l] MODES OF INFECTION II 



merely require to complete part of their larval development 

 in the tissues of the mosquito. 



The manner in which the infective agent re-enters the 

 vertebrate host is of considerable interest and varies in dif- 

 ferent cases. 



As a rule the infective agent is directly inoculated into the 

 body when the infected invertebrate feeds on some vertebrate 

 host. The parasites may be present in the proboscis of the 

 infected arthropod or leech, or contained in the salivary fluid, 

 but in either case the result is the same and the infection is 

 passed directly into the blood. 



In the case of Filaria, the parasite is contained in the 

 proboscis of the infected insect and when it feeds escapes on 

 to the surface of the skin. The parasite subsequently bores 

 its way through the skin and thus reaches the general circula- 

 tion of its vertebrate host. 



Sometimes the infective agent is merely deposited on the 

 surface of the skin in the faeces of the invertebrate host. 

 From this site the parasite may reach the blood circula- 

 tion by means of the open wound caused by the bite of the 

 invertebrate host (as in the case of Spirochcsia duttoni), or 

 through an excoriated part of the skin. 



Sometimes the same result is obtained by the host licking 

 up the parasite from the surface of its body. This is one of 

 the modes of infection of Trypanosoma lewisi, in which the 

 infection reaches the circulation through the mucous membrane 

 of the alimentary canal of the rat. 



The transmission of SpirochcBta recurrentis by the louse is 

 comparable with that of S. duttoni by the human tick, for in 

 the former case the parasites, set free only by crushing the 

 insect, pass through the excoriations produced by scratching 

 with the finger-nails. As lice are frequently crushed between 

 the nails, it is easy to see how the infective agent can reach 

 the circulation. 



Occasionally it is necessary for the vertebrate to swallow 

 the parasite, which subsequently bores its way through the 

 alimentary canal. The best known example of this method is 

 that of Filaria medinensis, which develops in the body of 



