.3.90 



HOST-PARASITE RELATIONSHIPS 



Fig. 49. Survival of mice following acute exposure to different quantities of the 

 pathogen, Salmonella typhimurium, and during the course of the disease. 



\5 x 10 \ 



V\2xiO b 



2* I0 6 \ 



S 



5 10 15 



DAYS SURVIVED 



20 



10 to 90 per cent survival of the treated population, as it is within these limits that 

 most information on disease processes may be obtained. 



Possibly the most satisfying evidence for progress in selecting for resistance to 

 disease, beginning with an unselected population, comes in the changes which appear 

 in days survived in survival curves for different dosages. Figure 50 illustrates these 

 curves as obtained by Schott 1167 and Hetzer. 573 



The selected mice were unquestionably more resistant to the S. typhimurium when 

 tested by each of these three different dosages than the unselected population from which 

 they came. These tests were made in 1936. The S strain was further selected to the 

 nineteenth generation. Selections were discontinued at that time and the strain 

 maintained by sibling matings. Today the mice are as resistant as those shown by the 

 data of figure 50. The lapsed time over which the S mice have retained their resistance 

 is 2 1 years. This period represents a turnover of 63 ± generations, or in terms of 

 generation time in man about 1 ,900 years. 



The syndrome by which the disease was first described also changed during the 



1 1 generations of selection. The relative dosage effects of the different quantities of 

 the bacteria have remained in similar positions in both the unselected and selected mice, 

 but fewer mice from the eleventh-generation population die. In the unselected mice 

 the lower dose causes somewhat less mortality than the intermediate dose. The high 

 dose is much more lethal than the two lower dosages. The selected mice show greatly 

 increased resistance. The low-dose treatment now has 87 per cent survivors, whereas 



