RECOMBINANT VACCINES AGAINST OVINE FOOTROT

For the past 20 years footrot vaccines have evolved from simple bacter ins to highly specific recombinant DNA (rDNA) fimbrial vaccines. The d evelopment of these vaccines has left a trail of discoveries, challeng es and solutions; these processes continue as we move closer to unders tanding the requirements of a footrot vaccine. The initial whole cell vaccines were unsuccessful due to the short duration of immunity and i ncorporation of limited serotypes. A multistrain vaccine eliminated th e problem of serotype inclusion, although the duration of immunity in many cases is still inadequate. The proteases of Dichelobacter nodosus appear to be cross protective, however, little is known of their abil ity to protect sheep against footrot. The major protective immunogen i s the bacterial fimbriae, which also forms the basis for the K-aggluti nation serotyping system. K-agglutinin titre correlates directly with resistance to challenge. The protective fimbrial epitope is conformati onally dependent, suggesting little advantage in the development of sy nthetic peptide vaccines. To enhance the efficiency of vaccine product ion D. nodosus fimbrial genes were eventually cloned and successfully expressed in Ps. aeruginosa. Monovalent vaccines based on recombinant fimbriae are omnipotent, inducing high levels of agglutinins and long lasting immunity. In multivalent vaccines, on the other hand, incorpor ation of each additional serogroup into the vaccine results in reduced efficacy both in terms of reduced K-agglutinin titres and reduced pro tection following challenge. The least effective are multivalent formu lations representing all major serogroups. In addition, considerable g enetic variation has been observed in the ability of sheep to respond optimally to each serogroup in a multivalent vaccine. Results show tha t the limitation of the sheep to mount an effective immune response, r ather than the quality or quantity of the immunogen, limits the effica cy of current footrot vaccines. Studies are being undertaken to examin e in detail the immune response of sheep to potentially highly effecti ve footrot vaccines.