available, penicillin-novobiocin, intramammary infusion product and a solution of procaine penicillin G (1.2 X 10(6) IU) in 10 ml of sterile saline solution were evaluated for their comparative efficacies against Streptococcus agalactiae mastitis in 3 California dairy herds. After composite milk samples from each cow in each herd were bacteriologically cultured, cows infected with S agalactiae (n = 228) were assigned randomly to 2 treatment groups. Milk samples were reevaluated bacteriologically 21 to 25 days after treatment. Both preparations were highly effective against S agalactiae in first-lactation cows and in cows scored negative or trace by use of the California Mastitis Test. Efficacy was significantly decreased in cows with California Mastitis Test scores of 1, 2, or 3. Herd and treatment were associated significantly with treatment success or failure. Most treatment failures were in one herd in cows that were given procaine penicillin G in sterile saline solution. Milk production and lactation stage were not associated with success or failure of treatment.