Hydrophobia  (rabies,  lyssa)  is  an  infectious  disease  resulting  from  the  bites
of  animals,  es[)ccially  the  dog,  cat,  and  wolf.  The  specific  micro-organism
has  not  been  isolated.  The  virus  is  found  in  the  saliva,  in  the  central
nervous  system,  and  occasionally  in  the  lachrjTnal  gland,  pancreas,  and  mam-
mary gland.  It  is  not  found  in  the  blood,  and  further  resembles  the  toxin
of  tetanus  in  that  it  has  a  marked  affinity  for  tlie  central  ner\-ous  system,  to
which  it  is  conveyed  by  the  nerves.  As  in  letanus,  the  wound  is  often  punctured,
and  may  heal  before  the  onset  of  symptoms,  and  again  become  pamful  as  the
disease  develops.  Between  10  and  25  per  cent,  of  those  bitten  by  rabid  animals
subsequently  develop  hydrophobia.  Cases  have  been  reported  in  which  the
disease  has  followed  the  mere  licking  of  the  hand  by  a  rabid  dog.  Bites
through  clothing,  which  may  wipe  the  virus  from  the  teeth  of  the  animal,  are  _
less  dangerous  than  those  on  exposed  parts,  while  bites  in  parts  richly  supplied  |
by  ncr\'es,  such  as  the  face  and  hands,  are  the  most  dangerous.  The  virus  is
present  in  the  saliva  for  several  days,  sometimes  as  long  as  eight,  before  the  de-
velopment of  symptoms,  thus  sustaining  the  popular  belief  that  hydrophobia
may  follow  the  bite  of  a  dog  which  later  becomes  rabid.  The  period  of  incuba-
tion in  man  varies  from  a  few  weeks  to  several  months,  the  average  being  forty
days.  The  disease  is  most  frequent  during  the  summer  months.  The  changes
usually  found  after  death  are  those  of  congestion  of  the  brain  and  membranes;
microscopically,  degenerative  lesions  are  discovered  in  the  ganglia  of  the  central
nervous  system,  especially  in  the  plexiform  ganglia  of  the  pneumogastric  nerve
and  Gasscrian  ganglion .  Normally  the  nerve  cciis  of  the  ganglia  are  surrounded
by  a  capsule  consisting  of  a  single  layer  of  endothelial  cells.  In  animals  dying
of  rabies  the  nerve  cells  are  destroyed,  and  their  places  occupied  by  the  prolif-
erated endothelial  cells.  These  changes  have  been  regarded  as  character-
istic, so  that  a  positive  diagnosis  may  be  made  by  the  examination  of  an
animal  after  death.    In  order  that  these  changes  may  be  allowed  to  occur,


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HYDROPHOBIA.
