Dynamics of intramammary infections and their association with quarter-level somatic cell score, milk yield, and milk components. Part 1: Staphylococcal species

On Canadian dairy farms, staphylococcal species, including Staphylococcus aureus and NAS, cause most intramammary infections (IMI) in lactating cows. However, knowledge remains limited on their impact on quarter-level somatic cell score (SCS), milk yield, and composition, and their lactational incidence and elimination rates. This study aimed to describe staphylococcal species-specific IMI association with SCS, milk production, and milk composition in primiparous and multiparous cows, and to describe their prevalence, incidence, and elimination rates. Five commercial dairy farms with automated milking systems in Quebec, Canada, were visited biweekly to collect aseptic quarter-milk samples. Cows were recruited at 3-14 d in milk (DIM) and sampled every other week until 305 DIM. Bacteriological culture and MALDI-TOF mass spectrometry were used for staphylococcal identification. A total of 21,467 samples from 1,537 quarters of 387 cows (307 multiparous, 80 primiparous) were analyzed. The most prevalent staphylococci in mammary gland quarters were S. chromogenes (4.0%), S. epidermidis (2.6%), S. haemolyticus (2.1%), and S. aureus (1.9%). S. auricularis, S. chromogenes, and S. equorum IMI were more frequent in primiparous cows, while S. aureus, S. epidermidis, and S. haemolyticus were more common in multiparous cows. S. haemolyticus and S. epidermidis showed the highest incidence, whereas S. chromogenes and S. simulans had elimination rates similar to or lower than S. aureus. Many lactational staphylococcal IMI were transient, with high species-specific elimination rates. Associations with quarter-level SCS, milk yield, and milk components were evaluated using 7,703 to 9,967 test-day samples from over 1,300 quarters of 369 cows. In primiparous cows, quarters with IMI by S. aureus, S. chromogenes, S. epidermidis, or S. simulans had higher SCS than healthy quarters. In multiparous cows, SCS was elevated in quarters infected by S. aureus, S. chromogenes, S. epidermidis, S. haemolyticus, S. simulans, or S. xylosus. Milk yields in primiparous cows were generally unaffected by IMI status, while in multiparous cows, only S. aureus IMI reduced milk yield (7.9 vs. 10.2 kg/d in healthy quarters). No changes in milk fat were detected. Protein content was higher in S. aureus IMI quarters of multiparous cows. Lactose reductions occurred in primiparous cows' quarters with S. chromogenes or S. simulans IMI, and in multiparous cows' quarters with S. aureus, S. epidermidis, or S. simulans IMI. In conclusion, in primiparous cows, IMI by S. aureus, S. chromogenes, S. epidermidis, and S. simulans increased SCS, with S. simulans also reducing lactose, but no changes in milk yield, fat, or protein. In multiparous cows, higher SCS occurred with several staphylococcal species, milk yield was reduced only with S. aureus IMI, and lactose was reduced in quarters infected by S. aureus, S. epidermidis, and S. simulans.