FIELD TRIALS OF A VACCINE AGAINST BOVINE MASTITIS .2. EVALUATION IN 2COMMERCIAL DAIRY HERDS

A vaccine against bovine mastitis was developed. The vaccine was based on inactivated, highly encapsulated Staphylococcus aureus cells; a cr ude extract of Staph. aureus exopolysaccharides; and inactivated unenc apsulated Staph. aureus and Streptococcus spp. cells. In this study, t he vaccine was evaluated in 164 cows from two commercial dairies (A an d B) during a 4-mo period. Two doses of the vaccine were administered subcutaneously to 82 cows in the brachiocephalicus muscle of the neck within a 4-wk interval. The results of this trial revealed significant ly fewer intramammary infections caused by Staph. aureus at various le vels of severity (clinical, subclinical, and latent) in cows that were vaccinated. The odds ratios of all types of intramammary infections c aused by Staph. aureus for dairies A and B, which were determined by a logistic model, were 1.84 and 1.89, respectively, for quarters of vac cinated cows and quarters of control cows. The colony counts for Staph . aureus in milk from infected quarters of vaccinated cows were signif icantly lower than those in milk from infected quarters of control cow s. Also, the somatic cell counts per milliliter in milk from vaccinate d cows were significantly decreased when the initial somatic cell coun t was <500,000 cells/ml at the start of the trial. The vaccine had no observable effect on fat production in milk or on streptococcal infect ions.