398 HOST-PARASITE RELATIONSHIPS 



observe effects like those of irradiation unless very small dosages are administered. 



When analyzed for X irradiation alone or typhoid resistance alone, the strain 

 effects were highly significant. For typhoid resistance, differences between strains were 

 more marked in those groups where the irradiation had been more detrimental to sur- 

 vival. In only a few of the exposure groups did the radiation lower the resistance of 

 the S strain to murine typhoid. For Z mice, the previous irradiations lowered survival 

 to typhoid infection in all treatment groups. The Q strain, with its greater ability to 

 withstand radiation, showed somewhat more reduction in survival from typhoid 

 following the exposures of 320 r than of 480 r or 640 r, except in the whole-body treat- 

 ment groups. Murine typhoid disease following 640- r, whole-body exposures was 

 lethal to all the strains. Similarly typhoid following 480-r, whole-body treatments 

 was lethal to all strains except S. The 480-r, regional exposures were effective in 

 lowering the resistance levels of the Z and K mice. The average values of the regional 

 effects on the four strains show that resistance to S. typhimurium was decreased as the 

 exposure doses were increased. These observations support those of Gowen and 

 Zelle 469 who found that X irradiation 15 days before contact with typhoid resulted in 

 reduced survival for the 6 inbred strains of mice tested. 



The high resistance level of the S strain to murine typhoid made the S strain the 

 better indicator for differences between the treatment groups. The full range of 

 survival from 100 per cent at 0-r to per cent at the 640-r, whole-body exposure was 

 covered by the S strain after typhoid infection. The range of survival was limited for 

 the other strains by their greater susceptibility to typhoid even without X irradiation. 

 The natural resistance of the S mice was most severely affected by the whole-body 

 exposures followed by the head-mid exposures. The mid-rear exposure treatments 

 at the three dosage levels were next in order of decreasing survival following typhoid 

 infection. The other regional combinations were, in order of decreasing effects: the 

 mid, head-rear, rear, and head exposures. The latter two exposure treatments had little 

 effect on the S mice. The three other strains, Z, K, and Q,, reacted in a similar manner 

 for the different regional irradiations. The Ba strain was so susceptible even to 200 

 S. typhimurium organisms that it contributed no useful information on the effects of 

 previous X-ray treatments. 



The typhoid-resistance response was affected most severely by the previous 

 irradiations to the whole body. Of the three body regions taken separately, resistance 

 was lowered most by irradiation to the mid region. Irradiation to the head region 

 reduced resistance somewhat more than irradiation to the rear region. The typhoid 

 response following the X-ray treatments showed the head-mid exposure as somewhat 

 more detrimental to natural resistance than the mid-rear exposure. 



Full expression of the effects of body cells in the different regions is exhibited by 

 the S mice. Survival decreased with exposure of any part of the body to X rays at the 

 640-r dose. Exposure of the head and rear regions decreased survival by 4 per cent; 

 exposure of the mid region decreased survival by 1 3 per cent. Treatment of any two 

 of the regions still further decreased survival proportionate to the regions exposed. 



