38 
Tuberculosis. 
bacilli  are  certain  to  be  passed  outwards  through  the  nose  or 
mouth  when  the  animal  coughs.  Many  of  the  coughed-up 
bacilli  are  swallowed  by  the  patient  and  eventually  discharged 
with  the  feces.  The  milk  seldom  contains  tubercle  bacilli 
when  the  udder  is  not  itself  diseased,  and,  similarly,  the  bacilli 
escape  from  the  genital  passages  of  the  cow  only  when  there 
is  tuberculous  disease  of  the  womb. 
8.  The  two  principal  channels  by  which  the  bacilli  enter 
the  body  are  the  nose  and  the  mouth.  When  the  bacilli  are 
suspended  in  the  air  in  a confined  space,  such  as  a cow-byre, 
they  are  apt  to  be  inhaled,  reach  the  lungs,  and  set  up  disease 
there.  When  they  are  swallowed  they  may  set  up  disease  in 
the  intestine  or  the  lymphatic  glands  attached  to  it.  Experi- 
me'nts  have  shown  that  fewer  bacilli  are  necessary  to  infect 
when  inhaled  than  when  swallowed,  and  this  is  in  agreement 
with  the  fact  that  in  the  great  majority  of  cases  in  cattle  the 
disease  begins  in  the  lungs  or  the  lymphatic  glands  attached  to 
them. 
9.  When  tuberculous  and  healthy  animals  are  kept  together 
the  rapidity  with  which  the  disease  spreads  depends  mainly 
on  three  things,  viz.  : (1)  the  number  of  animals  affected, 
(2)  the  proportion  of  such  animals  in  which  the  disease  is  in 
an  advanced  stage,  and  (3)  the  closeness  of  the  enforced 
association  between  diseased  and  healthy.  Close,  con- 
tinuous confinement  in  badly-ventilated  houses  is  highly 
favourable  to  infection,  because  it  increases  the  opportunities 
for  the  passage  of  bacilli  from  the  diseased  to  the  healthy. 
Association  in  the  field  is  less  dangerous,  because  the  chances 
that  bacilli  passed  out  by  the  diseased  will  enter  the  bodies 
of  the  healthy,  especially  by  inhalation,  are  diminished. 
10.  Although  tuberculosis  is  usually  contracted  during 
direct  association  of  diseased  with  healthy  animals,  it  may 
be  spread  indirectly  by  materials  containing  or  contaminated 
with  the  bacilli.  Milk  is  probably  the  commonest  medium 
by  which  such  indirect  infection  is  brought  about,  but  any 
material,  such  as  feces,  litter,  or  fodder,  removed  from  a 
building  in  which  tuberculous  animals  have  been  kept,  maj’ 
carry  bacilli  with  it. 
11.  Although  tuberculosis  is  in  many  cases  not  manifested 
by  any  outward  symptoms,  or  any  appreciable  disturbance  of 
the  general  health,  the  tuberculin  test  when  properly  carried 
out  is  a very  reliable  method  of  ascertaining  whether  an 
animal  is  affected  or  not. 
Keeping  these  facts  in  mind,  one  may  now  proceed  to 
consider  what  are  the  measures  necessary  to  eliminate  tuber- 
culosis from  a herd. 
