CLIMATOIOGY 



1143 



the body to manufacture protective substances, which keep the 

 growth of the parasite in check. Anything, therefore, which inter- 

 feres with the production of these checking materials will enable 

 the parasite to grow and multiply, and will thus become a predis- 

 posing cause of the disease. Such conditions are: (i) chills; 

 (2) starvation or overfeeding; (3) the onset of another acute disease; 

 (4) the presence of some chronic ailment, which may often be but 

 slight. 



{c) Meteorological. — We have already drawn attention to the 

 relationship between the temperature of the external air and the 

 development of the parasites in the mosquito. It now remains to 

 point out that a similar relationship exists between that condition 

 and the development of the parasite in the human being. 



Ross is of the opinion, not merely fromx observations upon man 

 infected with malaria, but also upon birds infected with Halteridium, 

 that high air-temperatures are favourable to the increase of the 

 nialarial parasites in man. High air-temperatures are therefore a 

 cause of the relapses met with so frequently in the hot dry season 

 of the tropics. 



The reverse is also true; hence the benefit of sanatoria at high 

 altitudes in the tropics, and also of sending a fever-stricken patient 

 to cooler climates, provided the change from the hot to the cold 

 climate be not sudden, but gradual. 



Climatology. — The geographical distribution of malaria is deter- 

 mined by a combination of the conditions suitable for the production 

 of large numbers of mosquitoes capable of carrying the germ, and of 

 those suitable for the development of the parasites in the mos- 

 quitoes, together with the presence of human beings with numbers 

 of gametocytes in their blood. 



The virulence in one region more than in another may depend 

 upon the type of parasite. Thus, Laverania malarice, which is a 

 more virulent parasite ih^ji Plasmodium malaricB or P. vivax, being 

 very common on the West Coast of Africa, renders that area 

 peculiarly dangerous. 



. Generally speaking, malaria is most prevalent in the region of the 

 Equator, and diminishes gradually north and south till the Arctic 

 and probably Antarctic Circles are reached. The malarial area lies 

 between 63° north latitude (mean summer isotherm of 15° to 16° C.) 

 and 35° south latitude. Its geography may alter considerably in 

 the course of years. Thus regions in England, Holland, France, 

 Germany, and Austria-Hungary are much less malarial than they 

 formerly were. On the other hand, it has been known to affect 

 countries which at one time were immune. Thus Ross says that 

 he believes that Mauritius was infected in the early sixties of last 

 century by the introduction of a mosquito capable of spreading the 

 disease, and hence the epidemic which occurred in that island. 

 Reunion is another example. Barbados is said to have no malaria 

 and no anophelines. Therefore the geography to be described is that 

 known to exist at present. 



