INFECTION AND TRANSMISSION 



875 



and which spread the trypanosome disease, ' el debab — by this 

 method, because they acquire a full meal from several individuals, 

 and because the camels resent their attentions. 



Infection may also be contaminative. In this method the parasite, 

 escaping with the carrier's fgeces, enters the new host via wounds, 

 either pre-existing or caused by the carrier itself. 



In the case of the Arthropoda infection is generally ingestive, as 

 it sucks the blood of the vertebrate and so obtains the blood parasite. 



Finally, just as the fly is an intermediary or temporary host for the 

 amoeba of dysentery, so blood-sucking flies may take up some 

 blood-containing parasites and immediately pass them on to a 

 healthy animal in a second feed; but while biting through the skin 

 they inject this second animal with the parasite, and infection 

 results. 



We will now assume that the definitive and intermediate hosts 

 have been living together for long periods undisturbed, and that 

 they have adjusted themselves to the parasite and the parasites 

 to them. Under circumstances such as these there will be little or 

 no sign of disease in the intermediate host, which has now become 

 a ' reservoir ' for the parasite. 



Assuming that the definitive host is a blood-sucking arthropod, 

 and that man enters such an area as a new-comer, and is bitten by 

 the blood-sucker, then several things may happen to the parasite. 



A. It may be killed off and no infection follow. 



B. It may find in man a suitable intermediate host, and cause— 



(1) Acute epidemic disease. 



(2) Chronic endemic disease. 



(3) No disease, only infection. 



The second is obviously better for the parasite than the first, 

 and the third than the second. 



Therefore, in studying the carriage of an animal parasite of man 

 by some other animal, we must know — 



1. The parasite. 



2. The definitive hosts: — 



{a) Reservoirs. 

 [h] Non-reservoirs. 



3. The method of infection. 



4. The intermediate hosts: — 



(a) Reservoirs. 

 ih) Non-reservoirs. 



5. The method of transmission. 



But there is another question which it is necessary to consider 

 when man is the definitive host. This question is whether the inter- 

 mediate host is new, and if so, whether any trace of the old original 

 life-cycle still persists. It should be remembered that man evolved 

 later than blood-sucking insects, as evidenced by tsetse-flies found 

 in geological formations in America — so Sambon informs the writers. 



