Field trial of a Pasteurella multocida extract vaccine in rabbits.

Pasteurella multocida is a bacterial pathogen that can cause significant disease and subsequent effects on research activities involving rabbits. Although several vaccines have been tested under laboratory conditions, field trials of vaccines for the control of P. multocida in rabbits are few. We used a potassium thiocyanate extract (PTE) produced from P. multocida serotype D:3,12,15 to vaccinate Pasteurella-free rabbits at their introduction into a colony having endemic infection with P. multocida serotype A:3. Groups of 15 rabbits were vaccinated either SC or IN with 1.0 mg PTE once weekly for 3 wk. In addition a control group was sham-vaccinated IN with saline. After the last vaccine dose had been administered, rabbits were housed with the general colony of a facility with endemic pasteurellosis. Serum samples obtained before and 5 and 24 wk after the first dose of vaccine were evaluated by ELISA for anti-PTE IgG. Rabbits were euthanized if found in poor clinical condition, and all remaining rabbits were euthanized 24 wk after initial vaccination. Clinical disease typical of P. multocida infection was observed in 10 of 15 saline-vaccinated rabbits, 4 of 15 IN PTE-vaccinated rabbits, and 1 of 15 SC PTE-vaccinated rabbits. Bacterial culture of the nasopharynx at the time of necropsy was positive for P. multocida in 10 of 15 control rabbits, 5 of 15 IN PTE-vaccinated rabbits, and 1 of 15 SC PTE-vaccinated rabbits. Anti-PTE serum IgG activity had developed in both IN- and SC-vaccinated rabbits by 5 wk, with significantly lower activity by 24 wk after initial vaccination. IgG activity was significantly greater in rabbits vaccinated SC compared with controls or those vaccinated IN. In summary, PTE can be used to stimulate protective immunity to a heterologous strain of P. multocida, with stronger immunity generated by SC than IN vaccination.