Antibiotic treatment of mastitis: comparison of intramammary and intramammary plus intramuscular therapies.

Efficacy of two therapy regimens for subclinical Staphylococcus aureus bovine mastitis was evaluated. Forty-nine lactating cows with a total of 78 subclinically infected quarters were included. Cows were divided into two treatment groups. Group 1 received intramammary infusion at each milking for six milkings with a commercial lactating cow product containing 62.5 mg of amoxicillin. Group 2 received the same intramammary infusion regimen plus one intramuscular injection daily of 9,000,000 U procaine penicillin G for 3 d. The combination of intramammary and intramuscular treatment resulted in bacteriologic cure of 51.4% of quarters and 48% of cows compared to 25% of quarters and 30.4% of cows for intramammary infusion alone. Quarters where infections were not cured supported more growth of S. aureus during treatment than glands that were cured. Mean antibiotic concentration in fore-milk samples during treatment were lower in quarters that were cured. Somatic cell counts of milk from quarters that were not cured were higher prior to initiation of treatment than those cured. Data suggest that further evaluation of new regimens for treatment of subclinical S. aureus mastitis is warranted.