Non-aureus staphylococci (NAS) have become the most prevalent cause of intramammary infections (IMI) in dairy cows in most parts of the world. The current body of literature on IMI caused by NAS has reported debatable and contradictory conclusions on their relevance for udder health and milk production. The aims of this thesis were to precisely estimate the impact of NAS IMI on somatic cell count (SCC) and milk yield (MY) at the quarter-level.
A longitudinal study was conducted on 3 dairy herds equipped with an automatic milking system. This allowed us to precisely measure MY at the quarter-level. Milk samples of each quarter were collected within 1-4 days in milk (DIM) followed by another 9 consecutive samplings at 14-day intervals until 130 DIM. Also, SCC from all quarter milk samples was measured.
In the first 4 DIM, 21% of the quarters were infected with NAS. These quarters had a higher SCC during the first 4 months of lactation compared with noninfected quarters. During the first 130 DIM, MY was not different between these NAS-infected and noninfected quarters. Also, the milk prolactin concentration did not differ.
Based on culturing results at 1-4 DIM and 15-18 DIM, the spontaneous cure rate was 70% of quarters being NAS-infected at 1-4 DIM. Staphylococcus chromogenes was the most prevalent species. Quarters infected with S. chromogenes had a higher SCC during later lactation, whereas there was no difference between quarters infected with the group of other NAS species and noninfected quarters. Persistent IMI and new IMI at the second sampling day resulted in a higher qSCC during the first 4 months of lactation, but transient IMI had no effect on later SCC. However, none of the different types of infections with NAS was associated with qMY in the first 4 months of lactation.
The IMI status was also determined for each of the 10 sampling days. A total of 304 NAS isolates belonging to 17 different species were cultured from 116 quarters of 64 out of the 82 heifers. This indicated that the prevalence of NAS IMI is high during the first 4 months of lactation. The most prevalent species was S. chromogenes (52% of the isolates). 45% of IMI caused by S. chromogenes persisted with an average duration of 110 days compared with 9.8% of IMI caused by other NAS species with an average duration of 70 days. Quarters that were persistently infected with NAS had the highest SCC, whereas the impact on udder health was smaller if an IMI was only present at 1 sampling point (i.e., a transient infection). Analysis of the associations between quarter infection status and qMY during the first 130 DIM did not reveal a positive or negative relation between transient or persistent NAS infections and MY.
In general, the status of NAS as minor pathogens, resulting in a moderately elevated SCC without a negative impact on MY, was substantiated in early-lactation dairy heifers.
Dr. Dimitri Valckenier graduated as a veterinarian in 2011 from Ghent University, Belgium. After completing a one-year internship training at the European College of Veterinary Public Health, he started working as an academic assistant at the Faculty of Veterinary Medicine at the same university. Here he also started his doctoral research on the impact of intramammary infections with NAS on udder health and milk production in dairy heifers. Furthermore, he was engaged in the clinic of reproduction and obstetrics, the ambulatory clinic, and doing herd health visits on multiple dairy farms. From 2015 to early 2017 Dimitri also worked as a veterinary advisor at the Dutch-speaking Regional Council of the Board of Veterinarians. Since January 2017, he is active as Project & Concept Manager at (MEX™).
Text and picture: Dimitri Valckenier
