CHAPTER VII 



ARTHROPODS AS ESSENTIAL HOSTS OF PATHOGENIC ORGANISMS 



We now have to consider the cases in which the arthropod acts 

 as the essential host of a pathogenic organism. In other words, 

 cases in which the organism, instead of being passively carried or 

 merely accidentally inoculated by the bite of its carrier, or vector, is 

 taken up and tmdergoes an essential part of its development within 

 the arthropod. 



In some cases, the sexual cycle of the parasite is undergone in the 

 arthropod, which then serves as the definitive or 

 primary host. In other cases, it is the asexual stage 

 of the parasite which is undergone, and the arthropod 

 then acts as the intermediate host. This distinction 

 is often overlooked and all the cases incorrectly 

 referred to ' as those in which the insect or other 

 arthropod acts as intermediate host. 



We have already emphasized that this is, the most 

 important way in which insects may transmit disease, 

 for without them the particular organisms concerned 

 could never complete their development. Exter- 

 minate the arthropod host and the life cycle of the 

 parasite is broken, the disease is exterminated. 



As the phenomenon of alternation of generations, 

 as exhibited by many of the parasitic protozoa, is a 

 complicated one and usually new to the student, we 

 shall first take up some of the grosser cases illustrated 

 by certain parasitic worms. There is the additional 

 reason that these were the first cases known of arthro- 

 pod transmission of pathogenic organisms. 



113. Dipylidium 



caninum. The 



double pored 



tapeworm of the 



'dog. 



Insects as Intermediate Hosts of Tapeworms 



A number of tapeworms are known to undergo their sexual stage 

 in an insect or other arthropod. Of these at least two are occasional 

 parasites of man. 



Dipylidium caninum (figs. 113 and 114), more generally known as 

 Taenia cucumerina or T. elliptica, is the commonest intestinal parasite 

 of pet dogs and cats. It is occasionally found as a human parasite, 

 70 per cent of the cases reported being in young children. 



175 



