Dynamics and impacts of staphylococcal intramammary infections during dairy cows' nonlactating periods

Staphylococcal species are frequently isolated from dairy cow milk and are a very common cause of IMI. First-lactation cows may already have an IMI at calving, and these may be acquired during different stages of their development. Moreover, the dry period is another critical "nonlactating" period regarding the acquisition of new IMI, and these may persist until calving. The first objectives of this observational longitudinal cohort study were as follows: to describe staphylococcal species-level IMI prevalence at calving in quarters from primiparous cows; to describe staphylococcal species-level IMI prevalence before dry-off and following calving; and to report the species-level incidence and persistence of staphylococcal IMI over the dry period in quarters from multiparous cows. A second objective was to report the species-level effect of early-lactation staphylococcal IMI on the following lactation milk production and SCS in quarters from primiparous and multiparous cows. Quarter-milk samples were collected from each recruited cow on 5 dairy farms using automated milking systems (AMS; QC, Canada). First-lactation cows were sampled 2 times between 3 and 14 DIM. Older cows were sampled 2 times, around 28 and 14 d before dry-off, and 2 times after calving, at ∼3 and 14 DIM. Staphylococcal species were identified in milk samples using bacteriological culture followed by MALDI-TOF MS. Quarter-milk samples were collected for SCC measurements every other week up to 305 DIM, and daily quarter-milk production was extracted from the AMS software up to 305 DIM. To estimate staphylococcal IMI dynamics, results for 301 quarters of 76 first-lactation cows at calving and 1,750 quarters of 447 cows before dry-off and after calving were available. First-lactation cows had a higher prevalence of staphylococcal IMI at calving than multiparous cows. In first-lactation cows, the most prevalent staphylococcal IMI in early lactation was Staphylococcus chromogenes IMI (10.3%, 95% CI [95CI]: 4.4-21.9), which was leading to substantial elevation of SCS (mean SCS of 3.9, 95CI: 3.1-4.6) during the following lactation in comparison to healthy quarters (1.7, 95CI: 1.2-2.3). However, quarter-level milk production was not affected by S. chromogenes IMI in primiparous cows. Staphylococcus aureus, Staphylococcus xylosus, Staphylococcus chromogenes, Staphylococcus simulans, Staphylococcus epidermidis, and Staphylococcus haemolyticus were able to persist throughout the dry period, although persistency rates varied by staphylococcal species. Acquisition of new staphylococcal IMI was also observed during the dry period. Finally, postcalving IMI by S. aureus, S. simulans, S. epidermidis, S. chromogenes, S. xylosus, and S. haemolyticus were all associated with higher quarter SCS in the subsequent lactation. Postcalving IMI by S. aureus and S. epidermidis were associated with quarter-milk yield reduction during the following lactation with losses of 1.9 kg/quarter-d (95CI: 0.1-3.7), and of 1.7 kg/quarter-d (95CI: 0.2-3.2), respectively. Although many NAS IMI were relatively frequent at calving in first lactation and older cows, could persist throughout the dry period and were associated with a measurable inflammation during the following lactation, few NAS species were actually associated with a measurable milk yield reduction.