298 Mi. A. Kdiiiiitnii. bvuth Afn"> // / 



establishment of protection, whereas the higher degree of insuscepti- 

 bility of the other animals resisted infection, and in this way evaded 

 the onset of protection. This phenomenon forms the greatest barrier 

 to protective inoculation, and has contributed to the enormous trouble 

 I have experienced in devising a practical method of protective 

 inoculation. 



The fresh infected blood of cattle, sheep, and goats is still more 

 variable in its results than that obtained from the donkey. 



Numerous other experiments of the same nature have been made, 

 all of which result in showing 



(a.) That donkeys, oxen, goats, and sheep possess a very irregular 



susceptibility to the disease. 

 (/>.) That the blood of donkeys which do not react may produce no 



effect when inoculated into the horse. 

 (>'.) That the blood of donkeys which have evinced moderate 



reaction may produce intense reaction in some horses and 



practically none in others. 

 (il.) That a mild reaction in the donkey furnishes no definite 



assurance as regards the reaction which its blood may set up 



in horses. 



Owing to the variable quality of the infection possessed by infected 

 donkey's blood in the fresh state, I experimented with blood taken 

 from donkeys and oxen which, after having l)een received, was pre- 

 served in the manner already described. 



A large number of experiments carried out by this means furnished 

 the following results : 



1. Protection was only obtained where a definite amount of fever 



had been produced on several occasions, but unless the reaction 

 was severe, the animal did not resist the inoculation of 1 c.c. 

 of preserved virulent blood at a later period. 



2. The susceptibility of horses to such a weakened or attenuated 



virus varies enormously. Of two animals inoculated with the 

 same dose of the same virus injected directly into the jugular 

 vein, one had good reaction, the other very feeble. Neither 

 were found to be protected when subsequently inoculated with 

 virulent blood. 



One inoculated with the same amount of the same preserved 

 material two months later died from the primary inoculation, thus 

 showing that even the attenuated virus can be satisfactorily preserved 

 for a considerable period of time. 



In the case of another animal which was inoculated intravenously 

 with this virus no result followed. Fifteen days later the same inocu- 



