Milk Lactose as a Biomarker of Subclinical Mastitis in Dairy Cows

SIMPLE SUMMARY: Early bovine subclinical mastitis detection may improve treatment strategies and milk yield and reduce the use of antibiotics. Scientific research is therefore focusing on the identification of new and less expensive biomarkers. One of the biomarkers with the greatest potential is milk lactose. To establish whether lactose could be used as an indicator for subclinical mastitis, milk samples from seventy-two herds were screened for bovine subclinical mastitis agents. This study evaluated the prevalence of subclinical mastitis pathogens, their seasonal occurrence, and the relationship with milk lactose concentration. Milk samples were mostly found to be infected with mixed microbiota, coagulase-negative Staphylococcus and Staphylococcus aureus. Yeast, Enterococcus spp. and coliform bacteria such as Escherichia coli were found during the outdoor grazing period. Increases in somatic cell count and decreases in lactose concentration were directly related to the presence of bovine subclinical mastitis agents. These findings suggest that changes in lactose content could be tracked as a diagnostic method in subclinical mastitis prevention in cows. ABSTRACT: Bovine subclinical mastitis can cause great harm to dairy herds because of its negative impact on milk production and quality and cow health. Improved diagnostic tools are needed to maximise the control of subclinical mastitis distribution and ensure the high quality of milk as an industrial product. Between 2015 and 2020, seventy-two dairy herds were screened for bovine subclinical mastitis causative agents to identify the relationship between seasons, lactose levels and subclinical mastitis infection. The predominant species found in the milk samples were mixed microbiota, coagulase-negative Staphylococcus and Staphylococcus aureus. Yeasts were found exclusively in autumn, while Enterococcus spp. and Escherichia coli were only found in summer and autumn. A negative correlation was detected between milk lactose and number of somatic cells in milk (−0.471; p < 0.001). The lactose levels in milk were closely associated with the prevalence (%) of subclinical mastitis pathogens, such as Streptococcus agalactiae (y = −1.8011x + 10.867, R(2) = 0.9298), Staph. aureus (y = −3.5216x + 25.957, R(2) = 0.8604) and other Streptococci (y = −0.5956x + 7.6179, R(2) = 0.6656). These findings suggest that milk lactose may be used as a biomarker of suspected udder inflammation in modern health prevention programmes for cows to reduce the prevalence of subclinical mastitis pathogens in dairy cattle herds.