EVALUATION OF THE VIRULENCE AND IMMUNOGENICITY OF VACCINE STRAINS OF SALMONELLA AND SHIGELLA BY DETERMINING THE MEAN (50%) DOSES (LD50)
ARMY BIOLOGICAL LABS FREDERICK MD
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The following requirements are minimum for the virulence of vaccine strains of salmonella and shigella The LD50 of typhoid, paratyphoid B and Sonne dysentery should not exceed 50 million and Flexners dysentery c and paratyphoid A -- 275 million during infection in a physiological solution. During infection in 0.4 agar the LD50 of paratyphoid A and Flexners dysentery c strains should not exceed 20 million. The median 50 immunizing dose during infection of 2.5 - 3 LD50 of a test-culture in typhoid and Sonne dysentery strains with a single immunization should not exceed 30 million, and in paratyphoid B strains - 60 million. In Flexners dysentery c and paratyphoid A with a double immunization and infection of 2.5 - 3 LD50 of a test-culture it should not exceed 125 million. The index of effectiveness for typhoid, and Sonne dysentery strains with a single immunizing dose of 100 million would be no lower than 3, and for paratyphoid B strains -- no lower than 2.5. For Flexners dysentery and paratyphoid A strains with a double immunization with a dose equal to 200 million the index should not be lower than 3. Cultures of typhoid and paratyphoid B strains of a 6-hour growth process a higher virulence than 20-hour cultures. The virulence of typhoid strains depends on the relationship of the antigens of the culture. In checking the virulence and immunogenicity of salmonella and shigella strains it is necessary to pay particular attention to standardization of the conditions of cultivation and storage, and also the testing of the strains.