Cancer 255 



Saul (1910) and Dahl (1910) go much further, since they attribute 

 the production of the malignant growth to the presence of mites 

 which Saul had found in cancers. These Dahl described as belonging 

 to a new species, which he designated Tarsonemus hominis. These 

 findings have since been confirmed by several workers. Neverthe- 

 less, the presence of the mite is so rare that it cannot be regarded as 

 an important factor in the causation of the disease. The theory 

 that cancer is caused by an external parasite is given little credence 

 by investigators in this field. 



In conclusion, it should be noted that the medical and entomolog- 

 ical literature of the past few years abounds in suggestions, and in 

 unsupported direct statements that various other diseases are insect- 

 borne. Knab (1912) has well said " Since the discovery that certain 

 blood-sucking insects are the secondary hosts of pathogenic para- 

 sites, nearly every insect that sucks blood, whether habitually or 

 occasionally, has been suspected or considered a possible transmitter 

 of disease. No thought seems to have been given to the conditions 

 and the characteristics of the individual species of blood-sucking 

 insects, which make disease transmission possible." 



He points out that "in order to be a potential transmitter of human 

 blood-parasites, an insect must be closely associated with man and 

 normally have opportunity to suck his blood repeatedly. It is not 

 sufficient that occasional specimens bite man, as, for example, is the 

 case with forest mosquitoes. Although a person may be bitten by a 

 large number of such mosquitoes, the chances that any of these 

 mosquitoes survive to develop the parasites in question, (assuming 

 such development to be possible), and then find opportunity to bite 

 and infect another person, are altogether too remote. Applying 

 this criterion, not only the majority of mosquitoes but many other 

 blood-sucking insects, such as Tabanidse and Simuliidse, may be 

 confidently eliminated. Moreover, these insects are mostly in 

 evidence only during a brief season, so that we have an additional 

 difficulty of a very long interval during which there could be no prop- 

 agation of the disease in question." He makes an exception of 

 tick-borne diseases, where the parasites are directly transmitted from 

 the tick host to its offspring and where, for this reason, the insect 

 remains a potential transmitter for a very long period. He also 

 cites the trypanosome diseases as possible exceptions, since the causa- 

 tive organisms apparently thrive in a number of different vertebrate 

 hosts and may be transmitted from cattle, or wild animals, to man. 



