INSECTS AND DISEASE 3 



Indirect infection. This form of infection relates chiefly to 

 enteric diseases in the causation of which the pathogenic organism 

 is deposited upon the food of the higher animal by the insect. 

 Thus the food is first infected, and through this the pathogenic 

 organism is implanted within the alimentary canal of the victim; 

 in this way the insect is concerned only indirectly. The housefly 

 which is quite certainly one of the grossest transmitters of 

 enteric diseases is so only because of accident of habit and struc- 

 ture, feeding as it does indiscriminately upon excrement and 

 upon the food of higher animals. The structure of the proboscis 

 and feet is such as to make it quite difficult not to carry particles 

 of the excrement to the food. Thus if disease producing " germs ' ' 

 are present the result is inevitable. 



Insects possessing mouthparts not adapted to piercing the 

 skin (whether biting or sucking) can relate only to this form of 

 infection, and indeed any insect or arachnid may be an indirect 

 carrier by accident. Furthermore insects ordinarily relating 

 only to indirect infection may produce direct infection of certain 

 kinds, where there is access to an open wound; for example, the 

 transmission of gangrene through the agency of the common 

 housefly from a diseased animal to another animal which presents 

 an open wound, cut or sore. 



Internal parasitism. There are no insects as far as is known 

 which spend their entire life history in the form of internal 

 parasites. There are, however, a number which pass their larval 

 period (period of growth) within the alimentary canal or in the 

 muscle tissue of higher animals. The best known representatives 

 of this group are the botflies and the warble flies, the former 

 found mainly in the stomach of equine animals, while the latter 

 are found in the muscle tissue of bovine and equine animals, 

 rodents and sometimes man. The damage done by internal 

 parasites is of various kinds; first, disturbed nutrition, and 

 secondly, irritation caused by the burrowing parasites in the 

 muscles or by the attachment of hooks to the intestinal lining 

 for the purpose of prehension. 



