Thesis defense Kitty Exel

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Udder inflammation, also known as mastitis, is a major problem in dairy cows due to lower milk production, treatment costs, culling and damage to welfare. Mastitis is mainly caused by bacterial infections, of which Staphylococcus aureus is a common species. This bacterium, an opportunistic pathogen often present in the microbiota, can cause several diseases in addition to mastitis in many different animal species. Strains of S. aureus show great variability, they can be grouped into clonal complexes (CCs). It is known that strains from different CCs vary in properties, for example the preferred host and whether they occur mainly in the udder or in other sites. Current control measures are largely based on biosecurity and antibiotic treatment, but their effectiveness is limited, mainly due to the diversity of strains. Therefore, the aim of this thesis was to investigate the effect of strain differences on the epidemiology of S. aureus mastitis and to develop better control measures, such as vaccination, against S. aureus mastitis in dairy cattle.

Results from a stochastic bioeconomic simulation model showed that removal of infected cows was the most effective measure for all strains, with lower incidence of intra-mammary infections (IMI), fewer treatment days and higher net income than other measures based on antibiotic treatment. Other measures showed a strain-dependent effect, where for example the spill-over strain, which has a source outside the udder, was less sensitive to certain measures than other strains.

Body sites can be a source of S. aureus mastitis, especially the hock of cows is often colonized. In dairy goats, the nose in particular appeared to be colonized, which shows that colonization patterns differ between animal species. These insights may help in the development of specific measures to control S. aureus mastitis on different farms and in different animal species.

The use of wall teichoic acid (WTA), a cell wall specific sugar structure, as a possible vaccine antigen was investigated, as existing vaccines do not provide sufficient protection against the variety of strains. Genetic variation in the tar genes, which decorate WTA with N-acetylglucosamine (GlcNAc), was found between strains, which may influence immunological recognition. This will have to be elaborated in further research.

The work presented in this thesis has shown that strain variation has a major impact on the epidemiology, antigen expression and the impact of various control measures against S. aureus infections. Strain variation is therefore important and must be taken into account when implementing measures against S. aureus mastitis.