Evaluation of protein A and a commercial bacterin as vaccines against Staphylococcus aureus mastitis by experimental challenge.

Protein A and a commercial staphylococcal bacterin were evaluated by experimental challenge with Staphylococcus aureus (ATCC 29740). Thirty cows in first lactation were in three treatment groups, protein A, bacterin, and nonvaccinated controls. Studies were through three lactations and included bacteriological and cytological analyses of quarter milk samples. Rate of intramammary infection with Staphylococcus aureus was similar for vaccinated and unvaccinated cows. Rates of spontaneous cure within each lactation were significantly higher for vaccinated cows. For all three lactations, spontaneous cure rates were 83, 73, and 47% for protein A, bacterin, and control cows. Somatic cell counts were significantly lower for vaccinated cows for quarters infected with Staphylococcus aureus, but no differences were demonstrated for milk production by lactation. Incidence of clinical mastitis was higher in unvaccinated cows, but too few developed for a valid comparison.