Dynamics and impacts of staphylococcal intramammary infections during dairy cows’ nonlactating period

Staphylococcal species are frequently isolated from milk samples of dairy cows. Among the 20 staphylococcal species commonly recovered, Staphylococcus aureus is the most studied. Staphylococcus aureus can cause clinical or subclinical mastitis and often results in persistent infections. The prevalence of S. aureus intra-udder infection (IUI) is important, as a significant percentage of apparently healthy quarters can be infected. Generally, these IUI induce persistent SCC increases in the affected quarter (average of 333,000 cells/mL) and can also reduce milk production by 0.3 to 0.7 kg of milk/cow/day. More than 10% of udder quarters from apparently healthy cows are infected with non-aureus staphylococci (NAS), resulting in more frequent recoveries of this type of IUI than S. aureus IUI. The fact that they are so prevalent explains, in part, the growing interest in NAS. Some of the most common staphylococci recovered from dairy cow milk are Staphylococcus chromogenes, S. haemolyticus, S. epidermidis, S. simulans, and S. xylosus. Although these bacterial species have been considered as minor pathogens, research has revealed that 15 NAS species have been associated with either clinical mastitis cases or high SCC, or both. Nevertheless, the information on NAS is limited. Therefore, the objectives of the study cited were to: (1) describe staphylococcal species–level IUI prevalence at calving in quarters of first-lactation cows; (2) describe staphylococcal species–level IUI prevalence before dry-off and at the following calving in quarters from multi-lactation cows; (3) report the species-level cumulative incidence and persistence of staphylococcal IUI over the dry period; and (4) report the species-level effect of early-lactation staphylococcal IUI on the following lactation milk production and SCC in quarters from first-lactation and multi-lactation cows.

In the study, quarter-milk samples were collected from each recruited cow on five dairy farms using automated milking systems. First-lactation cows were sampled twice between three and 14 days in milk (DIM). Older cows were sampled twice, around 28 and 14 days before dry-off, and twice after calving, at three 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 IUI dynamics, results for 301 quarters of 76 first-lactation cows at calving and 1750 quarters of 447 cows before dry-off and after calving were analysed.

First-lactation cows had a higher prevalence of staphylococcal IUI at calving than multi-lactation cows. In first-lactation cows, the most prevalent staphylococcal IUI in early lactation was S. chromogenes, which was leading to substantial elevation of SCC during the following lactation in comparison to healthy quarters. However, quarter-level milk production was not affected by S. chromogenes IUI in first-lactation cows. Staphylococcus aureus, S. xylosus, S. chromogenes, S. simulans, S. epidermidis, and S. haemolyticus were able to persist throughout the dry period, although persistency rates varied by species. Acquisition of new staphylococcal IUI was also observed during the dry period. Finally, post-calving IUI by S. aureus, S. simulans, S. epidermidis, S. chromogenes, S. xylosus, and S. haemolyticus were all associated with higher quarter SCC in the subsequent lactation. Post-calving IUI by S. aureus and S. epidermidis were associated with quarter-milk yield reduction during the following lactation with losses of 1.9 kg/quarter/day and 1.7 kg/quarter/day, respectively.

In summary: First-lactation cows had a higher prevalence of staphylococcal IUI at calving than multi-lactation cows. In first-lactation cows, the most prevalent staphylococcal IUI in early lactation was S. chromogenes IUI, which led to a substantial elevation of SCC during the following lactation. However, quarter-level milk production was not affected by S. chromogenes IUI in first-lactation cows. Some staphylococcal species were more frequent in older cows. Staphylococcus petrasii, S. pasteuri, S. microti, S. intermedius, S. devriesei, and S. delphini were only present immediately before the dry-off period, and S. warneri, S. saprophyticus, S. pettenkoferi, S. kloosii, S. cohnii, S. arlettae, S. gallinarum, and S. epidermidis were only present in multi-lactation cows during early lactation. Staphylococcus aureus, S. xylosus, S. chromogenes, S. simulans, S. epidermidis, and S. haemolyticus were able to persist throughout the dry period, although the persistency rates varied by species. Acquisition of new staphylococcal IUI was also observed during the dry period. Finally, post-calving IUI by S. aureus, S. simulans, S. epidermidis, S. chromogenes, S. xylosus, and S. haemolyticus were all associated with higher quarter SCC in the subsequent lactation, and postcalving IUI by S. aureus and S. epidermidis were associated with quarter-milk yield reduction during the following lactation.