660 DISEASES CAUSED BY FILTRABLE VIRUS 



and because of its spontaneous occurrence almost invariably upon the 

 teats of cows, although both males and females are equally susceptible 

 to experimental inoculation. 



The relationship of variola to chicken-pox or varicella has been more 

 easily determined. Chicken-pox does not protect against smallpox nor is 

 this the case vice versa. The two diseases are unquestionably quite distinct. 



The Production of Vaccine. During the early days of vaccination, 

 it was customary to inoculate human beings with the matter obtained 

 from the pustules of those previously vaccinated. While this method 

 was perfectly satisfactory for the immediate purposes in view, practical 

 difficulties and the occasional accidental transmission of syphilis have 

 rendered this practice undesirable. In consequence, at all institutes 

 at which vaccine is produced for use upon man,, the virus is obtained 

 from animals. Horses and mules, both extremely susceptible to vac- 

 cine, have been employed, and goats have, at times, been chosen because 

 of their insusceptibility to tuberculosis. Rabbits have also been used 

 more recently by Calmette and Guerin. 1 



The animals almost exclusivelyemployed at the present day, how- 

 ever, are calves, preferably at ages of from six months to two years. 

 Very young suckling calves are unsuitable because of the great speed of 

 development and small size of the lesions produced. The animals should 

 be healthy and at some institutes (Dresden) are subjected, before use, 

 to the tuberculin test; although, according to Paul, 2 this produces a 

 hypersusceptibility to the vaccine, and can be omitted without danger 

 when careful supervision is observed. Some observers prefer to use light- 

 colored animals rather than dark-skinned or black ones, both for reasons 

 of greater ease of cleanliness and because the former are supposed to be 

 more susceptible than the latter. This contention is denied by others. 

 The sex of the animals seems to be immaterial. 



During the period of use, the calves are fed, according to age, with 

 either an exclusive milk diet, or they are given, in addition, fresh hay. 

 The greatest cleanliness in regard to the bedding and stalls must be 

 observed and separate stables should be available for the animals under 

 treatment and those under observation before treatment. These stables, 

 if possible, should be so built that they can be easily scoured and flushed 

 with water, and stalls should be disinfected after occupation. If possible, 

 stables should be artificially heated and a comfortable temperature 

 maintained. Halters and fastenings should be so arranged that the 



1 Calmette and Guerin, Ann. de 1'inst. Pasteur, 1902. 2 Paul, loc. cit. 



