﻿348 
  C. 
  F. 
  M. 
  SWYNNERTON. 
  

  

  first 
  through 
  debilitated 
  persons, 
  in 
  order 
  to 
  attain 
  the 
  same 
  specific 
  virulence 
  and 
  

   produce 
  the 
  disease. 
  Taute's 
  evidence 
  shows 
  that 
  it 
  is 
  vastly 
  more 
  likely 
  that 
  the 
  

   Lindi-Kilwa 
  foci 
  were 
  set 
  going 
  by 
  infected 
  man 
  from 
  the 
  Rovuma 
  or 
  Lujenda 
  than 
  

   that 
  each 
  arose 
  independently 
  from 
  local 
  brucei 
  infection. 
  And 
  it 
  would 
  seem 
  

   perfectly 
  probable, 
  though 
  not 
  necessary, 
  that 
  the 
  Usukuma 
  outbreak 
  itself 
  arose 
  in 
  

   the 
  same 
  way 
  from 
  man-borne 
  infection. 
  

  

  Density 
  of 
  Fly 
  not 
  in 
  itself 
  a 
  Cause. 
  

  

  A 
  point 
  that 
  seems 
  clear 
  is 
  that 
  mere 
  density 
  of 
  the 
  fly, 
  as 
  such, 
  has 
  nothing 
  to 
  

   do 
  with 
  the 
  incidence 
  of 
  human 
  trypanosomiasis. 
  In 
  other 
  places 
  in 
  the 
  territory 
  

   the 
  fly 
  (to 
  judge 
  from 
  the 
  evidence 
  of 
  the 
  puparia) 
  is 
  just 
  as 
  numerous, 
  yet 
  in 
  these, 
  

   so 
  far 
  as 
  we 
  know, 
  there 
  is 
  no 
  sleeping 
  sickness. 
  The 
  fly 
  in 
  the 
  latter 
  areas 
  is 
  living 
  

   largely 
  on 
  game 
  (as 
  the 
  proportions 
  of 
  the 
  sexes 
  show 
  strongly), 
  so 
  that 
  the 
  fly 
  that 
  

   bites 
  a 
  man 
  now 
  will 
  next 
  bite 
  a 
  zebra, 
  and 
  though 
  males 
  — 
  and 
  many 
  males 
  — 
  may 
  

   temporarily 
  follow 
  a 
  caravan, 
  there 
  is 
  little 
  intensive 
  feeding 
  ; 
  whereas 
  the 
  fly 
  on 
  the 
  

   Simiyu 
  was 
  quite 
  largely 
  living 
  on 
  man, 
  obtaining 
  feed 
  after 
  feed 
  from 
  man 
  possibly 
  

   for 
  weeks 
  (as 
  the 
  proportions 
  of 
  the 
  sexes 
  again 
  suggest), 
  because 
  the 
  game 
  in 
  that 
  

   place 
  had 
  been 
  reduced. 
  From 
  this, 
  on 
  Duke's 
  view, 
  would 
  have 
  followed 
  a 
  

   development 
  or 
  an 
  elevation 
  of 
  specific 
  virulence, 
  a 
  conquest 
  of 
  man's 
  usual 
  

   resistance 
  to 
  trypanosomiasis 
  and, 
  if 
  all 
  other 
  circumstances 
  favoured 
  it, 
  an 
  

   epidemic. 
  It 
  is 
  continuous 
  density 
  in 
  relation 
  to 
  man 
  that 
  matters, 
  and 
  ready 
  

   feeding, 
  and 
  these 
  depend 
  entirely 
  on 
  the 
  presence 
  or 
  absence 
  of 
  game. 
  

  

  Density 
  of 
  Population 
  unessential. 
  

