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Degree of Immunity Induced by Killed Vaccines to Experimental Salmonellosis in Mice

Department of Microbiology, University of Hawaii, Honolulu, Hawaii 96822

ABSTRACT

Killed vaccines, deoxycholate-extracted or heated, were shown to induce an effective degree of immunity which protected against death (100%), prevented extensive multiplication, and left the mice with low residual salmonella populations in spleen and liver after intravenous (iv) or intraperitoneal (ip) challenge with virulent Salmonella typhimurium. Protection was most effective against the ip challenge route and less effective against the iv route. A study of the kinetics of the population of bacteria in the spleens and livers of immunized animals showed that after ip challenge there was an initial reduction of 99% at 6 hr after challenge, maintenance of levels of less than 10³ bacteria per organ, and a final population of 10² to 10³ per organ at 21 days. With iv challenge, after an initial reduction of 90% at 6 hr, growth ensued to levels above 10⁶ bacteria per organ until 8 days, followed by a steady decline yielding residual populations of 10³ to 10⁴ in some cases. Organ hypertrophy correlated with bacterial population. Morbidity was prevented (as measured by gain in body weight) by immunization against ip challenge but not against iv challenge. Killed vaccines protected by their ability to induce an immune state which reduced the initial challenge population, prevented extensive multiplication, yet allowed "cellular immunity" to develop due to response to the living challenge infection itself. The consequence was a low-level carrier state similar to that induced by recovery from sublethal virulent infection.

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