Evaluation and cluster analysis of inflammatory reactions of dairy cattle mastitis pathogens in milk samples submitted for microbiological examination

Aim of study: We have classified into homogenous groups a wide spectrum of mammary pathogens according to their frequency of isolation in clinical mastitis and their somatic cell counts in non-clinical mastitis. Area of study : The study was conducted in Galicia (NW Spain) Material and methods : 163,741 dairy cattle quarter milk samples were analyzed. We identified mastitis pathogens to the species level and performed a cluster analysis to classify these microorganisms by their median of Linear Score (mLS), percentage of isolation in clinical mastitis samples (%ICS) and percentage of isolation in samples with somatic cell counts under 100,000 cells/mL (%ISU100). Main results : Forty-three different species were isolated. Cluster analysis identified 4 groups of pathogens; mLS and %ICS progressively increased from Group I to Group IV and %ISU100 decreased. However, several pathogens included in groups II and III showed %ISU100 values higher than 2% and 3%. Minor pathogens were mainly clustered in Group I (e.g., Corynebacterium spp. and most of Staphylococcus spp.), while known major pathogens were included in Groups II, III y IV. Species of the same family, genus or microbiological group like Enterobacteriaceae or Enterococcus spp. were frequently separated into different groups, thus showing heterogeneity among the members of these groups. Research highlights: Results obtained here may aid in assessing the pathogenicity of sporadic pathogens in relation to more well-known pathogens and suggest that the traditional classification between minor and major pathogens is an oversimplification of the reality, especially for the latter category.