220 PROCEEDINGS ENTOMOLOGICAL SOCIETY 



objections raised by Dr. Brues, I wrote a more extended 

 discussion of the subject and touched (too lightly, it now 

 appears) upon the contradictions. This paper appeared in a 

 well-known entomological periodical 1 nearly two months in 

 advance of the discussion criticized and should have come to 

 Dr. Brues's notice. I find it necessary to reply to Dr. Brues's 

 criticism, as he has somewhat confused the subject by intro- 

 ducing the mechanical conveyance of other classes of disease 

 into the discussion. Such diseases very obviously do not 

 depend upon such close association of particular insects and 

 may be acquired in a variety of ways. In the case of true 

 blood-parasites the association is necessary, not only that, 

 as Dr. Brues puts it, transmission may take place, but in 

 order that the host-relation may become established. I am 

 fully aware that it has been experimentally proved that cer- 

 tain blood-diseases can be transmitted mechanically by blood- 

 sucking insects which are not hosts of the disease-producing 

 parasite. In these experiments positive results were only 

 obtained when the partly fed insect was at once transferred 

 to a healthy animal and the bite followed immediately. This 

 combination of circumstances must occur very rarely, if at 

 all, in nature. Even should it occur, the transfer of the para- 

 site will not be, by any means, always effected. In a practical 

 consideration of blood-disease transmission it is certainly a 

 negligable factor. 



The case of the African sleeping-sickness requires some 

 explanation, as it is in a different category from those dis- 

 eases caused by true blood-parasites of man. Here we have 

 to do with a disease caused by hamatozoans not normally 

 parasites of man. These protozoans are normally the para- 

 sites of the wild animals which abound in those parts of Africa; 

 they do not seriously affect their normal hosts, but they be- 

 come highly pathogenic when introduced into the human 

 circulation. That this is the true condition with sleeping- 

 sickness is now generally recognized. In this respect, then, 

 the parasites of sleeping-sickness differ from most other blood- 

 parasites, that they may become established in a vertebrate 

 host other than their normal one. In the case of most other 

 blood-parasites, as those of malaria, yellow fever, pappataci 

 fever, and filariasis, it has been determined beyond question 

 that they can thrive only in a single vertebrate host. But 

 with trypanosomiasis, as with the diseases just mentioned, 

 the continuance of the disease depends upon the association 



'Unconsidered factors in disease transmission by blood-sucking insects. 

 Journ. Econ. Ent. vol. 5, pp. 196-200 (April, 1912). 



