Bovine Tuberculosis. 113 



the extending of the disease will necessarily be slow, but when ten herds 

 are infected from this one the progress of the disease will be ten times as 

 rapid, and when five herds are infected from each of the ten, the disease 

 will, other factors being equal, spread at fifty times the original rate of 

 progress. 



3. Period of incubation. 



In case of many of the infectious diseases, the time that elapses be- 

 tween the exposure (infection) of the individual and the time when the 

 disease appears is short and more or less uniform. This makes it possible 

 to quarantine suspected animals until after this period has passed and thus 

 ensure safety in placing them with the home stock. With tuberculosis 

 this period is not regular and it is not known how long it may be. Our 

 present knowledge of the subject shows that it varies from a few days to 

 as many months. Tuberculin (see page 115) docs not give a reaction dur- 

 ing this period. It is necessary, therefore, for safety that cattle which do 

 not react when purchased should be tested again in from three to six 

 months, as it is possible they were bought after they hail become infected 

 but in the period of incubation. This precaution is of great importance 

 in protecting a dairy. The newly purchased cows should, if possible, be 

 kept apart from the herd until after the second test. 



4. The duration of the disease. 



Tuberculosis is a disease of very slow progress. It often requires 

 years for it to destroy its victim. The tubercle bacilli multiply and pene- 

 trate into the organ to which they were first carried and gradually destroy 

 it. It often happens that the tubercle germs pass into the blood or lymph 

 and are carried to other parts of the body where each germ may start a 

 new tubercle. This is the condition known as generalized tuberculosis. 

 If general and acute, running a rapid course, it is called '* miliary tubercu- 

 losis " or " quick consumption." Fig. 129 illustrates a case of chronic 

 generalized tuberculosis. 



When the diseased tissues are restricted to one organ, the condition is 

 known as localized tuberculosis (Fig. 130). When the organs in two of 

 the cavities, such as the lungs in the pleural cavity, and the liver in the 

 abdominal cavity, are affected the condition is known as generalized 

 tuberculosis. This is very important, as the meat inspection regulations 

 of this and other countries permit the flesh of animals suffering from local 

 tuberculosis to be used for food but when the disease is generalized it 

 must not be so used."" 



* Following: are the United States regulations concerning the use of flesh of 

 tuberculous animals : 



"Generalized" tuberculosis refers to that form of the disease in which the 

 bacilli have been disseminated through the blood and lymph, and in which a 

 number of organs are affected. " Extensive "' tuberculosis refers entirely to the 



