Is milk cortisol a biomarker of subclinical mastitis and acute phase response in dairy cows?

The aim of the trial was to assess if the concentration of the milk cortisol is related to health disorders and plasma acute phase response in lactating cows. One thousand and forty one lactating cows were sampled from 6 Simmental (IS) and 4 Holstein (IH) commercial farms in Italy. For the study, lactating cows were preferentially chosen from 70 to 250 days in milk (DIM) and with parity from 2 to 6. The day of official milk recording, 100 ml of milk samples were collected from each cow at the morning milking. An aliquot was used for milk composition and for somatic cell count (SCC) determination with mid infrared spectroscopy. A second aliquot of milk was analysed for -hydroxybutyrate (BHB) and cortisol. Blood was sampled after the morning milking and before the morning meal and plasma samples were analyzed for total protein (TP), albumin, globulin, ceruloplasmin (CuCp), haptoglobin (Hp) and paraoxonase (PON). The day of sampling, the body condition score (BCS) was recorded. Cows were classified in 3 classes according to SCC: 1, SCC<200,000; 2, 200,000<SCC<400.000; 3, SCC>400,000 cells/ml. Statistical analysis considered class of SCC and breed as fixed factors and DIM as covariate. BCS was significantly lower (P<0.05) for both breeds in class 3 and, as expected, IS cows had higher values (P=0.000) than IH cows. Milk cortisol was significantly higher (P<0.05) in class 3 for both breeds and no differences were shown between breeds (Figure 1). The SCC were also related to markers of acute phase response. Positive acute phase protein (CuCp and HP) and globulins had significantly (P=0.000) higher plasma values in class 3. Instead, albumins and PON content resulted significantly lower (P=0.000) in this class. Moreover, TP, globulins and Hp were significantly (P<0.05) higher and albumins and PON were significantly (P<0.01) lower for IH cows respect to IS cows. The preliminary data indicate that the variation of milk cortisol concentration parallels acute phase response indicators in plasma. The results of this study suggest that milk cortisol can be a candidate index to diagnose subclinical health disorders, in particular mastitis, in mid lactating cows.