HOST-PARASITE RELATIONS: INTESTINAL PROTOZOA 



Incidence. Only about 200 cases of human coccidiosis 

 have been reported. This apparent low incidence of in- 

 fection may be due to several factors: (i) the small 

 number of oocysts usually passed by the human host 

 (Wenyon, 1926) ; (2) the fact pointed out by Andrews 

 (1927) that cysts do not appear until the symptoms have 

 disappeared and hence are probably frequently over- 

 looked, and (3) the possibility that /. hominis may be a 

 natural parasite of some lower animal and only occa- 

 sionally brings about infection in man. To what extent 

 these and probably other factors influence the incidence 

 of infection in man can hardly at present be estimated. 



Development and escape of sporozoites. When swal- 

 lowed, the oocysts are carried with the food or drink 

 into the stomach and then into the intestine. No one 

 knows where they hatch nor what is the primary site of 

 infection. Presumably, however, the sporozoites escape 

 in the small intestine and immediately penetrate the 

 epithelial cells ; this we know to be true of Isospora felis 

 in the cat. They are thus always pathogenic although 

 probably large numbers must be ingested before symp- 

 toms are produced. Several investigators have recently 

 attempted to determine what factors are responsible for 

 the development of the sporozoites within the oocyst and 

 for their subsequent liberation in the intestine of a new 

 host. The oocysts of Isospora hominis are passed either 

 before or after the division of the protoplasmic contents 

 into two sporoblasts. In Egypt, Wenyon and O'Connor 

 (1917) found that complete development took place at 

 room temperature in one day; in England (Wenyon, 

 1926) it requires 3 to 4 days. The oocysts of the rabbit 

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