Natural Infection. 515 



found tlie expressed muscle juice of two out of 15 tuberculous cows infectious. 

 Kastners arrived at negative results in all cases as long as he confined his experi- 

 ments to the muscle juice obtained from tuberculous cows at the abattoir of Munich 

 that were passed for beef. However, when he turned his attention to 7 cows, 6 

 of which had been condemned on account of advanced generali2ed tuberculosis, he 

 produced 5 positive results. According to this view, meat may contain bacilli when 

 the internal organs are affected with cheesy, purulent foci, while dry or calcified 

 lesions, do not produce infection. Swierstra found bacilli in the muscle juice of 

 7 out of 18 cowB and of 2 out of 8 swine affected with advanced tuberculosis. 

 Bongert found the muscle juice virulent in 5 out of 13 cattle affected with softened 

 tuberculous foci. Hoefnagel arrived at negative results in cases of advanced chronic 

 tuberculosis and Westenhofer obtained only one positive result of acute miliary 

 tuberculosis. 



It seems that the feeding of meat from tuberculous animals produced positive 

 results almost exclusively at the time of the discovery of the tubercle bacillus (Gerlach, 

 Giinther, Harms). More recently only the reports of Forster & Brown give positive 

 results, while those of Galtier, Nocard, MaeFadyean & Leclainehe give results 

 similar to the earlier ones of Perroncito which had been negative without exception. 

 Vander Sluys also was unable to infect pigs by means of feeding large quantities 

 of muscle from animals affected with advanced tuberculosis, having produced only 

 one positive result and in that case only by adding tuberculous organs and bones 

 to the muscles. 



Natural Infection. Observations in regard to the occur- 

 rence and dissemination of tuberculosis point to the fact that 

 under normal conditions tuberculosis is transmitted only after 

 prolonged exposure to the contagion, or after repeated infection 

 with the virus. It seems that a single light infection usually 

 produces only a local tuberculous lesion which terminates in 

 recovery and in addition increases the resisting power of the 

 individual to the disease. In other words, it produces a certain 

 temporary immunity. (For further information see chapter on 

 Pathogenesis.) 



In the vast majority of cases infection occurs through the 

 medium of ingested food or of inhaled air. 



In alimentary infection the milk of diseased animals fed 

 in bulk or suckled by the young is of the greatest importance. 

 In addition to this the skim milk obtained from skimming sta- 

 tions also plays an important role in the dissemination of the 

 disease. Skim milk is particularly infectious when the slimy 

 residue which collects in the separator bowl is added to it, as 

 this slime contains large masses of tubercle bacilli when infected 

 milk is centrifuged (Kjerrulf). 



The infectiousness of milk containing tubercle bacilli bears an in- 

 timate relation to the occurrence of tuberculosis in swine which are fed 

 with skim milk furnished by skimming stations. 



For example, in North America where swine were at one time fed almost 

 exclusively with grain, statistics for 1894-1895 show that of 13,616,539 swine 

 slaughtered in the larger abattoirs only 579 head were tuberculous, that is 0.0004%. 

 On the other hand in 1908 when skim milk from skimming stations came into 

 general use for the feeding of swine, the percentage increased to 2.05%. Of 

 35,113,077 swine that were slaughtered 719,309 were tuberculous. Thus the per- 

 centage of tuberculous swine has increased five thousand fold (Salmon). The 

 annual loss caused by tuberculosis in swine in the United States exceeds $3,000,000. 

 In some abattoirs of Holland (Amsterdam, Eotterdam) the disease is observed 

 in 10 to 15% of all slaughtered swine; in Denmark the disease was formei-ly present 

 in 15 to 18% of all swine; in Copenhagen 20% of all swine were infected 

 (Pederson) ; in Germany some shipments averaged 50 to 60%, and in Hamburg 



