HOST-PARASITE RELATIONS'. INTESTINAL PROTOZOA 



Among the variable conditions within the intestine 

 are those due to the character of the diet. Carnivorous 

 animals are very seldom parasitized by intestinal pro- 

 tozoa (Hegner, 1923a, 1924b) and omnivorous species 

 such as the rat and man can be relieved of some of these 

 organisms if fed on a carnivorous diet. Such a change of 

 diet brings about many profound changes in the intes- 

 tinal contents, which apparently make them unfit as a 

 medium for the growth and multiplication of certain 

 protozoa. The character of the intestinal contents result- 

 ing from the normal dietary of the host may thus pre- 

 vent a foreign species from initiating an infection even 

 if it succeeded in reaching the normal location in the 

 host unharmed. 



The character of the digestive juices, failure of cysts 

 to excyst, the character of the diet, and various other 

 factors may, therefore, encourage or prevent parasites 

 that gain access to the body of the host from setting up 

 an infection. 



(4) How MAY WE ACCOUNT FOR LABORATORY INFEC- 

 TIONS IN FOREIGN HOSTS? It is possible in certain cases 

 to bring about an infection in a certain host species in 

 the laboratory that appears never to become parasitized 

 in nature. Several explanations suggest themselves to 

 account for this phenomenon. In the first place, the host 

 or intermediate host may behave in such a way as never 

 to encounter the infective stages of the parasite in na- 

 ture. For example, we would hardly expect an animal 

 that does not live in association with man to become in- 

 fected with human parasites, although it might be sus- 

 ceptible as indicated by laboratory experiments. The 



SO 



