Prevalence of mastitis pathogens in herds participating in a mastitis control program.

Bacteriological data from herd surveys by the Massachusetts Mastitis Laboratory were analyzed. Comparisons of percent cows and quarters with major mastitis pathogens and types of bacteria isolated were between Streptococcus agalactiae-positive (1105 cows from 17 herds) and Streptococcus agalactiae-negative herds (1088 cows from 17 herds). Major mastitis pathogens were isolated from 58.5% of cows and 37.0% quarters in Streptococcus agalactiae-positive herds. The most frequently isolated bacteria were Streptococcus agalactiae and Staphylococcus aureus, and together these accounted for 87% of organisms isolated. In contrast, major mastitis pathogens were isolated from only 26.3% of cows and 10.2% of quarters in Streptococcus agalactiae negative herds. Streptococci other than Streptococcus agalactiae, Staphylococcus aureus, and coliforms were the predominant organisms isolated. Seventeen additional Streptococcus agalactiae negative herds were surveyed annually over 6 yr. An average of 25.8% of cows and 10.3% of quarters were positive for major mastitis pathogens during this time. The predominant bacteria isolated were streptococci other than Streptococcus agalactiae, Staphylococcus aureus, and coliforms. There was little variation between years.