CHAPTER VI 

 ARTHROPODS AS DIRECT INOCULATORS OF DISEASE GERMS 



We have seen that any insect which, like the house-fly, has access 

 to disease germs and then comes into contact with the food or drink 

 of man, may serve to disseminate disease. Moreover, it has been 

 clearly established that a contaminated insect, alighting upon 

 wounded or abraded surfaces, may infect them. These are instances 

 of mere accidental, mechanical transfer of pathogenic organisms. 



Closely related are the instances of direct inoculation of disease 

 germs by insects and other arthropods. In this type, a blood- 

 sucking species not only takes up the germs but, passing to a healthy 

 individual, it inserts its contaminated mouth-parts and thus directly 

 inoculates its victim. In other words, the disease is transferred 

 just as blood poisoning may be induced by the prick of a contami- 

 nated needle, or as the laboratory worker may inoculate an experi- 

 mental animal. 



Formerly, it was supposed that this method of the transfer of 

 disease by arthropods was a very common one and many instances 

 are cited in the earlier literature of the subject. It is, however, 

 difficult to draw a sharp line between such cases and those in which, 

 on the one hand, the arthropod serves as a mere passive carrier or, 

 on the other hand, serves as an essential host of the pathogenic 

 organism. More critical study of the subject has led to the belief 

 that the importance of the r61e of arthropods as direct inoculators 

 has been much overestimated. 



The principal reason for regarding this phase of the subject as 

 relatively unimportant, is derived from a study of the habits of the 

 blood-sucking species. It is found that, in general, they are inter- 

 mittent feeders, visiting their hosts at intervals and then abstaining 

 from feeding for a more or less extended period, while digesting their 

 meal. In the meantime, most species of bacteria or of protozoan 

 parasites with which they might have contaminated their mouth- 

 parts, would have perished, through inability to withstand drying. 



In spite of this, it must be recognized that this method of transfer 

 does occur and must be reckoned with in any consideration of the 

 relations of insects to disease. We shall first cite some general 

 illustrations and shall then discuss the r61e of fleas in the spreading 

 of bubonic plague, an illustration which cannot be regarded as typi- 

 cal, since it involves more than mere passive carriage. 



