Nonlactating-cow therapy with a formulation of penicillin and novobiocin: therapeutic and prophylactic effects.

An experimental product incorporating 500,000 IU of procaine penicillin G and 600 mg of sodium novobiocin in 2% aluminum monostearate-peanut oil gel (10-ml dose) was used to treat all quarters of 56 cows which were infected in at least 1 quarter at time of final mild-out at end of lactation. Treatment was withheld from 89 cows uninfected in all quarters. Quarter infection was determined by bacteriologic culturing of milk samples collected at the last regular milking, at intervals up to final milk-out (7 or 12 days later), at calving, and 1 week later. Clearance rates against Staphylococcus aureus, Staphylococcus epidermidis, streptococci other than Streptococcus agalactiae, and coliform bacteria in treated quarters were 83, 94, 88, and 71%, respectively. Subtraction of the spontaneous clearance rate of about 50% in untreated quarters resulted in values of 35 to 45% for drug efficacy against existing staphylococcal and streptococcal infections. Prophylactic efficacy was examined. In cows entering the true nonlactating period with 1 or more quarters infected, new infection rates across the period aming quarters uninfected at the beginning were 36.0% among untreated cows and 6.3% among treated cows (P less than 0.005). The comparable rates for cows entering the nonlactating period uninfected in all quarters were 5.7 and 0%. Staphylococcus aureus and streptococci, which comprised 38.5% of new period infections among untreated cows, were completely lacking among treated cows (P less than 0.025). Within the treated group of cows, 83.1% of infected quarters were cleared, and new infection rate in the non-lactating period was 50% less than the rate among untreated cows. Because the frequency of intramammary infection in this herd was quite low at "drying-off" (10.5% of quarters), the net effect on herd health of selective therapy of cows infected at end of lactation was a reduction in total quarter infection from 19.8 to 13.6%.