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 role 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 role of fleas in the spreading 
of bubonic plague, an illustration which cannot be regarded as typi¬ 
cal, since it involves more than mere passive carnage. 
