HOW INSECTS CAN CARRY OR CAUSE DISEASE 23 



Many disease organisms are transmitted bj insects which exercise 

 apparently only a mechanical role. Principal among these are bacteria 

 and certain parasitic worms. Many of the bacteria may be taken up by 

 fly and beetle larvae, and by adult flies, beetles, roaches, and ants, and be 

 carried on the body or ingested and passed through the body and out 

 in the feces without modification or multiplication. A number of species 

 of parasitic worms may be taken up in the egg stage by insects and 

 deposited in the insect's feces. If such infested feces happen to be 

 deposited on food, contamination and infection may conceivably follow. 



Certain other organisms which are carried by insects to food pass 

 part of their life history in the insects. Such are some of the nematodes 

 that may be ingested by coprophagous insects, which in turn are eaten 

 by the animals that serve as final hosts of the parasites. 



2. Diseases carried by insects to wounds. — We can make the same 

 division of these diseases into mechanical and biological carriage. The 

 transmission of anthrax, leprosy, ophthalmia, and such diseases, from 

 sore to sore or from excreta to sore is purely mechanical. When the 

 organism passes part of its life cycle in the insect we might call the 

 transmission biological. As examples of such types of transmission we 

 may cite European relapsing fever and trench fever, louse-borne diseases 

 which gain access to the body by the scratching in of fragments of the 

 lice or their excreta. 



3. Diseases gaining access through direct attack of insect. — ^Most of 

 the protozoal diseases and some of the parasitic worms gain access to 

 the body of the vertebrate host by direct inoculation, or indirectly, at 

 the time of feeding. When the organism is taken up by the insect 

 it begins its development in the insect body and finally reappears in the 

 salivary glands or some other position adjoining the mouth parts, the 

 inoculation occurring during the blood feast. Such is the inoculation of 

 malaria, .yellow fever, and Rocky Mountain spotted fever. But other 

 disease organisms pass through the intestinal canal of the insect and out 

 in the feces and yet obtain access to the wound by being washed into it by 

 body secretions of the insect, as is the case of the organism of African 

 relapsing fever inoculated by the tick Ornithodoros moubata. 



WHY IT IS NECESSARY TO KNOW HOW INSECTS CARRY DISEASE 



In the foregoing discussion I -have attempted to analyze the methods 

 by which insects can cause or carry disease. There is also a practical 

 side of the question. We must know the why and the wherefore and the 

 what to do. 



Without a conception of the role of the insect we cannot give suf- 

 ficient force to our arguments, or reasons for taking a particular course 



