A potassium thiocyanate extract vaccine prepared from Pasteurella multocida 3:A protects rabbits against homologous challenge

Potassium thiocyanate extracts of a virulent Pasteurella multocida 3:A rabbit isolate were prepared and used as a vaccine in rabbits. The extract contained protein, carbohydrate, hyaluronic acid, lipopolysaccharide, DNA, and RNA. The protein and lipopolysaccharide profiles of the extract were similar to those of the P. multocida cell membrane. Rabbits were vaccinated intranasally (i.n.) or intramuscularly (i.m.) four times at 1- or 3-week intervals and challenged i.n. with the homologous P. multocida 2 weeks after the last vaccination. Rabbits vaccinated with the extract by the i.n. route developed persisting serum immunoglobulin G (IgG) and nasal IgA antibodies, whereas rabbits immunized by the i.m. route produced persisting serum IgG and transient nasal IgA antibodies. The extract prevents the death of rabbits which were vaccinated by either route and challenged. Vaccination by the i.n. route in rabbits reduced the numbers of virulent P. multocida in nasal cavities and lungs and the prevalence and severity of rhinitis and pneumonia. These i.n.-vaccinated rabbits were also resistant to virulent P. multocida colonization in liver, spleen, uterus, and tympanic bullae. Similarly, i.m. vaccination in rabbits resulted in a reduction in the severity of rhinitis; the numbers of virulent P. multocida in lungs; and the prevalence of colonization in liver, spleen, uterus, and tympanic bullae. Vaccination by the i.n. route was superior to that by the i.m. route in that there was a significant reduction in the severity of pneumonia and numbers of virulent P. multocida in nasal cavities and lungs. Rabbits vaccinated with the extract without challenge showed no lesions.