162 



THE TELOSPORASIDA AND THE COCCIDIA PROPER 



Once in a glandular epithelial cell, 

 each sporozoite rounds up and becomes a 

 first generation schizont. By a process 

 of asexual multiple fission (schizogony), 

 each schizont forms about 900 first gen- 

 eration merozoites, each about 2 to 4/j. 

 long. These get their name from the 

 Greek word for mulberry, which they re- 

 semble before they separate. They break 

 out into the lumen of the cecum about 2. 5. 

 to 3 days after infection. Each first gen- 

 eration merozoite enters a new host cell, 

 and rounds up to form a second generation 

 schizont, which lies above the host cell 

 nucleus. By multiple fission it forms 

 about 200 to 350 second generation mero- 

 zoites about IGfi long. These are found 

 5 days after infection. Some of them 

 enter new intestinal cells, round up to form 

 third generation schizonts, which lie be- 

 neath the host cell nuclei and produce 4 to 

 30 third generation merozoites about 7ji 

 long. 



Most of the second generation mero- 

 zoites, however, enter new host cells and 

 begin the sexual phase of the life cycle, 

 known as gametogony. Most of these 

 merozoites turn into female gametes (ma- 

 crogametes), which simply grow until they 

 reach full size. Some of the merozoites 

 turn into male gametocytes (microgameto- 

 cytes). Both the macrogametes and mi- 

 crogametocytes lie below the host cell 

 nuclei. Within each microgametocyte a 

 large number of tiny biflagellate micro- 

 gametes are formed. These break out 

 and fertilize the macrogametes. 



The resultant zygote lays down a wall 

 around itself in the following way: The 

 macrogametes contain one or two layers 

 of eosinophilic plastic granules in their 

 cytoplasm; these are composed of muco- 

 protein (Kheisin, 1958). They pass to the 

 periphery, flatten out and coalesce to 

 form the oocyst wall after fertilization. 

 The formation of this wall marks the 

 transition of a fertilized macrogamete into 

 an oocyst. According to Monne and Honig 

 (1954), the outer layer of the oocyst wall 

 is a quinone-tanned protein and the inner 

 layer is a lipid coat firmly associated 

 with a protein lamella. 



The oocysts then break out of their 

 host cells, enter the intestinal lumen, and 

 pass out in the feces. The prepatent per- 

 iod, from the time of infection to the ap- 

 pearance of the first oocysts in the feces, 

 is 7 days. Oocysts continue to be dis- 

 charged for a number of days thereafter, 

 due to the fact that the sporozoites do not 

 all enter the host cells immediately but 

 may remain in the lumen for some time, 

 and also because many of them are retained 

 in a plug of material in the ceca for some 

 days before they are eliminated. 



In the absence of reinfection, coccidial 

 infections are self-limiting. Asexual re- 

 production does not continue indefinitely as 

 it does, for example, in Plasmodium. In 

 E. tenella, 3 generations of merozoites are 

 produced; in other species there may be 1, 

 2 or 4. After this, the life cycle enters its 

 sexual phase; the oocysts are formed, 

 eliminated from the body, and the infection 

 is over. Reinfection may take place, but 

 the host develops more or less immunity 

 following primary infection. 



The number of oocysts produced in an 

 animal per oocyst fed depends in part on 

 the number of merozoite generations and 

 the number of merozoites per generation. 

 A single oocyst of E. tenella containing 8 

 sporozoites is theoretically capable of pro- 

 ducing 2, 520, 000 second generation mero- 

 zoites (8 X 900 X 350), each of which can 

 develop into a macrogamete or micro- 

 gametocyte. 



In E. bonis of cattle, there is only a 

 single asexual generation, but a giant 

 schizont containing about 120,000 mero- 

 zoites is formed (Hammond et al. 1946). 

 In the rat, E. nieschidzi is theoretically 

 capable of producing 1, 500,000 oocysts 

 per oocyst fed, E. miyairii 38,016, and 

 E. separata only 1536 (Roudabush, 1937). 

 In E. nieschulzi there are 4 generations of 

 merozoites, while in the latter two species 

 there are only 3, and fewer merozoites are 

 usually produced in each than in E. nie- 

 sclmlzi. In the rabbit, E. magna produces 

 800,000 oocysts per oocyst fed, E. media 

 produces 150,000 and £. coecicola 100,000 

 (Kheisin, 1947, 1947a). 



