Comprehensive evaluation of milk biomarkers as indicators of intramammary infection in dairy goats across lactation

We evaluated the milk biomarkers cathelicidin (Cath), haptoglobin (Hp), milk serum amyloid A (M-SAA), N-acetyl-β-D-glucosaminidase (NAGase), lysozyme (LZ), and the half-udder milk somatic cell count (H-SCC), for their potential association with intramammary infections (IMI) in dairy goats across lactation stages. Half-udder milk from 105 goats was collected during early (39 ± 13 days in milk, DIM), mid (136 ± 12 DIM), and late (269 ± 12 DIM) lactation stages and analyzed for bacteriological culture (BC), H-SCC, and potential biomarker reactivity. Predictive performance was evaluated with BC as the outcome, and Dairy Herd Improvement records provided contextual production data. Cath positivity, assessed by western blot, showed the strongest association with positive BC in early lactation and performed well also in mid-lactation. A dynamic H-SCC threshold was reliably correlated with positive BC in early and mid-lactation. Hp positivity, assessed by Western blot, was associated with BC in mid-lactation, while NAGase, assessed by enzymatic assay, showed no consistent association. M-SAA, assessed by ELISA, suffered technical limitations due to a lack of dilution linearity, and LZ, assessed by enzymatic assay, showed no association with BC. Our data suggest stage-dependent associations and support further validation of Cath, Hp, and SCC as potential components of combined diagnostic panels, particularly in early and mid-lactation. However, a semi-quantitative Western blot assessment for Cath and Hp, as well as technical limitations for M-SAA, warrant cautious interpretation and further analytical validation.