﻿174 STRONG. 



the inoculation of equal amounts of the virulent pest bacillus, if the 

 susceptibility of the different animals varies, then different resiilts in 

 mortality must occur both for the reason that different degrees of im- 

 munity will arise in the different animals from the primary inoculation 

 of the same dose of the vaccine and for the reason that in testing the 

 immunity, the fixed amount of the virulent organism employed will 

 represent in the more susceptible animals a greater multiple of the lethal 

 dose than it does in the less susceptible ones. This variation in suscep- 

 tibility to the action of the plague bacillus has been found to be much 

 more marked in the monkeys I have used than it is with other laboratory 

 animals, and it seems not unlikely that the conditions relating to sus- 

 ceptibility in these animals approach nearer to those which exist in man 

 than they do to those which are present in such animals as guinea pigs, rats, 

 mice, etc. During the past year we have had numerous opportunities to 

 observe the differences in immunity obtained in different human beings by 

 the inoculation of the same sized dose of our cholera prophylactic, by study- 

 ing the blood serum from individuals vaccinated against Asiatic cholera. 

 These variations in some instances have been very decided and have been 

 much greater than those which have been observed in series of guinea 

 pigs or of rabbits, all inoculated with an equal dose. Hence, the limit 

 of value of a method for the immunization of man against plague can 

 probably better be studied in monkeys than in any of the other lower 

 animals. It is particularly for this reason and because of the fact that 

 monkeys suffer with forms of plague analogous to those observed in 

 human beings, that they have been extensively used in my experiments 

 in testing the final value of methods of pest inoculation which have 

 proved effective in the ordinary laboratory animals. Moreover, another 

 reason for the extensive use of monkeys has been that it might be argued 

 that the value of a method of immunization against plague in man should 

 not be judged by its action in experiments upon such animals as guinea 

 pigs and mice alone, an argument which has already been advanced. 



Guinea pigs and rabbits. — Two methods of infection were employed in 

 guinea pigs, for the purpose of testing their immunity following the 

 prophylactic injections in the various experiments. The first consisted of 

 the suspension of a 48-hour agar culture of the strain "Pest Virulent" 

 in 5 cubic centimeters of bouillon. Five large oesen of this suspension 

 were then rubbed over a shaved area of the abdomen of the animal and 

 the skin scarified with a scalpel. The other method of infection less 

 commonly employed consisted of massaging over a shaved area of the 

 abdomen of the guinea pig a portion of the spleen of a second one just 

 dead from acute pest infection with the strain "Pest Virulent." By 

 either of these methods the guinea pig, unless previously immunized, 

 invariably succumbed to acute infection. Rabbits were only employed in 



