FIELD TRIALS OF A VACCINE AGAINST BOVINE MASTITIS .1. EVALUATION IN HEIFERS

A vaccine was developed against bovine mastitis based on inactivated, highly encapsulated Staphylococcus aureus cells; a crude extract of St aph. aureus exopolysaccharides; and inactivated, unencapsulated Staph. aureus and Streptococcus spp. cells. This vaccine was tested on 30 he ifers during a 7-mo period. The 30 heifers were randomly assigned to t hree groups of 10 heifers each. The prepartum group received two injec tions of the vaccine at 8 and 4 wk before calving, and the postpartum group received two injections at 1 and 5 wk after calving. The control group received two injections of a placebo at 8 and 4 wk before calvi ng. The vaccine or the placebo was administered subcutaneously in the brachiocephalicus muscle of the neck. The frequencies of intramammary infections caused by Staph. aureus were reduced from 18.8% for heifers in the control group to 6.7 and 6.0% for heifers in the prepartum and postpartum groups, respectively. This protective effect was maintaine d for at least 6 mo. The relative risk of mastitis caused by Staph. au reus was 0.31 and 0.28 for heifers in the prepartum and postpartum gro ups, respectively, compared with that for heifers in the control group . The results of the trial indicated the effectiveness of the vaccine in decreasing the incidence of intramammary infections caused by Staph . aureus. A slight but nonsignificant increase occurred in fat product ion in the milk of vaccinated cows. The vaccine had no observable effe ct on somatic cell count or streptococcal infections.