In dairy goats, non-aureus Staphylococcus spp. (NAS) are the most common cause of subclinical mastitis. Many NAS species can cause intramammary infections (IMI) in goats, and pathogenicity seems to differ between NAS species. Data from cows also suggest that NAS species may occupy different ecological niches. However, more species-specific data are needed, which also requires more reliable species identification of NAS isolates, such as genotyping or other molecular-based methods.
The objectives of this PhD dissertation included the validation of MALDI-TOF mass spectrometry as a method for reliable species identification of staphylococci isolated from goats, the evaluation of the persistence of IMI during lactation and over the dry period using speciation and strain-typing, and the evaluation of the role of teat skin as a potential reservoir for IMI. A series of studies evaluating goats during lactation and over the dry period were conducted in three herds ranging in size from 42 to 1,400 lactating goats.
The typeability of MALDI-TOF for identifying staphylococcal species isolated from goats’ milk was 84%, with some v
ariation between species. When compared to partial sequencing of rpoB, tuf, or 16S rRNA, the accuracy of MALDI-TOF using a score of ≥ 2.0 for identification at the species level was 100%.
Staphylococcus chromogenes (goat-level), and Staphylococcus epidermidis and Staphylococcus simulans (udder-half level) were associated with a higher somatic cell score (SCS). During lactation, species that were associated with more persistent IMI (defined at the species and strain level) were Staphylococcus caprae and S. simulans. Over the dry period, no significant difference between species was detected with regard to persistence.
Subspecies strain-typing using pulsed-field gel electrophoresis demonstrated that 7-20% of infections that would have been considered persistent based on species identification alone were not persistent. There was a higher odds of new IMI when the contralateral half had a NAS IMI suggesting the possibility of contagious spread, which was further supported by strain-typing for some species of NAS.
When comparing species isolated from pre-partum teat skin swabs to those isolated from milk collected at the onset of lactation, S. epidermidis was more frequently isolated from milk, S. caprae, S. simulans, and Staphylococcus xylosus were commonly isolated from both milk samples and teat skin swabs, and Staphylococcus arlettae, Staphylococcus equorum, and Staphylococcus lentus were more commonly isolated from teat skin swabs than from milk. Additionally, pre-partum skin colonization by S. caprae appeared to be a risk factor for S. caprae IMI at the onset of lactation.
The possible confounding effect of small ruminant lentivirus (SRLV) infection on the association between NAS IMI and milk SCS was investigated, but was not supported by the data from the study herds.
Overall, based on evidence of persistence and association of IMI with SCS, the NAS species most consistently identified as more pathogenic in goats were S. caprae, S. simulans, and S. epidermidis.
Dr. Véronique Bernier Gosselin received her Doctorate of Veterinary Medicine and Master’s degree from the University of Montréal. She completed a residency in food animal medicine and surgery and a doctoral program at the University of Missouri under the supervision of Dr. John Middleton. She is now a diplomate of the American College of Veterinary Internal Medicine, Large Animal Internal Medicine specialty, and completed her PhD in 2018. She currently works in a private dairy practice in Québec, Canada.