Microbiological examination was carried out in 6,315 milk samples collected from all mammary quarters of 1,609 lactating cows from 48 herds, located in the regions of Zona da Mata and Campo das Vertentes, Minas Gerais State, Brazil. At sampling time, the udders were clinically examined and milk from all mammary quarters were evaluated by California Mastitis Test (CMT). A total of 3,919 agents were isolated, being 3,637 from quarters with single infection and 283 from mixed infection with two types of microorganisms. The mastitis agents found and the percentages of isolations were: Staphylococcus aureus, 19.2%, coagulase negative Staphylococcus sp. (SCN), 12.4%, Streptococcus agalactiae, 6.9%, esculin-positive Streptococcus sp. (ESCPOS), 4.0%, esculin-negative Streptococcus sp. (ESCNEG), 2.1%, Corynebacterium sp., 55.2%, yeast, 0.1% e Pseudomonas sp., 0.1%. Negative cultures were found in 2,463 (39%) samples and 216 samples were contaminated. Corynebacterium sp. was present in all 48 herds, with mammary quarters infection levels between 1.5% and 58.6%. S. aureus was isolated from 47 herds; in 37 of these there were up to 20% of infected mammary quarters. S. agalactiae was recovered from 29 herds (60%) and in 24 out of these the average of infected quarters was 2.7%. S. aureus, S. agalactiae, SCN, ESCPOS, ESCNEG and Corynebacterium sp. were isolated from mammary quarters with and without signs of inflammatory reaction (positive and negative scores on CMT, respectively). Comparing with minor pathogens (SCN and Corynebacterium sp.), the isolation frequency of major pathogens was significantly higher from quarters with positive scores on CMT (P<0.001). The results indicated that mammary quarters negative on CMT should also be considered when selecting samples for microbiological examination and that negative cows, infected with major pathogens, can be a source of infection for the others animals in the herd. The high prevalence of S. aureus, S. agalactiae and Corynebacterium sp. suggests that current control measures for contagious mastitis were not correctly practised in most of the herds.
Bovine; mastitis; source of infection; herd infection pattern
