Sensitivities to antibiotics and seasonal occurrence of mastitis pathogens

In a 2-yr survey of the Clemson University's dairy herd, each sample of mastitic milk was cultured, and mastitis pathogens were isolated. Causative organisms then were subjected to disc assay of sensitivity to antibiotics. Antibiotics were penicillin, streptomycin, erythromycin, oxytetracycline, cephalothin (cephalosporin), ampicillin and novobiocin. The more widely used antibiotics--penicillin, streptomycin, and erythromycin--were relatively ineffective for bacterial sensitivity in vitro, whereas bacterial sensitivity in vitro for oxytetracycline and cephalothin was considerably more. These results also were true in vivo in treatment of animals in the University herd. Incidence of mastitis under our conditions of weather, housing, and management increased greatly during summer months with less increase during winter months.