XVIII PREVENTION OF TUBERCULOSIS 327 



From the facts given in Chapter VII. it is clear that 

 since it is impossible to recognise when a tuberculous cow 

 not excreting tubercle bacilli may become a case of open 

 tuberculosis, this method cannot eradicate Iwvine tuberculosis. 

 By removing and eliminating the most infectious animals it 

 can mitigate the extent of bovine tuberculosis, but it is not 

 more than a palliative. As regards the diminution of the 

 risk of tubercle bacilli in milk, it is probably more efficacious. 

 Vaccination against Tuberculosis.- — Numerous investigators 

 have shown that the resistance of cattle to infection by the 

 tubercle bacillus may be increased by vaccination with living 

 tubercle bacilli of virulence too low to cause infection. 

 Theoretically the vaccination of young stock offers therefore a 

 method for combating bovine tuberculosis. 



In 1902 Von Behring introduced bovo-vaccine for this 

 purpose. This substance is a culture of human tubercle 

 bacilli of diminished virulence. It is sold as a dried and 

 powdered substance. The most reliable results are claimed 

 when the animals are vaccinated quite young {i.e. one to three 

 months old), two injections being made three months apart, 

 the first being with a weaker vaccine than the second. 



Further investigation by Eber and others has shown that 

 the immunity is unfortunately not lasting, disappearing 

 within one to two years after vaccination. The general results 

 seem to show that the degree of immunity produced is not 

 sufficient to protect against infection for any length of 

 time, and is not worth the trouble and expense involved in 

 obtaining it. 



L. Pearson (Philadelphia) has extensively practised the 

 vaccination of cattle. He remarks that the degree of 

 immunity is proportional, in large measure, to the extent of 

 vaccination, i.e. to the number of times vaccinated, the dosage, 

 and the period of time recovered. The duration of resistance 

 is from one to three years. The vaccine may be administered 

 subcutaneously or intravenously. Judgment on its practical 

 value must be suspended pending further results. 



Heymans, at the Washington Tuberculosis Congress (1908), 

 reported that in Belgium about 40,000 animals had been 

 vaccinated by his method. In this method Heymans intro- 

 duces a capsule containing tubercle bacilli under the skin. 



