Cows with diverging haplotypes show differences in differential milk cell count, milk parameters and vaginal temperature after S. aureus challenge but not after E. coli challenge

Background

In dairy cattle, mastitis causes high fnancial losses and impairs animal well-being. Genetic selection is used to breed cows with reduced mastitis susceptibility. Techniques such as milk cell fow cytometry may improve early mastitis diagnosis. In a highly standardized in vivo infection model, 36 half-sib cows were selected for divergent paternal Bos taurus chromosome 18 haplotypes (Q vs. q) and challenged with Escherichia coli for 24 h or Staphylococcus aureus for 96 h, after which the samples were analyzed at 12 h intervals. Vaginal temperature (VT) was recorded every three minutes. The objective of this study was to compare the diferential milk cell count (DMCC), milk param‑ eters (fat %, protein %, lactose %, pH) and VT between favorable (Q) and unfavorable (q) haplotype cows using Bayes‑ ian models to evaluate their potential as improved early indicators of diferential susceptibility to mastitis.

Results

After S. aureus challenge, compared to the Q half-sibship cows, the milk of the q cows exhibited higher PMN levels according to the DMCC (24 h, p<0.001), a higher SCC (24 h, p<0.01 and 36 h, p<0.05), large cells (24 h, p<0.05) and more dead (36 h, p<0.001) and live cells (24 h, p<0.01). The protein % was greater in Q milk than in q milk at 0 h (p=0.025). In the S. aureus group, Q cows had a greater protein % (60 h, p=0.048) and fat % (84 h, p=0.022) than q cows. Initially, the greater VT of S. aureus-challenged q cows (0 and 12–24 h, p<0.05) reversed to a lower VT in q cows than in Q cows (48–60 h, p<0.05). Additionally, the following fndings emphasized the validity of the model: in the S. aureus group all DMCC subpopu‑ lations (24 h-96 h, p<0.001) and in the E. coli group nearly all DMCC subpopulations (12 h-24 h, p<0.001) were higher in challenged quarters than in unchallenged quarters. The lactose % was lower in the milk samples of E. colichallenged quarters than in those of S. aureus-challenged quarters (24 h, p<0.001). Between 12 and 18 h, the VT was greater in cows challenged with E. coli than in those challenged with S. aureus (3-h interval approach, p<0.001).