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Milk Prolactin Response after Experimental Infection with Different Coagulase-Negative Staphylococci in Dairy Heifers
Coagulase-negative staphylococci (CNS) are the most common group of bacteria involved in subclinical bovine mastitis. CNS-infected dairy heifers seem to produce more milk than uninfected heifers but the underlying mechanism is yet unclear. This study investigates the response of prolactin (PRL) in milk as a potential mediator of milk yield (MY) after experimental challenge with different CNS species. Eight Holstein-Friesian heifers in mid-lactation (126 DIM ± 66) were challenged in a split-udder design with 3 different CNS isolates: one S. fleurettii isolate from sawdust and two phenotypically dissimilar S. chromogenes isolates. The first S. chromogenes isolate originates from a chronic intramammary infection while the other is cultured from a teat apex. Three quarters were simultaneously inoculated with 1,0 x 106 colony forming units. The remaining quarter was infused with sterile phosphate-buffered saline and served as a control. Milk samples were obtained for measuring PRL (by radioimmunoassay) at various time points starting 24 hours pre-inoculation until 72 hours after challenge. Furthermore, quarter MY was recorded. Milk samples were cultured to evaluate bacterial clearance. A linear mixed regression model, using heifer and quarter as random effects, was built to evaluate the PRL response after infection with sampling time and inoculation type as fixed effects. Pre-inoculation data were not included in the analysis. None of the quarters developed clinical symptoms and none of the heifers showed signs of illness. Milk culture results revealed that all CNS were eliminated before the end of the trial. Even though this study did not focus on MY, a decreased production was observed in all quarters. The infection status did not have a demonstrable effect on milk PRL concentration: no significant difference was found between infected and control quarters, or between different CNS-isolates (P = 0.40). However, milk PRL generally changed over time (P < 0.05). These findings suggest that milk PRL is not a likely candidate to explain any potential increase in milk production after a subclinical infection caused by S. chromogenes or S. fleurettii.
Keywords: Prolactin – Mastitis – Coagulase-negative staphylococci