l] DIRECT TRANSMISSION 3 



well as animal infections may be occasionally carried in this 

 manner, especially in those cases where the bacteria are present 

 in the peripheral circulation in considerable numbers, e.g. 

 anthrax, Mediterranean fever. Moreover, by feeding sufficient 

 numbers of any species of biting-fly on an animal containing 

 large numbers of some parasite in its blood, and subsequently, 

 without any interval, on a normal susceptible animal, it is 

 possible to obtain experimentally the direct transmission of 

 practically any blood-inhabiting parasite. Employing such 

 methods the transmission of sleeping sickness and relapsing 

 fever may be effected by means of the bites of Stegomyia, and 

 Mediterranean fever by the bites of Culex ; it is almost certain, 

 however, that such transmission rarely, if ever, occurs in nature. 

 Nevertheless, the possibility must not be ignored and all 

 biting-flies should be regarded with suspicion from the point 

 of view of preventive medicine. 



Although under experimental conditions it is comparatively 

 easy to demonstrate the direct transmission of certain diseases 

 it is becoming more and more evident that, compared with in- 

 direct transmission, this mode of infection plays a relatively 

 unimportant part in the spread of disease. The pathogenic 

 agent of the disease, even under the most favourable conditions, 

 can only survive for a very limited time (at most two to three 

 days) on the mouth-parts of the biting-fly and unless the latter 

 feeds on another host before the expiration of this period no 

 infection is produced. As a rule a fly which has had a full meal 

 of blood rarely desires to feed again for some days and it is 

 only those flies which are interrupted during their feeding that 

 are liable to bite another host within a short space of time. 

 El Debab, a trypanosomiasis of camels occurring in North 

 Africa, is one of the best examples of a disease which seems to 

 be transmitted in this manner. Edmond and Etienne Sergent 

 have shewn that the outbreaks of this disease can be explained 

 on the supposition that it is directly transmitted from infected 

 to healthy animals by various species of tabanids, and they 

 note that in nature these insects frequently bite two or more 

 animals in quick succession, being disturbed whilst feeding, 

 through the efforts of their unwilling victims. 



