Nine dairy herds (mean size, 149 cows) with bulk-tank milk somatic cell counts of less than 300,000 cells/ml and greater than 80% of cows with Dairy Herd Improvement Association linear somatic cell counts less than or equal to 4 were selected for study. Each herd was monitored for 12 consecutive months. Duplicate quarter-milk specimens were collected from each cow for bacteriologic culturing at beginning of lactation, cessation of lactation, and at the time of each clinical episode of
mastitis. Streptococcus agalactiae was never isolated and Staphylococcus aureus was isolated from less than 1% of all quarters. There were 554 episodes of clinical
mastitis. During the year of study, the incidence rate of clinical
mastitis varied from 15.6 to 63.7% of cows among the 9 herds. Mean costs per cow per year in herd for
mastitis prevention were: $10 for paper towels, $3 for nonlactating cow treatment, and $10 for teat
disinfectants. Mean cost associated with clinical
mastitis was $107/episode. Approximately 84% ($90) of the costs attributed to a clinical episode were associated with decreased milk production and nonsalable milk. Costs of medication and professional veterinary fees per clinical episode varied significantly among the 9 herds. Three of the herds did not have a veterinarian treat a clinical episode of
mastitis during the year of study even though 2 of these herds had the first and third highest incidence rates of clinical
mastitis. When calculated on a per cow in herd basis, mean costs of $40/cow/year were attributed to clinical
mastitis.(ABSTRACT TRUNCATED AT 250 WORDS)