408 CHRONIC INFECTIOUS DISEASES 



no clinical evidences of the disease are kept separated from 

 the cattle which do not react to the tuberculin test, and 

 the progeny of the herd is reared on milk which is free 

 from tubercle bacilli, either by feeding it sterilized or 

 allowing the calves to suckle only healthy dams or nurse 

 cows. 



(b) The Ostertag method differs from that of Bang only 

 in that the original herd is not tested with tuberculin and no 

 separation of the diseased from the healthy is made. Clinical 

 or known open cases are removed from the herd and all 

 calves are reared and kept entirely isolated. Twice a year 

 the herd is inspected by a veterinarian, but not tuberculined, 

 and clinical cases which may have developed in the interim 

 removed. The calves are tuberculin tested every six months. 

 Reactors are removed or forbidden to be bred. The milk 

 and feces are frequently examined bacteriologically. 



Protective Inoculation. — Depending upon the well-recog- 

 nized fact that most strains (not all) of tubercle bacilli of the 

 human type are little virulent to cattle, attempts have been 

 made to produce immunity against bovine tuberculosis by 

 inoculations with human tubercle bacilli. Other investi- 

 gators have used attenuated bovine tubercle bacilli or have 

 enclosed the bacilli in vehicles (colloidal sacs) to prevent their 

 being taken up by the organism. A degree of immunity 

 has thus been produced, but it is of short duration, nine to 

 eighteen months, and in practice, as yet, has contributed 

 little toward either the control or eradication of the disease. 

 A certain danger attends inoculating cattle with tubercle 

 bacilli of human type in that they sometimes produce lesions 

 of tuberculosis, or at any rate are so slowly eliminated from 

 the body (may remain alive in the body from two to two and 

 one-half years) that the use for food of the animal so inocu- 

 lated might lead to infection of human beings. There is a 

 further possibility that the milk of cows so treated might 

 contain human tubercle bacilli. The following methods of 

 bovovaccination are in vogue: 



1. von Behring's Bovovaccine. — The vaccine is made by 

 drying tubercle bacilli (typus humanus) in a vacuum and 



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