1 140 



THE MALARIAL FEVERS 



but remain near the place where they are developed. Occasionally 

 they can be carried long distances by ships, trains, coaches, carts, 

 etc. , but this is the exception, and not the rule. Winds do not appear 

 to carry them far, as they generally take shelter from a high wind. 

 The natural enemies of the Anophelinse are numerous, including all 

 insectivorous animals, such as bats and birds, together with fish, 

 which eat the larvae. 



The anopheline not merely carries the germ, but because of its 

 length of life, a single individual may be capable of infecting several 

 human beings; for it must be remembered that there is no proof 

 that the mosquito is in any way deleteriously affected by the 

 malarial parasite, and in this way it forms a reservoir of infection. 



3. Man. — The female anopheline requires blood for the purpose 

 of providing its eggs with sufficient nourishment, and will, therefore, 

 bite any vertebrate it may come across in order to obtain the same. 



The malarial parasites have, so far, only been found in man, and 

 hence, until they are found in some other vertebrate, we are not 

 justified in assuming that the anophelines can obtain them from 

 any other source. In the tropics the native population is un- 

 doubtedly the great source of the gametocytes by which the infec- 

 tion of the mosquito is brought about, because the majority of 

 them do not protect themselves against mosquito-bites. Of the 

 native population, the children are the greatest source of infection, 

 because, as has already been explained, the adults obtain a partial 

 immunity. But in considering the native as a source of infection, 

 care must be taken not to forget the European, whose blood is 

 sometimes swarming with gametocytes. 



Ross has pointed out that, in considering the a.mount of malaria in a par- 

 ticular locality, imported cases must be distinguished from indigenous cases, 

 and that the latter should be further classified according to the month in which 

 they are infected. He has also attempted, by mathematical formulae, to 

 investigate the infection-rate of the disease, which method he terms * patho- 

 metry ' ; but it is very difhcult to obtain the necessary data for these calcula- 

 tions. He points out that the number of infections in a given locality during 

 a given period depend upon — 



1. The number of persons with gametocytes in the blood. 



2. The number of anophelines which have bitten these people and become 

 infected. 



3. The number of infected Anophelines which live long enough to transmit 

 the infection — i.e., at least a week. 



4. The number of these surviving infected Anophelines which get the chance 

 of biting man again. 



It is by no means easy to determine, even approximately, in a district the 

 number of Anophelines capable of carrying malaria; but an attempt might 

 be made to follow Ross, and determine the output from one or two of the 

 more important breeding-places by enclosing a given area with mosquito- 

 netting, and counting daily the numbers of Anophelines which hatch out. A 

 calculation of the areas of the breeding-places, together with the numbers 

 hatching per diem, will give a rough estimate of the increase during that time. 

 Associated with this, a census may be taken of the Anophelines found inside 

 damaged mosquito-curtains very early in the morning. This is a method we 

 have used in Tropical Africa and found of service, for it supplies not merely 

 the number of the Anophelines, but also the percentage which are infected, 

 though of course the error is likely to be considerable. Ross proceeds to show 



