Repeated infusion of killed Staphylococcus aureus has increasingly negative effects on milk yield and composition

Mastitis reduces milk yield of affected mammary glands, but the mechanisms that reduce mammary gland productivity are unclear. We previously aimed to develop a reduced milk yield mastitis model via a single sterile intramammary infusion of oyster glycogen but failed to observe milk yield response even when milk SCC spiked greater than 3,000,000 cells/mL. The aim of this study was to assess the impact of repeatedly challenging mammary glands in a split-udder model to determine if duration of immune response would affect milk yields. Four mid-lactation primiparous cows were used. Quarters of a randomly selected udder half were infused with saline (SAL), whereas the quarters of the opposite udder half were infused with 2 billion cfu of formalin-fixed <i>Staphylococcus aureus</i> (FX-STAPH). Intramammary infusions were repeated every 3 d for 12 d; milk yields and composition were measured at each milking (2×/d). Milk yield, composition, and somatic cell scores were similar between udder half treatments before challenge. After the first challenge, cows only displayed subclinical mastitis, and after repeated challenges, bouts of mild clinical mastitis resulted. Somatic cell scores of FX-STAPH udder halves were greater than SAL udder halves at all postchallenge milkings. Milk fat content was unaffected by udder half treatment. Conversely, FX-STAPH udder halves had greater overall mean protein content and lesser lactose content than SAL udder halves. Milk yield of FX-STAPH udder halves were lower than that of SAL udder halves, and the magnitude of depression increased with study duration. Similarly, milk component treatment differences became more pronounced after repeated challenges. The results of this study indicate that the duration of somatic cell count elevation during subclinical or mild clinical mastitis affects the degree of milk compositional change and yield loss.