PROBLEMS AND PRINCIPLES OF MALARIA PREVENTION. I95 



and a certain number of Anopheles of the species capable of 

 transmitting malaria may exist in a locality, and yet the disease 

 be very infrequent. This is a fact of great practical impor- 

 tance to remember in an anti-malaria campaign. One need not 

 necessarily exterminate all Anophelinc-e or free from parasites 

 all human carriers, both probably practical improbabilities ; it is 

 sufficient to reduce both to what has been aptly designated by 

 Carter as " the non-infective minimum," and maintain them at 

 that level, and for practical purposes the campaign is won. 



One more point in connection with the mechanism of 

 malaria transmission before I proceed to the principles of pre- 

 vention, and this is one frequently missed even by medical men : 

 It is not the man in the acute stage of malaria that is dangerous 

 to the community ; he does not as a rule harbour parasites in 

 that stage of their development wherein, if sucked up by an 

 Anopheles capable of transmitting the disease, they are capable 

 of going on to development within the mosquito. It is the man 

 who is apparently well that harbours the sexual form of the 

 organism — the only form in which it can proceed tu full develop- 

 ment within the mosquito. Successive attacks of malaria tend 

 to ])roduce in the infected individual a progressively increasing 

 immunity to the disease, so that in a community where malaria 

 is endemic, a large number of individuals harbour the parasites 

 of malaria in its sexual form without showing any symptoms 

 of disease. For example, in 1905 a considerable number of 

 native children in the schools of Panama were examined, and 

 over 30 per cent, were found harbouring malaria parasites, and 

 yet none of these were ill. In Daressalam in the years 1912-13 

 over 19,000 adults were examined, none of whom showed any 

 svmptoms of disease, and yet 20 per cent, of these were infected 

 with malaria. In 1913 I examined 100 adults and 50 children 

 in Daressalam, taking them at random from the street, all 

 apparantly healthy, and yet of the adults 32 per cent., and of the 

 children 50 per cent., harboured parasites of malaria. Similar 

 observations were made in many parts of the world. 



Bearing in mind the facts in connection with the transmission 

 of malaria which I have partially outlined, it becomes apparent 

 that malaria prevention can be carried out along five lines : — ■ 



(1) The elimination of human carriers only; 



(2) The reduction to a non-infective minimum of the number of 

 AnopheHucT of the species capable of transmitting malaria ; 



(3) The protection of the individual against the bites of mosquitos; 



(4) The protection of the individual by proper medication against the 

 development of the parasite within his blood ; and, finally — 



(5) A combination of several or all of these methods. 



(i) The Elimination of Human Carriers. — An attempt to 

 reduce malaria along the lines of eliminating human carriers only 

 is fraught with almost insurmountable difficulties. It means the 

 systematic blood examination of all the members of a community 

 and intensive treatment of persons foimd afifected. The re- 

 nowned Robert Koch and his pupil and associate, Ollwig, were 



