Characterization of Non-aureus Staphylococci Isolated from Intramammary Infections and Extramammary Sites on Dairy Farms

Abstract

This thesis is an investigation of the diagnosis, udder health effects and sources of nonaureus staphylococcal (NAS) intramammary infections (IMI) on dairy farms. The first objective of this series of studies was to evaluate the diagnostic accuracy of different target genes (rpoB or cpn60) and reference databases (GenBank or Chaperonin) to identify NAS species. The use of both rpoB and cpn60 genes using either databases provided similar accuracy in the identification of NAS species, although the highest proportion of NAS species was correctly identified with the rpoB gene using the GenBank database. The second objective was to determine the effects of NAS species on udder health and milk yield compared with healthy quarters and quarters infected with a major pathogen. The majority of NAS species had higher somatic cell counts (SCC) than healthy quarters and lower SCC than S. aureus-positive quarters. These SCC values were within the range of what is considered a ‘healthy’ SCC level. There was no effect of NAS IMI on overall milk yield. The third objective was to characterize the distribution of NAS isolated from the environment, body sites and IMI on dairy facilities and to determine potential sources of NAS IMI by comparing isolates from milk samples and extramammary sites. Staphylococcus chromogenes was identified exclusively from milk samples and represented the largest proportion of isolates. Staphylococcus equorum was identified exclusively from body and environmental site samples. Staphylococcus xylosus was the only NAS species that was identified from all sample types and was present in both milk and extramammary sites from the same cow. The fourth set of objectives were to determine (1) the proportion of specific S. chromogenes genotypes on dairy farms; (2) the effect of these genotypes on SCC; and (3) the persistency of each genotype within quarters. Of all 4 S. chromogenes genotypes, Genotype 1 was the most predominant in the study and had the largest proportion of persistent IMI identified. There was no effect of SCC among all genotypes and SCC levels were considered to be healthy.