ii6 



TROPICAL DISEASES 



providing abundant suitable food material for the parasite, does not 

 produce such a quantity of chemical substances antagonistic to the 

 parasite as to seriously hamper or prevent its growth and development. 



Given such conditions as those just described, a parasite should 

 be capable of developing enormously in a given district; but there 

 are still other factors to be considered, and the first of these is the 

 fact that the intermediary host, if an animal, depends for its exist- 

 ence upon the presence of a suitable food-supply, as well as suitable 

 means for propagating its species. 



If anything untoward happens to these, the intermediary host 

 may die out, but the problem is not quite so simple as this, because 

 the intermediary host itself may be preyed upon by some other 

 animal, or may be affected by disease, and so reduced in numbers. 



If the intermediary host- is reduced in numbers, even without 

 being exterminated, the parasite will have difficulty in completing 

 its life-cycle, and is therefore faced with the problem of seeking 

 another intermediary host, or another entirely different method of 

 leaving the human host, or of being exterminated. 



The reduction in numbers of the intermediary host in a given area 

 is one of the bases of prophylaxis in malarial fever, yellow fever, 

 dengue fever, and sleeping sickness. Faced with these difficulties, 

 it would appear possible that the parasite can change its inter- 

 mediary, probably undergoing certain changes itself in so doing, 

 and this would appear to be a possible explanation of the slight 

 modifications of the various forms of spirochsetes causing the re- 

 lapsing fevers. 



On the other hand, the malarial parasite would seem to be less 

 capable of accommodating itself to a change of hosts, for it would 

 appear to be only capable of completing its life-cycle in the less 

 common anophelinse, and not in the more common culicinse — a most 

 important fact in malarial prophylaxis. 



Another factor to be considered is the effect of atmospheric 

 temperature upon the parasite, for the study of the life-cycle of the 

 malarial parasite has clearly shown that this has a marked effect 

 upon the development of the oocyst in the anopheles. It is also 

 quite possible that other physical and chemical factors, concerning 

 which we are at present ignorant, may play important parts in 

 controlling the life of parasites. 



Finally, the parasite itself may suffer from the attacks of another 

 parasite, a condition of affairs called ' hyperparasitism,' and, thus 

 becoming diseased, may be unable to complete its life-cycle, and so 

 become extinct. 



It will thus be seen that the problems connected with the appear- 

 ance and the disappearance of a disease in a locality are extremely 

 complex, and that next to the parasite itself the most important 

 factor is the intermediary host, its food, its life-cycle, and its habits 

 — in one word, its cecology. 



Hence, in studying an endemic parasitic disease, or a disease 

 thought to be possibly parasitic, it is not advisable to restrict one's 



