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 undergoes 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 
113. Dipylidium . . . . . 
caninum. The by certain parasitic worms. There is the additional 
double pored 
tapeworm of the reason that these were the first cases known of arthro- 
dog. . . . . 
pod transmission of pathogenic organisms. 
Insects as Intermediate Hosts of Tapeworms 
A number of tapev r orms are known to undergo their sexual stage 
in an insect or other arthropod. Of these at least tv r o are occasional 
parasites of man. 
Dipylidium caninum (figs. 113 and 114), more generally knovm 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 
