MAN 



1141 



mathematically that the increase or diminution of the malaria really depends 

 upon the ratio to the population of Anophelines capable of carrying the para- 

 site; and also how an epidemic gradually increases, and how it diminishes 

 when the Anophelines are reduced; and proves that, in order that there may 

 be a reduction of the malaria in man, there need only be a reduction in the 

 numbers, and not an extermination of the Anophelines, which is what would 

 appear to have happened in England, and to be the cause of the disappearance 

 of malaria therein. 



Investigation of an Endemic Region. — In investigating malaria in 

 an endemic region, it is necessary to find out — 



1. The specific diagnosis of the parasite or parasites causing the 

 malaria. 



2. The population of the area. 



3. The average number of infected persons. 



4. The average number of infected persons carrying the gameto- 

 cytes in their blood. 



5. The species of Anophelinae in the district, and their breeding- 

 grounds. 



6. The species which carry the parasite. 



7. The species in which the parasite is found in nature. 



8. The number of Anophelines in the affected area. 



No remarks need be offered with regard to i, 2, 4, 5, 6, and 7. 



Withregard t o 8, the method suggested by Ross should be adopted ; 

 and as regards 3, the quickest and the best method is to estimate 

 the spleen-rate, but it must be remembered that the spleen is palpable 

 in 1-07 to 2 per cent, of non-malarial children living in London. 



Ross suggests that the term ' endemic index,' which was intro- 

 duced by Stephens and Christophers to denote the percentage of 

 persons carrying parasites in their blood, should be extended to 

 include not merely that factor, but also the proportion of people 

 with spleens enlarged by malaria. 



He therefore proposes that the endemic index should be made up 

 of a parasite-rate and a spleen-rate, of which the latter is the more 

 convenient, and if applied to children under fifteen years of age, is 

 useful, provided that kala-azar does not occur in the locality in- 

 vestigated. Children are chosen because the adults have acquired 

 a partial immunity, as already explained. 



But the endemic index as defined by Ross is sure to be higher 

 than the mere spleen-rate, and therefore the blood must be ex- 

 amined in the children whose spleens are not enlarged, and the 

 parasite-rate must be added to the spleen-rate to obtain the true 

 endemic index. 



In all calculations such as the above, the statistical error must be borne in 

 mind, and Poisson's formula, or the modification by Pearson which Ross 

 recommends, must be applied. 



Let 2sr=total number of children under fifteen years of age in a locality. 

 w=number examined for spleen-rate. 

 ;ir=number with enlarged spleen. 



^ ioo=spleen-rate. 

 i%= percentage of error. 



