CHAPTER XXXV 



THE ANIMAL CARRIERS OF DISEASES 



Preliminary — Historical — Protozoal diseases — Helminthiasis — Myiasis — 

 Bacterial diseases — -Diseases of unknown causation — Chance trans- 

 mission — Imperfect carriage of parasites — Terms — References. 



PRELIMINARY. 



The present chapter is an attempt to put in concrete form the role 

 of the ' animal carrier ' of disease. 



Animals can produce traumatisms by their bites, and can cause 

 disease by injecting chemical, substances manufactured in their 

 bodies — e.g., American and Australian tick paralysis — but these 

 questions do not now concern us. The problems which we are 

 about to consider are those associated with the spread of diseases 

 known or suspected to be parasitic. Such diseases are divisible into 

 those caused by animal and those caused by vegetal parasites. The 

 latter are the simpler, as their carriage does not involve any great 

 morphological changes in the parasite, such as often accompanies the 

 carriage of an animal parasite, which, as it is the more complex, 

 we will consider first. 



A given animal parasite apparently has some form of sexual 

 generation in some stage of its life-history, and it is probably merely 

 our lack of knowledge which prevents us from acknowledging 

 this as a proven fact. 



The host in which the sexual generation takes place is called the 

 definitive host, and is probably the original host, in which, as a rule, 

 the parasite does not produce severe forms of disease and may 

 produce no ill effects at all. This shows that it and its host have 

 become so adjusted that it does not overproduce itself in the 

 host, which on its part does not poison or otherwise attack the 

 parasite. 



It is certainly not the object of the parasite to kill its definitive 

 host, but to leave it by some route which causes no great disturb- 

 ance of its tissues or functions. Hence intestinal parasites leave 

 by means of the faeces, but in so doing they arefiung into new dangers, 

 and therefore require protection by encystment. These cysts may 

 be eaten by another individual of the same species as the original 

 definitive host, and the cycle may begin again; but the dangers 

 of the outer world may be guarded against by entering some animal's 

 body in which no development occurs. Such an animal would be 



