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Short communication: Effects of Bos taurus autosome 9-located quantitative trait loci haplotypes on enzymatic mastitis indicators of milk from dairy cows experimentally inoculated with Escherichia coli.

The aim of this study was to investigate the effect of a quantitative trait locus associated with mastitis caused by Escherichia coli, with one haplotype being more susceptible (HH) and another being more resistant (HL) to E. coli mastitis, on the activity of 4 inflammatory related milk enzymes. In particular, we investigated the suitability of β-glucuronidase (GLU) as an early indicator of E. coli mastitis. Besides GLU, the enzymes l-lactate dehydrogenase (LDH), N-acetyl-β-d-glucosaminidase (NAGase), and alkaline phosphatase were included. The study was conducted in an experimental setup with 31 Holstein cows divided into 4 groups representing repeated experiments and, within group, divided according to quantitative trait locus haplotype. All cows were inoculated with viable E. coli, and milk samples were collected 27 times from -6 to 396 h post-E. coli inoculation (PI). Activity of the 4 enzymes in milk, somatic cell count (SCC), daily milk yield, viable E. coli counts, and results of a semiquantitative polymerase chain reaction for pathogen detection, were all analyzed with a repeatability model. The response variables all expressed a strong reaction to the E. coli infection. Daily milk yield decreased significantly at 12 h PI and bacteria counts increased 100-fold and peaked at 18 h PI, which was validated by PCR. Also, SCC started to increase at 12 h PI and increased on average 70 times; however, no significant differences in SCC level were detected between HH and HL cows at any sampling point. The enzymes LDH, NAGase, and alkaline phosphatase showed similar responses, with a significantly increased activity and higher peak values for the HH than the HL cows. Significant differences between HH and HL cows were detected at different time points for these 3 enzymes, but not after adjusting P-values for multiple testing. A different pattern was also observed for GLU, where HL cows expressed the highest peak activity. Indication of differences in GLU activity between the 2 haplotype groups was only seen at 60 h PI. It was concluded that HL and HH cows expressed similar response patterns after E. coli infection but with differences in the size and profile of the activity of the 4 enzymes. The enzyme GLU was an equally good indicator of E. coli mastitis compared with the other studied enzymes, although it showed a slower response compared with LDH and NAGase.