PROTECTION OF EXPERIMENTALLY INFECTED-PIGS BY SUILYSIN, THE THIOL-ACTIVATED HEMOLYSIN OF STREPTOCOCCUS-SUIS

Three groups of three pigs were vaccinated either with vaccine VAC-SLY , containing purified suilysin derived from Streptococcus suis strain P1/7 (serotype 2), or with vaccine VAC-SCF, containing most of the oth er extracellular antigens produced by strain P1/7 (but essentially fre e from suilysin), or with a placebo vaccine. The pigs were vaccinated twice at four weeks and six weeks of age and were challenged intraveno usly with S suis strain P1/7 at eight weeks of age. On the day of chal lenge, only the VAC-SLY vaccinated pigs showed an increase in haemolys in neutralisation titre, After challenge the placebo vaccinated pigs d eveloped severe clinical signs characterised by lameness involving sev eral joints, a depressed appearance, high temperatures and/or neurolog ical signs. The VAC-SCF vaccinated pigs showed the same clinical signs but less severely. The VAC-SLY vaccinated pigs were the least affecte d and showed only mild signs which subsided more quickly than those of the other groups. A post mortem investigation and histology of brain tissue samples confirmed the clinical findings; fibrinous arthritis wa s less severe and less frequently observed in the VAC-SLY vaccinated p igs than in the VAC-SCF or placebo vaccinated pigs, and none of the VA C-SLY vaccinated pigs had meningitis whereas two of the VAC-SCF and tw o of the placebo vaccinated pigs did so. All the samples of brain, lun g and tarsus taken from the VAC-SLY vaccinated pigs were sterile where as S suis was reisolated from most of these tissues from the other gro ups.