The main objective was to determine the prevalence of intramammary infections (IMI) in dairy cows in Flanders, Belgium. Data were obtained from quarter milk samples of dairy herds subjected to a mandatory yearly screening of all lactating cows. A total of 178,668 quarter milk samples were collected at 1087 cross-sectional dairy herd screenings performed in three consecutive years. Of the dairy cows, 40% had at least one culture-positive quarter. More than 50% of all IMI were caused by non-aureus staphylococci. Streptococcus agalactiae is almost eradicated in Flanders, whereas Staphylococcus aureus was isolated from 18% of the culture-positive quarters. In addition, the distribution of mastitis pathogens in quarter milk samples from selected dairy cows with an elevated somatic cell count (SCC) is described. From 6390 cows with a geometric mean composite SCC 250,000 cells/ml, nearly 65% had at least one culture-positive quarter. The majority of the IMI were caused by non-aureus staphylococci (41.1%), whereas Staph. aureus and aesculin-positive cocci were found in respectively 25% and 18% of the culture-positive milk samples. We conclude that more efforts are needed in the prevention and control of subclinical mastitis in Flanders. Non-aureus staphylococci are the predominant cause of IMI, warranting more research regarding the epidemiology and pathogenicity of those species.

This study examines the influence of parity, stage of lactation, and single isolations (i.e., the isolation of a microorganism that could not be reisolated in the same quarter in the next sampling) of staphylococci other than Staphylococcus aureus (coagulase-negative staphylococci), Corynebacterium bovis, or esculin-positive cocci other than Streptococcus uberis (referred to as esculin-positive cocci throughout) on the monthly log(e)-transformed somatic cell count (SCC) for 180 first, second, and third parity cows that were observed over a whole lactation. Repeated measures ANOVA was used to analyze the data. No significant effect was found for the infection variable. However, the results indicated that even single isolations of coagulase-negative staphylocci, C. bovis, or esculin-positive cocci resulted in a numerical or statistically significant increase in SCC. Least squares mean SCC (log(e)-transformed) for bacterio-logically negative cows and cows with single isolations of coagulase-negative staphylococci, C. bovis, or esculin-positive cocci were 3.90, 3.97, 4.08, and 4.17, respectively. Significant effects of parity, stage of lactation, and the interaction of parity and stage of lactation could not be found when only bacteriologically negative cows were considered. Least squares mean SCC for first, second, and third parity cows were 3.80, 3.93, and 3.97, respectively. However, the effects of parity, stage of lactation, and the interaction of parity and stage of lactation were significant when all 180 cows were included. Therefore, these effects must be due to factors that were present in the infected groups.

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