  

  Nor 
  does 
  massed 
  population 
  seem 
  completely 
  necessary 
  for 
  the 
  propagation 
  of 
  

   an 
  epidemic 
  — 
  I 
  do 
  not 
  say 
  of 
  a 
  great 
  epidemic, 
  for 
  an 
  epidemic 
  needs 
  fuel 
  in 
  order 
  

   to 
  grow 
  great. 
  In 
  this 
  instance 
  we 
  had 
  small, 
  family 
  villages 
  scattered 
  through 
  

   the 
  woodland 
  widely 
  or 
  closely, 
  but 
  not 
  densely. 
  All, 
  then, 
  that 
  is 
  needed 
  apparently 
  

   in 
  this 
  respect, 
  is 
  that 
  villages, 
  large 
  or 
  small, 
  should 
  lie 
  so 
  near 
  as 
  not 
  to 
  preclude 
  

   ordinary 
  social 
  contact, 
  and 
  that 
  the 
  fly 
  should, 
  in 
  considerable 
  numbers, 
  be 
  devoting 
  

   its 
  special 
  attention 
  to 
  man. 
  It 
  is 
  true 
  that 
  where 
  an 
  adequate 
  enforcement 
  of 
  the 
  

   game 
  laws 
  is 
  absent, 
  great 
  population 
  means 
  great 
  game 
  destruction 
  and 
  special 
  

   concentration 
  by 
  the 
  fly 
  on 
  man. 
  

  

  In 
  relation 
  to 
  a 
  fly 
  that 
  likes 
  man 
  as 
  well 
  as 
  it 
  likes 
  other 
  animals, 
  a 
  population 
  

   of 
  man 
  that 
  has 
  locally 
  merely 
  outstripped 
  that 
  of 
  the 
  other 
  food-animals 
  in 
  avail- 
  

   ability 
  might, 
  without 
  any 
  complete 
  destruction 
  of 
  the 
  latter, 
  attract 
  the 
  fly 
  rather 
  

   specially 
  to 
  itself. 
  The 
  matter 
  would 
  be 
  relative. 
  This 
  may 
  be 
  the 
  position 
  in 
  

   relation 
  to 
  G. 
  palpalis 
  everywhere 
  or 
  in 
  particular 
  localities, 
  and 
  I 
  am 
  convinced 
  

   that 
  it 
  could 
  be 
  the 
  position 
  as 
  regards 
  G. 
  swynnertoni 
  were 
  it 
  not 
  that 
  population 
  

   in 
  this 
  case 
  means 
  clearing 
  of 
  bush 
  and 
  reduction 
  of 
  fly. 
  It 
  may, 
  I 
  think, 
  be 
  said 
  

   that 
  the 
  extent 
  to 
  which 
  a 
  given 
  species 
  will 
  attack 
  man 
  decides 
  whether 
  that 
  species 
  

   will 
  act 
  as 
  a 
  vector, 
  and, 
  secondly, 
  that 
  the 
  length 
  of 
  time 
  during 
  which 
  a 
  fly 
  

   that 
  will 
  fairly 
  readily 
  attack 
  man 
  is 
  led 
  by 
  circumstances 
  to 
  concentrate 
  on 
  man 
  

   will 
  perhaps 
  decide 
  whether, 
  in 
  a 
  given 
  locality, 
  the 
  result 
  will 
  consist 
  in 
  isolated 
  

   cases 
  and 
  small 
  foci 
  or 
  (with 
  debilitation 
  at 
  first 
  present 
  ?) 
  an 
  epidemic. 
  G. 
  brevi- 
  

   palpis 
  and 
  G. 
  austeni 
  will 
  probably 
  never, 
  in 
  practice, 
  carry 
  human 
  trypanosomiasis. 
  

   G. 
  pallidipes, 
  also, 
  to 
  a 
  vastly 
  greater 
  extent 
  than 
  morsitans, 
  is 
  a 
  wild-mammal 
  fly 
  

   and 
  is 
  unlikely 
  to 
  become 
  of 
  importance, 
  though 
  the 
  relation, 
  if 
  any, 
  of 
  the 
  adjoining 
  

   pallidipes 
  belt 
  to 
  the 
  sleeping 
  sickness 
  outbreak 
  associated 
  with 
  G. 
  palpalis 
  on 
  the 
  

   two 
  rivers 
  in 
  south 
  Kavirondo 
  is 
  worth 
  investigating. 
  G. 
  morsitans, 
  with 
  human 
  

   infection 
  present, 
  will 
  probably 
  produce 
  occasional 
  cases, 
  even 
  where 
  extremely 
  

   localised 
  game 
  removal, 
  as 
  along 
  a 
  road 
  or 
  as 
  a 
  result 
  of 
  local 
  heavy 
  shooting, 
  

   makes 
  it 
  concentrate 
  on 
  man, 
  and 
  would 
  produce 
  an 
  epidemic 
  under 
  the 
  conditions 
  

  

  