130 BuKEAu OF Farmers' Institutes. 



from all chances of infection, rarely show disease. Heredity has 

 but small claim to attention now. 



Predispositwn. — We have been assured by all writers that pre- 

 disposition and conformation were most important factors. We 

 know to-day that no matter how narrow-chested and weak con- 

 stitutionally an animal may be, tuberculosis cannot occur with- 

 out infection, and it does not seem to select the puny ones 

 especially. 



Contagion. — We have seen that most of the old ideas about 

 tuberculosis have been dissipated, but its infectiveness is beyond 

 cavil. That it will spread among healthy cattle when they are 

 kept with diseased ones is well known. 



Tuherculin Test. — This has been stated by many to be an in- 

 fallible guide in the diagnosis of bovine tuberculosis. I may say 

 that it depends largely on the infallibility of the man who uses 

 it. If a correct selection of animals is made, leaving out any 

 that should not be tested, and the proper amounts of tuberculin 

 are used for the varying ages and conditions, then if the tem- 

 peratures are taken without errors we shall have tables that if 

 correctly interpreted, are very valuable in determining the num- 

 ber of animals that are free from tuberculosis and the number 

 that are not. But tuberculin is not an automatic machine; it 

 requires brains, judgment and experience to make a test and then 

 read the answers correctly. When used with these pre- 

 requisities, it is a highly efficient diagnostic agent. 



Percentage of Diseased Cattle in Herds. — In some herds of cows 

 of dairy age the percentage may run from twenty to ninety per 

 cent., and now and then a herd may be found in which every 

 animal is infected. In herds in which there is a goodly number 

 of young animals the percentage is smaller. Young animals are 

 comparatively free from it. Mature cows give the largest per- 

 centage of victims, those from a year and a half to three years 

 old the next, and those less than a year old the least. There 

 'seems to be diversity of opinion among physicians as to whether 

 tuberculosis is more common in adults or children. This point 



