Milk prolactin response and quarter milk yield after experimental infection with coagulase-negative staphylococci in dairy heifers

Coagulase-negative staphylococci (CNS) are the most common bacteria involved in subclinical mastitis in dairy cows. Remarkably, CNS-infected dairy heifers produce more milk than uninfected heifers. Because the lactation hormone prolactin (PRL) is also involved in mammary gland immunity, we investigated the milk PRL response and the mammary quarter milk yield following experimental CNS challenge. Eight healthy Holstein-Friesian heifers in mid-lactation were experimentally infected using a split-udder design with 3 different CNS strains: one Staphylococcus fleurettii (from sawdust bedding) and 2 Staphylococcus chromogenes strains (one isolate from a teat apex, the other isolate from a chronic intramammary infection). Three mammary quarters per heifer were simultaneously inoculated with 1.0 × 106 cfu, whereas the remaining mammary quarter was infused with sterile phosphate-buffered saline, serving as a control. An existing radioimmunoassay was modified, validated, and used to measure PRL frozen-thawed milk at various time points until 78 h after challenge. The mean milk PRL level tended to be higher in the CNS-challenged mammary quarters compared with the control mammary quarters (7.56 and 6.85 ng/mL, respectively). The increase in PRL over time was significantly greater in the CNS-challenged mammary quarters than in the control mammary quarters. However, no difference was found in the PRL response when comparing each individual CNS strain with the control mammary quarters. The mean mammary quarter milk yield tended to be lower in the CNS-infected mammary quarters than in the control mammary quarters (1.73 and 1.98 kg per milking, respectively). The greatest milk loss occurred in the mammary quarters challenged with the intramammary strain of S. chromogenes. Future observational studies are needed to elucidate the relation between PRL, the milk yield, and the inflammatory condition, or infection status, of the mammary gland.