Non-specific resistance of mice to Pasteurella haemolytica.

A technique for challenge infection of mice with Pasteurella haemolytica is described. Mice were challenged intraperitoneally with P haemolytica in gastric mucin, and viable counts were performed 6 h later on liver suspensions. Viable counts of P haemolytica in the livers of unvaccinated control mice had increased 6 h after infection. Mice were injected subcutaneously with a commercial pasteurella vaccine at various time intervals before challenge. In those given vaccine two weeks, one week, 48 h and 1 h before challenge, viable counts of P haemolytica in the liver increased as for control mice. However, if the time interval between administration of vaccine and challenge was 12 or 24h, viable counts of P haemolytica in the liver decreased after challenge. Subcutaneous inoculation with the nonbacterial components of the vaccine 24 h before challenge did not cause such an effect. Inobulation with the vaccine 24h before challenge with Escherichia coli resulted in an increase in the LD50 for that organism, indicating that the effect is probably non-specific.