874 



THE ANIMAL CARRIERS OF DISEASES 



the mosquito, and may have spread from mosquito to mosquito 

 by hereditary infection, as Schaudinn suggested, the sporozoites — 

 i.e., the infective agent — going all over the body, and so entering 

 the salivary glands, and subsequently, when the evolution of warm- 

 blood animals took place, becoming blood parasites, at first 

 accidentally. 



The passage from the intermediate host to the definitive host we call 

 transmission. 



It is never directly inoculative, like infection, but it may be — 



1. Ingest ive, via the alimentary canal, 



2. Penetrative, via the unbroken skin and mucous membranes. 

 In the first it is the arthropod which ingests the parasite; in the 



second the parasites come in contact with the unbroken skin or 

 mucosae, through which they force their own way, and so enter 

 the bod}^ of the intermediate host, which is usually the vertebrate 

 in this case. 



Following our life-cycle, we come to the passage from the de- 

 finitive to the intermediate host, and this may be called ' infection,' 

 because it is so often followed by disease in the intermediate host. 

 This generally takes place by the agency of the product of the sexual 

 generation — i.e., some descendant of the zygote, using this term in 

 the widest sense to mean the product of the fusion of male and female 

 elements. 



Infection of the vertebrate is usually inoculative, and may be 

 performed in two ways: — 



(a) The Direct. — In this the blood-sucker simply transmits the 

 parasite unchanged after holding it for a short time. 



(6) The Indirect. — In this the parasite undergoes development 

 in the blood-sucker. 



Infection and Transmission. 



Parasite. 



Definitive 

 Host. 



Infection . 



Intermediate 

 Host. 



Transmission. 



Plasmodium 

 malar icB. 



Anopheline 

 mosquitoes. 



Sporozoites 

 from zygote. 



Inoculative. 



Man. 



Micro- and 

 macro-game- 

 to cytes. 



Ingestive. 



Filaria 

 bancrofti. 



Man. 



Microfilaria 

 from egg. 



Ingestive. 



Culicine mos- . 

 quitoes. 



Larvofilariae. 

 Penetrative. 



The direct is unusual, as a blood-sucker as a rule takes a full 

 meal from one individual, and does not feed again for a day or so, 

 but it may take place- — as Edmond and Etienne Sergent have shown 

 to be the case of the tabanid flies, which bite camels in North Africa^ 



