Susceptibility to infection with Pasteurella tularensis and the immune response of mice exposed to continous low dose rate gamma radiation.

Abstract

Abstract : Mice were exposed to continuous low dose rate (1.4 R/hr) gamma radiation. Immediately after accumulating 1000 R, 2000 R, or 3000 R, they received a subcutaneous injection of the live vaccine strain (LVS) of Pasteurella tularensis. Although the avirulent strain caused no effect in normal mice, a fulminating infection occurred in irradiated mice. As the total radiation dose increased the LD50 of the organism decreased. Animals surviving irradiation and immunization were subjected to an aerosol challenge of either the avirulent LVS or the virulent SCHU S-5 strain of P. tularensis. The LVS strain is virulent for mice when administered as an aerosol, but not when injected subcutaneously. Although an aerosol of avirulent organisms was lethal for normal mice, immune mice, both non-irradiated and irradiated, survived an aerosol infection with the avirulent organism. Irradiated, immunized mice challenged with an aerosol of the virulent SCHU S-5 strain showed a decrease in immunity, especially at the higher levels of radiation. Bacterial counts of the lungs, liver, spleen, and lymph nodes of non-immunized mice exposed to a lethal aerosol dose of the avirulent P. tularensis (LVS) showed increasing numbers of bacteria after the 2nd post-infection day. This increase continued until the death of the animal on about the 10th day. Immune animals, however, were able to clear the lungs 5 days after infection with no involvement of the other tissues. Although continuous low dose rate gamma radiation does not produce the gross radiation pathology that one sees following acute radiation, exposure to a live avirulent Pasteurella tularensis vaccine produces an infection and an impaired immunity.