CHAPTER XII 



THE RELATION OF ARTHROPODA TO THEIR PARASITES 



THE fundamental conception underlying the relations between blood- 

 sucking arthropods and disease is now so well known that there is no 

 need to explain it in detail or to quote instances, many of which are 

 familiar to every medical man. Briefly, it rests upon the existence of 

 a number of protozoa, pathogenic to vertebrates in many instances, the 

 life cycle of which is made up of two phases, one of which is passed in 

 the tissues of the vertebrate and the other in the tissues of an inverte- 

 brate. The linking up of the two phases of the cycle is brought about, 

 in the great majority of cases, by the act of feeding, the puncture through 

 which blood is obtained serving as the point of communication between 

 the tissues of the two hosts. 



Dissemination of bacterial diseases, such as cholera, enteric fever, 

 dysentery, and plague, does not come within these limits, since there is 

 no suggestion that a sojourn in the body of an insect is of advantage to 

 the organisms. The role of the insect in these cases is to a large extent 

 a mechanical one, infection being brought about by mere surface contact ; 

 it may be partly due to the faculty which these bacteria possess for 

 existing, though probably not for multiplying to any great extent, in the 

 alimentary tract of the carrier. The transmission of plague by the flea, 

 for instance, is not due to any fundamental change undergone by the 

 bacillus in the alimentary tract, but to its capacity of multiplying and 

 retaining its virulence until it is passed out with the faeces at the time 

 of feeding, and therefore in the neighbourhood of an abrasion of the 

 skin. These cases have been discussed in their respective chapters as 

 far as they fall within the scope of the present work, and need not be 

 referred to again. The question of mechanical or ' accidental ' trans- 

 mission of protozoal parasites will be dealt with later. 



It follows from the nature of the case that the study of these diseases 

 is inseparably bound up with the study of the insect host of the causal 

 organism, and the great increase of interest in entomology which has 

 taken place within recent years bears witness to the recognition of this 

 connection. A full knowledge of the bionomics, life history and breeding 

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