USE OF LIPOSOME-IMMUNOPOTENTIATED EXOPOLYSACCHARIDE AS A COMPONENT OFAN OVINE MASTITIS STAPHYLOCOCCAL VACCINE

Experiments on the development of a vaccine against staphylococcal mas titis were carried out in ewes. The vaccine (Spanish patent no. 920022 3) has the following components. (i) inactivated (formalinized) bacter ia (Staphylococcus aureus and a coagulase-negative staphylococcal spec ies. Staphylococcus simulans) and S. aureus toroid in presence of an a djuvant (dextran sulfate, M(w) 500 000); and (ii) S. aureus exopolysac charide included within liposomes. High serum antibody titres were obt ained against whole cells from Staphylococcus aureus, Staphylococcus s imulans, Staphylococcus hyicus and Staphylococcus epidermidis strains. However, there was no response to cells from Staphylococcus warneri a nd Staphylococcus chromogenes strains. An immune response (serum IgG) against the inoculated exopolysaccharide was obtained when greater tha n or equal to 20 mu g of exopolysaccharide were included in liposomes and when greater than or equal to 20 mg of exopolysaccharide were adju vanted with dextran sulfate instead of liposomes. For experimental inf ection assays, ewes were vaccinated during pregnancy and challenged ei ther with a low virulence S. simulans strain or with a highly virulent S. aureus strain. In these assays, the incidence of S. simulans subcl inical mastitis and of S. aureus acute mastitis was significantly lowe ; in vaccinated animals than in unvaccinated controls. Specifically, o n challenge with S. simulans, two out of 14 glands became infected amo ng the vaccinated animals and nine out of ten glands in the unvaccinat ed group (p < 0.001). On challenge with S. aureus, no protection was d etected when component (ii) was omitted from the vaccine; nine out of ten animals developed mastitis (two mild, two moderate and five severe ). When both components were used, the prevalence of mastitis was lowe r (p < 0.001): five out of ten animals developed mastitis (four mild a nd one severe).