2 DIRECT TRANSMISSION [CH. 



shewn to transmit in addition to numerous infections of animals 

 the following human diseases : malaria, sleeping sickness, 

 yellow fever, three-day fever ; and evidence has been brought 

 forward also suggesting that dengue and epidemic polyomyelitis 

 are spread by these insects. Before proceeding to a discussion 

 of individual infections, however, and the part played by flies 

 in their transmission, it will be convenient to give a short 

 general account of some of the problems connected with this 

 subject. 



It has never been proved that, under natural conditions, 

 biting-flies normally transmit any other than animal parasites 

 from one host to another. In those cases in which the patho- 

 genic agent is unknown, e.g. yellow fever, three-day fever, the 

 clinical symptoms seem to indicate that these diseases are 

 also due to animal parasites and not to bacteria. There is no 

 a priori reason why biting-flies should not transmit bacterial 

 infections as well as animal, but certainly no bacterial disease 

 is known to be normally transmitted by these insects. Since 

 a biting-fly feeds on blood and can only become infected by 

 ingesting the parasite, it necessarily follows that the latter, at 

 least during some part of its life-cycle, must be present in the 

 blood of the vertebrate host, and the manner in which the 

 infection is conveyed by the insect may be either " direct " 

 or " indirect." 



(a) Direct transmission. 



When the pathogenic agent does not develop in the body 

 of the biting-fly, but is merely carried on the mouth-parts 

 and directly inoculated into the next host which the insect 

 feeds upon, the transmission is said to be direct or mechanical. 

 This kind of transmission somewhat resembles the method of 

 infecting a healthy animal by means of the prick of a needle 

 that has been thrust previously into an infected animal and 

 which is thereby soiled with the infective blood from the latter. 



The efficiency of any particular species to act as the direct 

 carrier of a disease agent obviously depends on such mechanical 

 details as the size and shape of the mouth-parts, the number 

 of parasites in the blood, etc. It is possible that bacterial as 



