NATURAL IMMUNITY 51 



ease in those that are cold-blooded, and vice versa. Thus frequent 

 attempts to produce anthrax in turtles, frogs, and other cold-blooded 

 species have failed. Also among warm-blooded animals differences 

 in body temperature have been shown to influence susceptibility. 

 Thus avian tuberculosis does not develop in mammals, nor do the 

 human and bovine types of tubercle bacilli infect birds. And this is 

 probably due to the fact that the avian bacillus has become adapted 

 to growth at from 40 to 45 C., about the normal temperature of 

 birds, while the mammalian bacilli cease to grow when the tempera- 

 ture is raised above 40 C. Another observation which clearly illus- 

 trates the influence of body temperature upon susceptibility is that 

 made by Gibier * upon anthrax. Frogs are ordinarily resistant to 

 this disease. When they are kept in water at 35 C. a fatal infec- 

 tion can be produced. Suttall's 2 experiments with plague infection 

 in lizards illustrate the same point. Kept at 16 C., no infection 

 could take place. Warmed to 26 C., they could be readily infected. 

 It is ordinarily assumed that these results are explicable upon the 

 basis of purely cultural and temperature considerations. And this, 

 indeed, is most likely. It is possible, however, that an additional 

 factor involved in this may be the lowering of the general resistance 

 of cold-blooded animals when warmed, just as warm-blooded animals 

 can be rendered susceptible by chilling. 



It is for similar simple cultural reasons, possibly, that diseases 

 which occur spontaneously in carnivora do not occur in purely 

 herbivorous animals. The relative resistance of dogs to anthrax 

 and to tuberculosis may possibly be accounted for in this way. 

 However, there are many micro-organisms which infect easily 

 both carnivorous and herbivorous animals, and it may well be that 

 the frequently cited cases we have mentioned above depend on fac- 

 tors more complicated than mere cultural conditions incident to 

 metabolic differences. In most cases of species resistance, indeed, 

 simple nutritional conditions alone do not serve as valid explana- 

 tions. 



Species resistance may be so perfect that it amounts to an ab- 

 solute immunity. This is apparently so in the cases cited above, 

 namely the immunity of the cold-blooded species to certain diseases 

 of warm-blooded animals. However, such examples are exceptional. 

 When we are dealing with diseases of warm-blooded animals only, 

 natural resistance, in all but a limited number of cases, is sufficient 

 only to prevent the spontaneous occurrence of the particular disease, 

 or to prevent infection when experimental inoculation with moderate 

 doses is practiced upon normal animals. In most of these cases, 

 however, when the dose experimentally administered is excessive, or 

 the resistance is lowered artificially, by chilling or by any other 



1 Gibier. Compt. rend, de I'acad. des sc., Vol. 94, 1882. 

 2 Nuttall. Centralbl. f. Bakt., Vol. 22, 1897. 



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