Effect of mastitis and related-germ on milk yield and composition during naturally-occurring udder infections in dairy cows

The effect of mastitis and related-germs on milk chemical composition (protein and lactose contents) and milk somatic cell count (SCC) was investigated in 501 milk quarter samples during two consecutive years in cows from three experimental herds. Each infected quarter was matched by a healthy one in the same udder, as a control. Milk protein and mineral assays were performed in a sub-sample of 128 milks. Staphylococci were the most frequently isolated germs (Staphylococcus aureus : 27%, coagulase-negative Staphylococci: 26%, Streptococci: 21%). Major milk pathogens ( Staphylococcus aureus, Streptococcus uberis or Escherichia coli) associated with clinical signs of mastitis were accompanied by higher SCC (+1.6 log.mL(-1), P < 0.01), lower lactose concentration (-7.6 g.kg(-1), P < 0.01), higher protein concentration (+3.3 g.kg(-1), P < 0.01) and higher soluble protein concentrations (IgG and BSA), hence a sharp decrease in the casein/protein ratio (-10 percentage points, P < 0.01). Changes were more marked when Escherichia coli was present. Corynebacterium bovis did not alter milk chemical composition whereas coagulase-negative Staphylococci slightly reduced lactose concentration (-1.8 g.kg(-1)) and increased SCC (+0.37 log.mL(-1)). Calcium and phosphorus milk contents were hardly modified by the presence of microorganisms. The decrease in milk yield during clinical mastitis varied from 1.6 kg.d(-1) in the presence of Staphylococcus aureus to 15 kg.d(-1) in the presence of Escherichia coli.