The capsular
polysaccharide (CP) of Pasteurella haemolytica serotype A1 is a poor immunogen for the prevention of
pneumonic pasteurellosis of ruminants. To improve CP immunogenicity,
vaccines were prepared with 1.0 mg CP dose-1 with and without the synthetic adjuvant,
muramyl dipeptide (MDP; range 0.2-1.0 mg) or a lipophilic derivative, MDP-sn-glyceryl-dipalmitoyl (
MDP-GDP; range 0.1-1.0 mg). The optimum effective concentration of adjuvant was first determined in lambs and calves and then the efficacy of CP +0.5 mg MDP and CP +1.0 mg
MDP-GDP was compared with that of two commercial
vaccines in calves. After immunization with CP, antibody titers in lambs and calves were typical of that seen with
polysaccharide immunogens and characterized by an early
IgM response followed by later
IgG1 and
IgG2 responses. CP + MDP or
MDP-GDP vaccines induced significantly higher
IgM,
IgG1, and
IgG2 titers. After transtracheal challenge of immunity with P. haemolytica serotype A1, extensive pulmonary consolidation containing P. haemolytica (10(6)-10(8) c.f.u. g-1) was seen in all lambs and calves vaccinated with CP alone and was not significantly different (P < 0.05) from the consolidation and concentrations of organisms in nonvaccinated challenge controls. In lambs,
vaccines containing 1.0 mg CP +0.05 mg MDP or
MDP-GDP significantly reduced pulmonary consolidation and concentrations of P. haemolytica in lung lesions. In calves,
vaccines containing 0.2 mg MDP, 0.5 mg MDP, or 1.0 mg
MDP-GDP also significantly reduced pulmonary consolidation and concentrations of P. haemolytica in lung lesions.
Vaccines containing CP +0.5 mg MDP and CP +1.0 mg
MDP-GDP induced high titer bactericidal
antibodies by 7 days and were more efficacious than two commercial
vaccines. Potentiation of CP with MDP or
MDP-GDP has great promise in furthering the potential of CP as a
vaccine immunogen for the prevention of
pneumonic pasteurellosis.