298 RESEARCH IN PROTOZOOLOGY 



dogs. More accurate information as to the extent of this host- 

 parasite specificity is needed. Cross-infectivity seems to be an easily 

 testable experiment to make, yet the results may be very mislead- 

 ing unless one bears in mind three things : ( i ) the natural coc- 

 cidial parasites of the host, (2) the interference of an acquired 

 immunity, and (3) mechanical carriage. The first difficulty 

 amounts to very little if the foreign parasites employed differ de- 

 cidedly in their morphology from the coccidia which are natural 

 parasites of the experimental host. The interference of immunity 

 by virtue of a previous infection is a phenomenon the significance 

 of which is hard to estimate at present. Varying degrees of re- 

 sistance to subsequent attempts at coccidial infection have been 

 indicated in cat, dog, and poultry coccidiosis. The length of time 

 that coccidial immunity lasts is not known, nor is it known whether 

 or not infection by one species can confer resistance to infection 

 by related species. It is clear, then, that there is a possibility that 

 a negative cross-infectivity experiment might indicate individual 

 immunity instead of species immunity. In regard to the third 

 caution, mechanical carriage, it is known that non-viable and un- 

 segmented oocysts will pass through the intestine unchanged. The 

 recovery of such oocysts must not be mistaken for evidence of 

 the establishment of an infection. Proof of the ability of one 

 species of coccidium to parasitize more than one species of host 

 involves the use of foreign hosts that are free and that are known 

 to have been free from coccidiosis from birth, and the demonstra- 

 tion not only of the discharge of oocysts from these foreign hosts 

 but of the presence of the asexual forms of the parasite in the 

 tissue of the host as well. 



The mechanism of host-parasite specificity offers many interest- 

 ing problems for investigation. At what point in the development 

 of the "infection" is the foreign host protected? Is there any 

 selectivity involved in excystation or, granting that sporozoites 

 are liberated, are they incapacitated before they can penetrate 

 cells? Are there such differences in the epithelial cells of various 

 animals that the sporozoites of a particular species of coccidium 

 can penetrate into the cells of a particular species of host and be 

 unable to enter the cells of other species of hosts? Or is the infec- 

 tion aborted after the intestinal cells have been entered? The rare 

 incidence of coccidiosis of man has been explained upon the as- 

 sumption that he was an accidental host to the coccidial parasite 



