CLINICAL MASTITIS IN DAIRY-CATTLE IN ONTARIO - FREQUENCY OF OCCURRENCE AND BACTERIOLOGICAL ISOLATES

The objective of this study was to describe the frequency of occurrenc e of clinical mastitis in dairy herds in Ontario. The study group cons isted of 65 dairy farms involved in a 2-year observational study, whic h included recording all clinical mastitis cases and milk sampling of quarters with clinical mastitis. Lactational incidence risks of 9.8% f or abnormal milk only, 8.2% for abnormal milk with a hard or swollen u dder, and 4.4% for abnormal milk plus systemic signs of illness relate d to mastitis were calculated for 2840 cows and heifers, Overall, 19.8 % of cows experienced one or more cases of clinical mastitis during la ctation. Tear injuries occurred in 2.1% of lactations. Standard bacter iology was performed on pretreatment milk samples from 834 cows with c linical mastitis. The bacteria isolated were Staphylococcus aureus (6. 7%), Streptococcus agalactiae (0.7%), other Streptococcus spp. (14.1%) , coliforms (17.2%), gram-positive bacilli (5.5%), Corynebacterium bov is (1.7%), and other Staphylococcus spp. (28.7%). There was no growth in 17.7% of samples, and 8.3% of samples were contaminated. Clinical m astitis is a common disease in dairy cows in Ontario; approximately 1 in 5 cow lactations have at lease one episode of clinical mastitis. Th ere is, however, considerable variation in the incidence of clinical m astitis among farms. The majority of Ist cases of clinical mastitis oc cur early in lactation, and the risk of clinical mastitis increases wi th increasing parity. Environmental, contagious, and minor pathogens w ere all associated with cases of clinical mastitis.