Effects ofStaphylococcus aureusMastitis after Endotoxin Application on Milk Yield and Composition during Subsequent Lactation of Guinea‐pigs

The effects of Staphylococcus aureus mastitis on milk yield and composition throughout subsequent lactation in lactating guinea-pigs and the role of endotoxin pretreatment on these phenomena were investigated. Primiparous lactating guinea-pigs were intramammarily inoculated with sterile saline (group 1), S. aureus strain UC 6097 (group 2), or with S. aureus UC 6097 after endotoxin pretreatment (group 3). Clinical signs and survival rate were monitored. During the second lactation, daily milk yield was measured and milk composition was determined. In primiparous inoculated guinea-pigs, moderate (group 3)-to-severe acute mastitis (group 2) was produced. During subsequent lactation, milk yield in the control group peaked on day 5 and then decreased. Concentrations of Na+ and Cl- in milk, and concentrations of fat, gradually increased, but lactose and K+ decreased. After an early decrease, NAGase in milk increased towards the end of lactation. Except for higher NAGase concentrations in group 3, milk yield and composition during the second lactation did not differ significantly between the mastitis and the control groups. Endotoxin pretreatment only plays a role in the determination of the severity of the infection.