Tuberculosis. 
37 
PART  II. 
The  Elimination  of  Tuberculosis  from  Infected 
Herds. 
Before  proceeding  to  consider  in  detail  the  methods  by 
which  tuberculosis  may  be  eliminated  from  an  infected  herd, 
it  may  be  well  to  set  out  the  facts  which  are  of  fundamental 
importance  in  this  connection  : — 
1.  Tuberculosis  is  a purely  contagious  or  infectious  disease.1 
Its  sole  cause  is  the  admission  of  tubercle  bacilli  into  the 
previously  healthy  body,  and  the  bacilli  which  serve  as  the 
starting  point  of  the  disease  in  any  given  case  have  always 
been  derived  from  an  antecedent  case. 
2.  Even  in  badly  infected  breeding  herds  not  one  calf  in  a 
hundred  is  born  diseased,  or  with  tubercle  bacilli  in  its  body. 
3.  All  the  common  breeds  of  cattle  are  susceptible  to 
tuberculosis,  and,  as  a working  proposition,  it  may  be  accepted 
that  no  individual  animal  is  immune  against  the  disease. 
Cattle  can  be  infected  at  any  age,  but  young  animals  are 
probably  more  easily  infected  than  adults. 
4.  Tuberculous  cattle  are  a source  of  danger  to  healthy 
cattle  kept  in  association  with  them  because  of  the  tubercle 
bacilli  which  escape  from  their  bodies.  Owing  to  the  small 
extent  or  the  position  of  the  lesions,  there  may  in  certain  cases 
be  no  outward  escape  of  bacilli,  but  at  any  time,  owing  to 
extension  of  the  disease,  such  a non-infective  animal  may 
become  infective.  Every  tuberculous  animal  must  therefore 
be  regarded  as  actually  or  potentially  a distributor  of  tubercle 
bacilli  in  its  neighbourhood. 
5.  In  the  great  majority  of  cases  bovine  tuberculosis  is  a 
chronic  non-fatal  disease,  spreading  slowly  throughout  the 
body,  or  even  coming  to  a stand-still,  and  not  sensibly  affecting 
the  animal’s  general  health. 
6.  In  a small  minority  of  cases  the  disease  is  either  rapidly 
progressive  from  the  outset,  or  assumes  that  character  after 
an  initial  chronic  stage,  and  in  either  case  it  proves  fatal  if 
allowed  to  run  its  natural  course.  Animals  affected  with 
this  progressive  type  of  the  disease  are  specially  dangerous 
because  of  the  great  number  of  bacilli  which  escape  from 
their  bodies. 
7.  The  principal  channels  by  which  the  bacilli  leave  the 
bodies  of  diseased  animals  are  the  nose  and  mouth,  the  bowel, 
the  udder,  and  the,  genital  passages,  but  especially  the  first 
three  of  these.  The  lungs  are  among  the  commonest  seats  of 
disease  in  cattle,  and  when  these  organs  are  affected  tubercle 
1 In  the  present  state  of  knowledge  no  useful  distinction  can  be  drawn 
between  the  terms  contagious  and  infectious. 
