IMMUNITY IN TUBERCULOSIS 245 



they are of too high and others, possibly, because they are of too low, 

 virulence for cattle. There is also need for comparison in immunizing 

 value of fresh cultures and cultures that have been dried in vacuum 

 and reduced to powder. Some observations appear clearly and strongly 

 to indicate that the fresh cultures are preferable. Although it has been 

 shown that vaccination can be practised so as to be entirely harmless 

 to the animals, yet, on the other hand, it is not always unattended with 

 danger. What is the shortest and most economical procedure for the 

 protection of cattle on a large scale is still to be established. Only 

 prolonged observation of carefully recorded results of vaccinations 

 practised on a large scale can settle this point. The question of dura- 

 tion of immunity is still an open one. It has been shown that the 

 immunity endures a year. To say, at the present stage of the studies, 

 that it will last during the entire life of an animal is to make a state- 

 ment for which there is no experimental proof. Modes of vaccination, 

 as illustrated by the intervals between the successive injections, differ 

 greatly. Behring recommends an interval of three months, while others 

 have obtained a high degree of immunity by repeated injection at short 

 intervals. As artificial immunity is relative and not absolute it need 

 not excite surprise that the immunity to the tubercle bacilli can be 

 overcome by the injection of large quantities of active bacilli. What 

 is desired in practise is a degree of immunity that will suffice to protect 

 animals from acquiring the disease under natural, and consequently 

 highly variable, conditions. In some herds, where the natural disease 

 prevails in a mild form, a lower degree of immunity may suffice than 

 in other herds in which the disease is more severe and wide-spread. We 

 are, therefore, at the beginning of this complex and highly important 

 subject. These are Dr. Pearson's conclusions. 



There is another aspect of this subject which demands attention. 

 When it is recalled that immunity in cattle is obtained by the injection 

 of living human tubercle bacilli the question arises whether this pro- 

 cedure is wholly free from danger to the consumers later of the flesh 

 and milk of these cattle. It would appear that the human bacilli do 

 not excite in cattle the tubercular lesions, in which doubtless the bacilli 

 are so enclosed as to be, to a considerable degree, protected from 

 perishing. It is equally true that as the living micro-organism can not 

 be replaced by dead ones in bringing about immunity, the immunizing 

 process is in some way bound up with their survival and even, pos- 

 sibly, with a restricted multiplication. Hence it is necessary that we 

 ascertain, first, how long the human bacilli survive in the organs of 

 the vaccinated animals, and second, whether they are ever eliminated 

 with the milk of cows. The observations already made upon these 

 points are so few as at present not to be useful for any scientific deduc- 

 tions. But before the method is too implicitly relied upon these ques- 

 tions should be answered. 



