130 Dr Stephens, The Prevention of Malaria. 



fever, and even when present they bear no comparison to the large 

 infections with the ring forms or with gametes of native children. 

 Thus of twenty-one Europeans who habitually slept without 

 mosquito nets, and were so exposed to the constant risk of infection, 

 only two shewed parasites and these were extremely rare. Of 

 another set of Europeans who used nets and adopted ordinary pre- 

 cautions none contained parasites. Again it is a peculiar fact that 

 in Europeans who are convalescing from an attack of fever in West 

 Africa, gametes are rarely found. This also is the experience of 

 Ziemann in Cameroons, so that they are actually not in the 

 necessary condition for the infection of mosquitoes. 



These facts then shew that the generally received idea that 

 Europeans derive malaria from pre-existing cases in Europeans 

 requires considerable modification. This factor it seems to us 

 sinks into complete insignificance beside that of infection derived 

 from native sources. The normal condition of native children is 

 one of almost continuous infection, and there are therefore many 

 thousands of cases of malaria in large towns. This enormous source 

 of infection has we believe so far entirely escaped recognition. 

 Whilst Europeans live in the midst of native quarters exposed to 

 infection on all sides, the isolation of such Europeans only as 

 have fever is manifestly a futile procedure. 



Malaria then is a contagious disease, the contagion being con- 

 veyed by the mosquito, and we deduce as the direct result of these 

 observations the conclusion that malaria can be avoided most 

 readily by avoiding this source of contagion and by living as far 

 removed as possible from native huts. 



Now it is notorious that among men employed on railway 

 works in W. Africa malaria is always rife and the explanation is 

 clear when we know the conditions under which they live. We 

 found without exception in all the railway camps examined by us 

 the following conditions. A single European house or a small 

 group surrounded by native huts, or with huts in the immediate 

 vicinity, and in these huts we always found, as already stated, 

 infected Anopheles. 



This dangerous source of infection can be however readily 

 avoided by locating the European dwellings at a distance from 

 native quarters, and on railways this can be effected with great 

 ease. A site then should be selected as far as possible from the 

 native villages. We should fix a mile as a perfectly safe limit 

 and we shall give reasons for believing that half-a-mile or even 

 less would be sufficient. 



We must consider here, as having a direct bearing on this 

 question of segregation, the distance of flight of Anopheles, for it 

 may be objected that even if segregation be effected there is 

 no certainty that infected Anopheles will not fly this distance. 



