Bovine Tuberculosis. 158 



to which the herd is subjected. The extent of cattle traffic is one 

 reason why there is greater danger from tuberculosis now than there 

 was a generation ago. 



If a tuberculous cow is placed iri . jnfinement with other cattle she 

 will convey the disease to thrm more certainly and more quickly 

 than when the animals are at liberty. As bearing upon this point, 

 it has been noted repeatedly that tuberculosis spreads more rapidly 

 in herds when they are confined in winter than when they are at 

 pasture in summer, and there is reason to believe that this difference 

 is due, not to the season, but to the intimacy of contact. Moreover, 

 tuberculosis once introduced, spreads with increasing rapidity as 

 the centers of infection are multiplied. So long as there is but one 

 infected herd from which it may spread in a district, the extending 

 of the disease will necessarily be slow, but when 10 herds are infected 

 from this one the progress of the disease will be 10 times as rapid, 

 and when five herds are infected from each of the 10, the disease will, 

 other factors being equal, spread at 50 times the original rate of 

 progress. 



3. The Period of Incubation, 



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

 between 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. Tuber- 

 culin (see page 89) does not give a reaction during this period. It is 

 necessary, therefore, for safety that cattle that do not react when 

 purchased should be tested again in from three to six months later, 

 as it is possible they were bought after they had become infected but 

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

 in protecting a dairy. The newly purchased cows should 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 

 penetrate into the organ in which they were first carried and gradu- 

 ally^ 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 



