and  the  Prevention  of  Rot. 
301 
October  is  the  most  fatal  time  for  their  district.  In  November 
the  germs  are  less  numerous,  and  rot  is  less  frequently  taken, 
though  there  are  one  or  two  instances  in  which  the  disease  has 
O # m 
been  clearly  proved  to  have  been  taken  as  late  as  the  beginning 
of  December. 
Some  farmers  are  inclined  to  place  the  dangerous  period 
earlier  in  the  year,  dating  from  the  time  of  a flood  in  June  or 
July.  It  may  be  quite  correct  to  attribute  the  outbreak  to  the 
flood,  but  the  date  of  the  flood  may  simply  be  the  time  when 
the  snails  are  infected,  and  not  when  the  sheep  take  the  rot. 
The  cercariae  would  probably  be  matured  some  two  months 
later,  so  that  the  true  date  of  the  infection  of  the  sheep  would 
be  August  or  September. 
In  the  case  of  snails  which  become  infected  late  in  the  year, 
, the  different  generations  will  not  have  time  to  grow,  multiply 
and  become  mature  before  the  cold  of  the  closing  year  drives  the 
snail  into  its  winter  quarters,  and  suspends  for  the  time  the  life 
of  the  parasites.  It  is  a question  of  some  interest  whether  the 
snail  and  its  included  parasites  can  together  survive  the  winter 
and  complete  their  growth  in  the  following  spring.  If  the 
parasites  are  numerous  or  are  far  advanced,  the  snail  is  so 
much  weakened  that,  fortunately  for  us,  it  will  probably  suc- 
cumb to  the  rigours  of  the  winter.  But  there  can  be  little 
doubt  that  some  few  will  retain  their  vitality  throughout  the 
winter,  and  with  the  return  of  milder  weather  a fresh  start  will 
be  made,  and  that  at  length  the  forms  destined  to  be  transferred 
to  the  sheep  will  be  matured.  Some  of  the  cases  of  early 
infection  in  June  are  probably  due  to  parasites  which  have 
survived  the  winter  in  the  snail. 
June  and  July  are  then  the  principal  but  not  the  only 
months  in  which  we  are  to  wage  war  against  the  embryos  ; the 
latter  part  of  August,  September  and  October,  are  the  months 
in  which  especially  to  destroy  the  germs  on  the  grass  ready  for 
transference  to  the  sheep. 
3rd  Condition.  A particular  snail  called  Limnceus  trunca- 
tulus  must  be  present — There  seems  to  be  only  one  snail  in 
England  * which  can  serve  as  host  to  the  intermediate  forms 
* The  snail  L.  truncatulus  has  a very  wide  distribution,  being  found,  according 
to  Dr.  Gwyn-Jeffreys,  throughout  Europe,  in  North  Asia,  Morocco,  Algeria, 
Madeira,  and  (doubtfully)  Guatemala.  The  liver-fluke,  however,  has  a much 
wider  distribution,  and  it  follows  that  there  must  be  in  certain  countries,  supposing 
L.  truncatulus  to  be  really  absent,  another  snail  which  can  replace  it  as  host. 
Thus  the  fluke  is  found  in  the  United  States,  where  L.  truncatulus  is  said  not  to 
occur,  though  several  other  kinds  of  snails  belonging  to  the  genus  Limnxus  are 
found.  In  Australia  the  fluke  occurs  both  in  sheep  and  in  human  beings. 
L.  truncatulus  is  stated  not  to  exist  there,  but  eleven  kinds  of  Limnxi  are 
recorded.  It  is  probable  that  in  both  these  countries  some  other  species  of 
Limnxus  serves  as  host. 
