Experiments were undertaken to enhance the immune response of mice, guinea pigs and sheep to C. pseudotuberculosis vaccine.
The antibody response of guinea pigs was not increased either by the simultaneous administration of formalin inactivated vaccine and heterologous live vaccine, or by vaccines grown on special media and extracted with ether : ethanol or sodium lauryl sulphate, or by endotoxin.
In mice and sheep the immunity was not enhanced either by the intravenous administration of inactivated vaccines or by the subcutaneous injection of live vaccines prepared from avirulent strains.
In sheep an inactivated vaccine, concentrated tenfold, caused a moderate increase in the antibody titre and there was a marked improvement in their immunity. Such sheep were markedly more resistant to the lethal effects of a subacute infection of living bacteria than those that received standard inactivated vaccine. It is concluded that concentrated C. pseudotuberculosis vaccine does not confer an absolute immunity but it would be of value when used shortly before possible exposure to infection, e.g. shearing.