30 Report oF DEPARTMENT OF ANIMAL HUSBANDRY OF THE 
tissue escape with the intestinal contents. If the disease becomes 
localized in the udder then the bacteria pass out with the milk. 
In cases where the lesions are restricted to subcutaneous glands 
or tissues, abscesses may be formed and the dead tissues laden with 
tubercle bacteria may escape through an opening in the skin. If 
the disease is localized the tubercle bacteria are liable occasionally 
to grow or eat their way into a blood vessel when they may be 
carried in the blood to any or all parts of the body and temporarily 
escape into the milk, intestinal contents or sputa. 
When the bacteria have escaped from infected individuals into 
mangers, watering troughs, or on to the grass in pastures, they are 
readily taken up by healthy cattle that drink, feed or graze after 
diseased animals... In case the bacteria are escaping with the milk, 
calves and swine fed with it are very liable to become infected. 
The period of incubation, that is the time between the entrance 
of the bacteria into the body and the appearance of the fresh tissue 
changes (tubercles) is variable. There is good evidence to show 
that it may be very short in some cases and very long in others. It 
is safe to assert that it may vary from a few days or weeks to as 
many months. Tuberculin does not give a reaction during the 
period of incubation. This condition requires for safety that ani- 
mals that do not react to tuberculin in herds where the disease 
exists and where they may have become infected must be tested 
several months later to be sure that they were not infected at the 
time the first test was made. 
The duration of the disease varies quite as much as the period 
of incubation. Tuberculosis may run a rapidly fatal course, the 
affected animal dying in a few months.* As a rule, however, the 
duration of the disease is much longer. The tuberculous tissue may 
become encapsulated and the disease remain stationary for a long 
time® and in some cases recovery may take place. In an experi- 
‘The cow Jane is an illustration of this point. She became diseased 
during 1901 and first reacted in October, 1901. She failed rapidly and was 
killed in June, 1902. The autopsy showed that her liver was badly diseased. 
* This is well illustrated by Chloe and Kate. They were tuberculous when 
tested as two-year-olds in 1900 but when killed and examined in 1905 both 
were so slightly diseased that their carcasses would both pass a Federal 
meat inspection examination. During these five years Chloe gave a positive 
reaction to tuberculin at eight different tests while Kate gave a positive 
reaction five times and failed to do so at three tests. 
