382 THE ETIOLOGY OF TROPICAL DISEASES 



subject next bitten. These zygotoblasts are the actual source then 

 of infection of man, and on arriving in the human blood the parasite 

 (as spores) attacks the blood cells, and thus commences the intra- 

 corporeal or human phase described above. The mosquito phase 

 occupies a time varying between six to sixteen days or longer, 

 depending on temperature and other factors. 



Such, in outline, is the mosquito theory of malaria. N'o one 

 supposes that the last word has been said. But sufficient is now 

 known to make it certain that the mosquito phase is a fact of 

 essential importance in the conveyance of the disease to man. In 

 the first place, the malarial parasite has been found repeatedly in 

 the body of the mosquito, and in the second place the crucial 

 experiment of inoculation has been performed, and has yielded a 

 positive result. Infected mosquitoes were brought from the Eoman 

 Campagna, and Dr Thorburn Mason and Mr George Warren con- 

 sented to be bitten, and thus contracted malaria. There yet remain 

 gaps to be filled up in our knowledge of the disease, but there can 

 be little doubt that future work will further establish and elaborate 

 the principles of the mosquito theory, and the lines of prevention 

 will of necessity follow the new facts now proved.* 



As concerns preventive medicine, the new facts may be sum- 

 marised in three propositions : — (1) Malaria is caused by a number 

 of microscopical parasites which live and propagate themselves in 

 the blood. (2) These parasites are carried from infected persons to 

 healthy ones by the agency of the genus of mosquitoes termed 

 Anopheles. (3) These mosquitoes breed chiefly in shallow and 

 stagnant terrestrial waters. 



Examination of Malarial Blood (see Appendix, p. 485). 



The Prevention of Malaria 



The new knowledge respecting malaria indicates the only 

 adequate preventive methods. The malarial parasite gains access 

 to the human subject by means of mosquito bites, and, as far as 

 is known, in no other way. Hence the methods of prevention must 

 be directed mainly against the mosquito : — 



1. The prevention of mosquito-breeding. 



2. The destruction of mosquitoes. 



3. Avoidance of being bitten by mosquitoes. 



4. The use of quinine. 



1. The Prevention of Mosquito-breeding.— In order to prevent 



the breeding of mosquitoes, it is necessary to eradicate all possible 



breeding-places. As we have seen, such places are tanks, cisterns, 



vessels of stagnant water, ditches, small pools, buckets, cocoa-nut 



* For full account of malaria, see Tropical Diseases (Manson), 1903, pp. 1-173. 



