Immunization of White Rats against Infections with Pasteurella tularensis

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Abstract
White rats (Rattus norvegicus) were found to be less susceptible to infection with P. tularensis than white mice, guinea pigs or domestic rabbits. Many rats infected with suspensions containing sufficient organisms to kill the other rodents tested became ill but subsequently recovered. In such animals specific agglutinins against P. tularensis could be demonstrated. The degree of susceptibility of white rats to infections with P. tularensis suggested that they might be used to test the value of vaccines against tularemia. Vaccines were prepd. from the yolk sacs of chicken embryos infected with P. tularensis or from suspensions of these organisms grown on; glucose-cystine blood agar. The organisms were killed with phenol or formalin. Vaccines made from yolk sac suspensions were extracted with ether and the aqueous phase used according to the method Topping and Shear developed in studying typhus fever. The results indicated that the yolk sac vaccines were immunogenic and protected white rats given this vaccine against subsequent infection with P. tularensis. The degree of protection was manifested by a decrease in mortality rate and an increase in survival time of vaccinated rats as compared to unvaccinated control rats. A soluble antigen was demonstrated in the aqueous phase of vaccines following extraction with ether. This antigen was capable of entering into a complement fixation reaction. Vaccines from which organisms were removed by centrifugation and which contained soluble antigen were found to be immunogenic.

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