Abstract
Four heifers were immunized in late pregnancy with two doses of attenuated, live Staphylococcus aureus and challenged during early lactation by intramammary infusion into one quarter of approximately 100 organisms of the same attenuated strain. Three unvaccinated control heifers were challenged similarly. At challenge immunoglobulins G1 and G2 antibodies against Staphylococcus aureus surface antigens were significantly greater in blood serum of vaccinated heifers than in controls. Also at challenge, serum from vaccinated heifers had a significantly greater opsonizing capacity for Staphylococcus aureus than did that of controls. The challenge dose of Staphylococcus aureus did not produce prolonged clinical signs of acute mastitis in any of the heifers; however, once of the control animals remained chronically infected. There was a decrease of milk production following challenge for controls but no such decrease for the immunized heifers. Taken together, results of clinical assessments, bacteriology, and measurements of milk production suggested that vaccinated heifers had higher resistance to the challenge dose than did controls.
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