Protection of pigs against challenge with virulent Streptococcus suis serotype Z strains by a muramidase-released protein and extracellular factor vaccine

The efficacy of a muramidase-released protein (MRP) and extracellular facto r (EF) vaccine in preventing infection and disease in pigs challenged eithe r with a homologous or a heterologous Streptococcus suis serotype 2 strain (MRP+EF+) was compared with the efficacy of a vaccine containing formalin-k illed bacterin of S suis serotype 2 (MRP+EF+). The enhancement of the immun e response by different adjuvants (a water-in-oil emulsion [wo] and an alum inium hydroxide-based adjuvant [AH]) and their side effects were also studi ed. The MRP and EF were purified by affinity chromatography. pigs were vacc inated twice at three weeks and six weeks of age and challenged intravenous ly with virulent S suis serotype 2 strains (MRP+EF+) at eight weeks of age. At challenge, the pigs vaccinated with MRP+EF/WO had high anti-Map and ant i-EF titres and were protected as effectively as pigs vaccinated with we-fo rmulated vaccines with bacterin. Eight of the nine pigs survived the challe nge and almost no clinical signs of disease were observed. The titres obtai ned with the MRP+EF/AH vaccine were low and only two of the five pigs were protected. Pigs vaccinated with either MRP or EF were less well protected; three of the four pigs died after challenge but the clinical signs of disea se were significantly less severe than those observed in the placebo-vaccin ated pigs. The protective capacity of the bacterin/AH vaccine was very low, and the mortality among these pigs was as high as in the placebo-vaccinate d pigs (80 per cent). Postmortem histological examination revealed meningit is, polyserositis and arthritis in the clinically affected pigs. The result s demonstrate that a subunit vaccine containing both MRP and EF, formulated with the WO adjuvant, protected pigs against challenge with virulent S sui s type 2 strains.