Hemorrhagic septicemia (HS) is a fatal systemic disease of cattle and buffaloes. In South Asia HS is caused by infection with Pasteurella multocida serotype B:2. Some control is achievedwith killed whole-cell vaccines injected subcutaneously, but these provide only short-termimmunity and require annual administration. For live attenuated strains to be used as vaccines,the mode of attenuation should be well defined. Two groups of 5 calves each were immunized intramuscularly (i.m.) in three weeks interval (does it means 3 injections at 0, 3 and 6 weeks) with two doses of a 10 ml of 4 h culture (2 - 4 × 109 CFU ml-1) of a derivative of P. multocida serotype B:2 Iranian native strain containing an inactivated aroA gene (P. m. MT1). Ten vaccinated calves and two unvaccinated control calves were challenged subcutaneously, 3 weeks after the last injection (it means 3 weeks after the last injection) with 2 different doses of a 4 h wild-type P. mmultocida culture. Five calves injected by one ml of pure culture (3.4 × 109 CFU ml-1) and other 5 and two control unvaccinated calves were taken one ml of 10 fold diluted culture (3.4 × 108 CFU ml-1). The vaccinated calves did not show any clinical signs of illness but the control calves were shown signs of illness such as rise in temperature, respiratory distress with nasal discharge, and increase salivation. Two control calves were killed within 20 h post challenge. This experiment showed that the aroA mutated P. multocida strain can act as an effective live-attenuated vaccine to protect calves against challenge with the virulent strain.
Key words: P. multocida, live vaccine, aroA mutant, Hemorrhagic septicemia
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