334  Annual  Report  for  1909  of  Royal  Veterinary  College. 
Probably  the  best  account  of  the  disease  is  the  one  found 
in  an  article  by  the  late  Professor  Walley/  who  quoted 
Simonds  to  the  above  effect,  but  declared  his  own  convic- 
tion that  the  disease  was  contagious  and  doubtless  due  to  a 
micro-organism,  though  possibly  favoured  by  severe  weather. 
As  there  is  abundant  clinical  evidence  that  the  disease  is 
a contagious  one,  there  cannot  at  the  present  day  be  any 
doubt  that  it  is  bacterial  or  micro-parasitic  in  its  nature, 
but,  as  is  usually  the  case  with  diseased  conditions  which 
have  their  starting  point  in  the  skin  or  lining  membrane  of 
the  mouth  and  do  not  cause  internal  lesions,  there  is  great 
difficulty  in  determining  what  particular  organism  is  the 
Head  of  lamb  affected  with  malignant  aphtha.  The  lines  diverging  from  A 
indicate  the  position  of  the  lesions  on  the  upper  and  lower  lips. 
actual  cause  of  the  disease.  The  nature  of  this  difficulty 
will  be  understood  when  it  is  said  that  both  skin  and  mucous 
membranes  are  always  contaminated  with  bacteria,  and  that 
these  often  invade  and  multiply  abundantly  in  superficial 
lesions  although  not  themselves  the  actual  cause  of  the 
disease. 
Cases  of  malignant  aphtha  may  be  met  with  at  any  season 
of  the  year,  but  serious  outbreaks  are  confined  to  the  winter 
and  early  spring,  and  that,  no  doubt,  led  to  the  supposition  that 
bad  or  severe  weather  is  a factor  in  the  causation  of  the  disease. 
There  is,  however,  another  explanation  of  the  seasonal  pre- 
valence of  the  disease,  viz.,  that  the  conditions  for  its  spread 
are  most  favourable  in  a flock  of  ewes  and  young  lambs. 
Indeed,  the  disease  is  very  rarely  the  cause  of  any  serious  loss 
among  other  classes  of  sheep. 
'■  Luc.  cit. 
