Abstract

The aims of this thesis were to describe the innate and adapted immune response during acute and chronic phase of bovine Staphylococcus aureus (S. aureus) mastitis, and to investigate why the infection often becomes persistent. The potential of the milk acute phase proteins (APP) haptoglobin and serum amyloid A (SAA) as indicators of chronic sub-clinical mastitis was also evaluated, as well as the preventive and therapeutic effects of the immunomodulator ß1,3-glucan against intramammary S. aureus infection. After intramammary inoculation of S. aureus, acute clinical mastitis developed and was transformed to chronic sub-clinical mastitis with controlled use of penicillin. Blood and milk samples from infected and healthy quarters were collected during five weeks, and analysed for APP and lymphocyte sub-populations. The most prominent features were increased APP concentrations in serum, and in milk from infected quarters, but not in milk from control quarters, during both acute and chronic phase of mastitis, and an increased proportion of B-lymphocytes and cellular expression of B-cell antigen in blood, infected and healthy quarters. The results indicate that both clinical and sub-clinical mastitis exert effects on local, as well as systemic, innate and adapted immune responses. The B-cell response could be one explanation why the immune system failed to eliminate the infection. When studying naturally occurring cases of chronic sub-clinical mastitis, a large variation in expression of APP in milk, and a discrepancy between the levels of APP and adenosine triphosphate (ATP), an indirect measurement of the milk somatic cell count, was observed. In most cases, healthy cows had undetectable levels of milk APP. The results indicate that milk haptoglobin and SAA can be used as indicators of udder health. Intramammary infusions of ß1,3-glucan failed to prevent experimental S. aureus infection at drying-off, and to eliminate S. aureus infection in cows with chronic sub-clinical mastitis. However, an immunostimulating effect was observed as the expression of MHC class II was increased on lymphocytes from S. aureus-infected quarters. Prevention and elimination of intramammary S. aureus infections using immunomodulators, like ß1,3-glucan, need further studies.