Determination of Milk and Mammary Tissue Concentrations of Ceftiofur After Intramammary and Intramuscular Therapy

Abstract

Twenty-five Staphylococcus aureus strains isolated from bovine mastitis were tested for their susceptibility to ceftiofur. Zone diameter for 30μg disks averaged 39mm, and minimum inhibitory concentrations ranged from .5 to 1μg/ml. Tissue and milk concentrations were determined from biopsy and quarter milk samples collected from eight cows treated with either intramammary infusion of 100 or 200mg of ceftiofur, one or two intramuscular injections of 500mg of ceftiofur, or combination therapy of intramammary infusion coupled with intramuscular injection. Three additional cows received two intramammary infusions of 200mg of cephapirin at 24-h intervals. Intramuscular injections of ceftiofur resulted in tissue and milk concentrations below detectable limits. Staphylococcus aureus was not eliminated from infected mammary glands by infusion of 100mg of ceftiofur or by injection of 500mg of ceftiofur by 48h after treatment. Combination therapy of 100mg of ceftiofur infused and 500mg injected reduced S. aureus numbers in milk and tissue markedly, as did infusion of 200mg of ceftiofur. Cows receiving intramammary infusion of 200mg of ceftiofur (two doses at 24-h intervals) had highest concentrations in milk (450μg/ml at 4 and 6h) and in tissue (.08μg/mg at 30h). These concentrations are similar to those obtained with two 200-mg doses of cephapirin at 24-h intervals. Histologic analysis of mammary parenchymal tissues showed that combination therapy resulted in higher percentages of alveolar luminal area and lower percentages of interalveolar stroma compared with infusion or injection alone. Histology of quarters receiving combination therapy was not different from that of quarters receiving cephapirin infusion alone.