124 ANNUAL REPORT OF THE Off. Doc. 



mental herd numbers something more than 100 cattle and consists 

 of cattle that have been vaccinated in various ways, tuberculous cows 

 that associate with the vaccinated animals and that furnish expo- 

 sure that tests their resistance and unvaccinated cattle that are 

 likewise exposed and that serve to measure the severity of exposure 

 and to furnish a basis for measuring the resistance of the cattle that 

 are vaccinated. The herd is so managed that exposure to infection 

 is practically continuous. The exposure is distributed as equally as 

 possible so that all vaccinated and control animals are exposed to 

 practically the same amount of infective material. This is done by 

 changing the animals from stall to stall in such a way that all are 

 brought into equal association with tuberculous herd mates. The 

 severity of the conditions of exposure is shown by the fact that all 

 of the unvaccinated animals that have been placed in the herd have 

 become infected in some degree, most of them extensively, and some 

 of them have died of tuberculosis acquired from this association. 

 Many .methods of vaccination have been tried. The differences 

 have consisted in the use of cultures of different origins and degrees 

 of virulence, of different dosages, varying numbers of doses and of 

 different intervals between doses. The effort has been to gain scien- 

 tific data of use in formulating a plan for vaccinating cattle at mini- 

 mum cost and of adequate efficacy. I do not say of maximum efii- 

 eacy, meaning the production of the highest attainable degree of im- 

 munity, because experience shows that it is not necessary to obtain 

 more than a certain amount of resistance to protect against a given 

 set of infective conditions. To secure higher immunity than is need- 

 ed is to prolong the course of treatment and increase the expense. To 

 make this point clear it should be said that any acquired immunity 

 or resistance to disease is not absolute, it is only relative. An ani- 

 mal vaccinated against any disease may still be caused to die of that 

 disease if it is inoculated with an overwhelming quantity of virus 

 of high virulence. For example, in the West where blackquarter pre- 

 vails extensively and where it is necessary to vaccinate cattle against 

 this disease, the effort is not to secure the highest possible immunity 

 but only so much resistance as is necessary to protect the young ani- 

 mal from the amount of blackleg virus it is likely to acquire under 

 usual conditions of exposure. It is found that this is accomplished 

 by a single vaccination. If a second vaccination were applied, the 

 second dose being stronger that the first, more resistance would be 

 developed. But, since one vaccination is sufficient, to apply a sec- 

 ond would be wasteful. While such animals are protected under 

 their actual conditions of life it is still possible for them to contract 

 blackquarter if they are inoculated with more virus than the amount 

 from which they are protected, the amount that experience teaches 

 they are normally exposed to. There is no utility in developing more 

 resistance than is necessary. 'Enough is plenty.' 



"After trying a kirge number of strains of tubercle bacilli of va- 

 rious types and degrees of virulence we have come to use a culture 

 that was isolated in 1897 from the sputum of a consumptive girl. It 

 is of rather low virulence for guinea pigs, is not virulent for rabbits 

 and has never been known to cause infection in cattle (unless given 

 in enormous dosage to young calves). This organism is grown on 

 veal broth and is made into a suspension by adding a weighed 



