44 
Tuberculosis. 
considerations  make  the  immediate  sale  of  these  animals  for 
butchers’  purposes  impossible,  complete  eradication  of  the 
disease  is  not  to  be  expected  for  many  years,  unless  the  herd 
is  a small  one,  and,  unfortunately,  it  is,  as  a rule,  the  large 
herds  that  are  worst  affected  with  the  disease.  The  only 
persons  who  can  expect  to  attain  immediate  and  complete 
success  are  those  who  are  able  to  dispose  forthwith  of  all  the 
animals  found  to  be  diseased  on  the  occasion  of  the  first  test. 
Even  these  may  not  be  immediately  successful,  but  by  taking 
advantage  of  the  combined  methods  of  testing  recently  intro- 
duced, it  will  generally  be  possible  to  weed  out  the  last  of  the 
diseased  animals  at  the  second  test,  that  is,  within  three 
months. 
When  two  separate  farms  are  available,  one  for  the  reacting 
and  the  other  for  the  non-reacting  animals,  the  owner  can  by 
the  same  procedure  within  three  months  be  practically  assured 
that  one  section  of  his  herd  is  healthy. 
In  all  other  circumstances  years  may  be  expected  to  pass 
before  even  the  non-reacting  section  of  the  herd  can  be  con- 
sidered absolutely  free  from  the  disease,  for  the  rule  is  that 
when  the  reacting  and  non-reacting  animals  are  kept  on  the 
same  premises,  but  in  separate  houses,  the  successive  tests  will 
reveal  a few  reactors  among  the  animals  previously  supposed 
to  be  healthy.  Disappointments  of  this  kind  may  be  caused 
by  (1)  one  or  more  infected  animals  having  failed  to  react  on 
the  occasion  of  the  preceding  test ; (2)  the  occurrence  of  a case 
of  congenital  tuberculosis  in  the  herd  ; or  (3)  infection  acci- 
dentally introduced  into  the  healthy  section. 
It  is  very  probable  that  in  the  past  the  first  of  these  causes 
has  often  been  x-esponsible  for  unexpected  reactions  in  the 
healthy  division  of  the  herd.  Hitherto  sufficient  account  does 
not  appear  to  have  been  taken  of  the  fact  previously  mentioned, 
that  an  animal  may  not  develop  the  power  to  react  until  some 
considerable  time  after  the  act  of  infection.  This  possibility 
must  always  be  kept  in  mind,  and  to  counteract  it  the  second 
test  must  be  carried  out  within  three  months  of  the  first. 
Moreover,  it  is  important  that  the  interval  between  the  subse- 
quent tests  should  not  be  longer  than  six  months. 
Provided  none  of  the  reacting  cows  kept  on  for  breeding 
purposes  show  any  outward  symptoms  of  disease,  there  is  very 
little  risk  that  any  of  the  calves  will  be  born  tuberculous,  but, 
again,  the  possibility  must  be  kept  in  mind,  and  young  calves 
that  appear  weakly  and  unthriving  should  be  isolated  until 
they  can  be  tested. 
There  can  be  no  doubt  that  the  third  of  the  before-mentioned 
causes,  viz.,  accidental  infection,  is  in  most  cases  responsible 
for  the  repeated  occurrence  of  reactions  in  the  healthy  section 
