HOST-PARASITE RELATIONS: INTESTINAL PROTOZOA 



as a ''contact" carrier. Spontaneous recovery or recov- 

 ery following- the administration of drugs does not nec- 

 essarily include the elimination of the amoebae from the 

 body but only the cessation of symptoms. 



As Walker (Walker and Sellards, 191 3) pointed out, 

 the acute stage is followed by the "convalescent" carrier 

 condition during which the amoebae still multiply within 

 the host and escape with the feces usually in the form of 

 cysts. Such hosts may suffer relapses during which acute 

 symptoms reappear. 



The period of infection. How long a host remains in- 

 fected, unless cured by the administration of drugs, it 

 is difficult to state. It seems probable that an infection 

 once established persists throughout the life of the host. 

 Various investigators have attempted to determine the 

 length of infections in particular hosts (Low, 19 16; 

 Wenyon and O'Connor, 1917; Dobell and Stevenson, 

 1 918), and although they seem to prove that infections 

 persist for many years, the chances of reinfection are so 

 favorable that no definite conclusion can be reached. 



Host-parasite relations during the carrier period. 

 There are three principal points of view with regard to 

 host-parasite relations during the carrier period. The first 

 is that the amoebae live as harmless commensals in the 

 lumen of the intestine. This was considered impossible 

 until recent cultivation experiments proved that tissue 

 elements are not necessary for the growth and repro- 

 duction of the trophozoites. 



The second view is that the amoebae require access to 

 the tissue of the intestinal wall; that they must have 

 tissue elements for their growth and reproduction; and 



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