INSECTS AND DISEASE 5 



HOW INSECTS CARRY DISEASE 



In the above section we have seen how insects relate to the 

 causation of disease. It is obvious that we may view the field 

 from a slightly different angle, i. e. our classification may be 

 based on transmission as well. 



The simplest way in which insects enter in as a factor in the 

 transmission of disease is through soiled feet and mouthparts. 

 Any insect might accidentally become contaminated with infec- 

 tive sputum or fecal matter and in turn might accidentally come 

 in contact with human foods, thus becoming an indirect infector, 

 as already explained. In this connection the normal habit of 

 the insect must be considered, i. e. its breeding habits, food 

 habits and general behavior. Thus the housefly enters in as a 

 factor in the transmission of such diseases as typhoid fever and 

 tuberculosis, because of its normal habits and is a carrier in the 

 simplest possible manner. 



A second purely mechanical method of transmission, though 

 more restricted, is by means of a soiled piercing proboscis, in cases 

 of certain blood diseases. In the foregoing class the type of 

 mouthparts does not enter as a restrictive factor, but in order 

 that the proboscis may become soiled with blood the mouthparts 

 must be capable of piercing the skin, thus coming in contact 

 with the blood and its contained parasites, if present. The 

 inoculation of the second host may be purely mechanical. In- 

 sects that belong to this class of carriers ordinarily have heavy 

 piercing mouthparts drawing considerable blood, are inter- 

 mittent parasites, and often go from host to host within a short 

 time. The horsefly (Tabanus) is a good representative of this 

 class in its chance relation to anthrax. 



A still more highly specialized method is involved in the trans- 

 mission of bubonic plague by fleas. In this case the carrier has 

 piercing mouthparts, is bloodsucking and an intermittent para- 

 site, but it does not inoculate the second host by means of a 

 soiled proboscis as far as is known. The plague bacilli when 



