-The aim of this paper was to evaluate the effects associated with intramammary infection (IMI) by a bacterium or a group of bacteria (Staphylococcus aureus, Streptococcus agalactiae, Streptococcus dysgalactiae, Streptococcus uberis, coliforms, Staphylococci other than S. aureus, and Corynebacterium bovis) on the somatic cell count (SCC) in quarter milk of dairy cows. Papers selected for analysis had to provide SCC values associated with the natural infection in quarters by different bacteria. Sampling for measurement of SCC and determination of the infection had to be done on the same day. Only papers published in English or in French after 1971 were considered. Twenty-one papers fulfilled the selection criteria. The animals sampled, the measurement techniques for SCC and the bacteriological identification, as well as the definition of the infection, all differed widely among the selected studies. The meta-analysis method was used to estimate both the mean SCC (arithmetic and geometric) value and the average increase on SCC of each type of infection. The geometric mean SCC in bacteriologically negative quarters was 68 000 c/mL. In case of IMI, the retained SCC was 357 000, 857 000, 547 000, 1 024 000, 1 151 000, 138 000 and 105 000 c/mL in quarters infected by Staphylococcus aureus, Streptococcus agalactiae, Streptococcus dysgalactiae, Streptococcus uberis, coliforms, staphylococci other than S. aureus and Corynebacterium bovis, respectively. The variation factors that could influence these SCC values and the bacteriological results are discussed. dairy cow / mastitis / somatic cell count / meta-analysis Résumé -Méta-analyse des effets de l'infection sur la concentration en cellules somatiques du lait de quartier chez la vache laitière. Le présent article a pour objectif d'évaluer l'effet associé à l'infection intra-mammaire (IIM) par une bactérie ou par un groupe de bactéries (Staphylococcus aureus, Streptococcus agalactiae, Streptococcus dysgalactiae, Streptococcus uberis, coliformes, 335

-The aim of this study was to determine if taking parity and stage of lactation of the dairy cow into account could improve the sensitivity (Se) and specificity (Sp) of a SCC-test to detect quarter intramammary infection (IMI). A total of 27 315 quarter milk samples were collected from 277 Holstein cows from one experimental herd from 1980 to 1994. Two analyses were carried out. In the first analysis (MIN+MAJ analysis), quarter samples showing at least a single isolation of a minor or a major pathogen were considered as infected. In the second analysis (MAJ analysis), only quarter samples showing at least a single isolation of a major pathogen were considered as infected. The SCC threshold value above which a quarter was defined positive was the one which maximised the Youden index (YI). Se, Sp and YI were calculated on (1) the whole study sample (herd level), and (2) 6 subsamples defined according to parity (primiparous versus multiparous) and stage of lactation (0 to 60 days, 61 to 120 days and 120 to 400 days). Se, Sp and YI were also recalculated at the herd level on the basis of Se and Sp obtained for each subsample. The SCC threshold selected on the whole study sample was 265 000 cells·mL -1 in the MIN+MAJ analysis and 420 000 cells·mL -1 in the MAJ analysis. At these thresholds, the Se and Sp values were 71.2% and 75.8% respectively (MIN+MAJ analysis) and 82.9% and 83.6% respectively (MAJ analysis). For both analyses, the SCC thresholds, Se and Sp varied according to parity and stage of lactation. Finally, the results showed that taking the cow characteristics into account had a very limited impact on the accuracy of IMI detection.