Interrelationships between the content of oxidative markers, antioxidative status, and somatic cell count in cow's milk

Bovine mastitis is a major disease affecting dairy cattle worldwide. The milk collected from cows with different type of inflammation, including mastitis, is characterized by an increased number of somatic cells (SCC), especially polymorphonuclear cells (PMN). It was proved that the antibacterial activity of PMN generates reactive oxygen species and nitric oxide-derived metabolites. The accumulation of the reactive species correlating with a decrease in the antioxidant activity specific for milk can lead to oxidative and nitrosative stress. The aim of this study was to evaluate the antioxidant/oxidant status in normal cow's milk and in subclinical mastitis milk using three parameters: total antioxidant status (TAS); levels of malondialdehyde (MDA) - a degradation product of lipid peroxidation; and levels of proteins oxidation expressed as carbonylated proteins analyzed in the reaction with dinitrophenilhydrazine (DNPH). Subclinical mastitis was diagnosed using an electrical conductivity method and by determining the number of SCC in milk. Comparative analysis of TAS in normal and mastitis milk revealed extremely significant differences (P = 0.0005), the average of this parameter was lower for mastitis milk samples. The results describing the antioxidant status were correlated with those on lipid and protein oxidative degradation. The average level of MDA in mastitis milk was higher compared with normal milk, the mean difference was 40.69 nmol/ml. A similar variation was observed for DNPH, the mean difference was 191.24 µmol/ml. The levels of SCC, MDA, and DNPH were significantly higher in subclinical mastitis milk compared to milk from healthy cows due to the occurrence of oxidative stress state in subclinically inflamed mammary gland.