330 



SARCOCYSTIS, TOXOPLASMA AND REL,\TED PROTOZOA 



are 0.02fi in diameter when they leave 

 the conoid and then thicken to form a club 

 or sausage-shaped structure 0.08 to 0.2ju 

 in diameter. In addition to these, there 

 are 1 or 2 central fibrils which frequently 

 form a large loop or run posteriorly in a 

 zigzag. 



The cytoplasm is somewhat vacuolated 

 and contains a number of osmiophilic gran- 

 ules about 0.5fi in diameter, mitochondria 

 and often a cluster of fine granules around 

 the nucleus. Goldman, Carver and Sulzer 

 (1958) found a mass of argyrophilic gran- 

 ules at the very posterior end. The nu- 

 cleus is usually round or oval, but lobed 

 and horseshoe shapes have also been seen 

 in electron micrographs. In the latter, 

 the open end faces anteriorly as in Sarco- 

 cystis. The nucleus is about 1.0 to 1. 5ji 

 in diameter when circular and up to 2 ^ in 

 diameter when elongated. Inside the nu- 

 cleus is a large endosome which can be 

 seen both in electron micrographs and 

 after silver protein staining. 



In addition to the above structures, 

 Goldman, Carver and Sulzer (1958) des- 

 cribed long, thread-like appendages in 

 trophozoites treated with dilute (0. 1 to 

 1.0%) formalin in saline before fixation. 

 These may have been detached pellicular 

 fibrils. 



The parasites occur within vacuoles 

 in their host cells. According to Gustaf- 

 son, Agar and Cramer (1954), there is a 

 definite space between the parasite and 

 the vacuole wall. The space often con- 

 tains a filamentous or granular precipitate, 

 and concentrations of mitochondria are 

 often present in the host cell at the edge 

 of the vacuole. 



As the parasites multiply, they form 

 a cyst-like structure. Frenkel (1956a) 

 emphasized that there is a difference be- 

 tween the terminal colonies which repre- 

 sent the final stage of parasitization in the 

 leucocytes and the cysts which are found 

 in the central nervous system, eye and 

 myocardium. The wall of the latter is 

 argyrophilic and weakly positive to the 

 periodic acid-Schiff stain (PAS), while 

 that of the former is not. Some authorities 

 believe that the wall is formed by the host. 



so that the "cyst" is actually a pseudocyst, 

 but Frenkel and Friedlander (1951) con- 

 sidered it likely that the wall is derived 

 from the parasite. Lainson (1958), too, 

 distinguished between the cyst-like struc- 

 tures formed in the acute and chronic 

 stages of the infection. The former he con- 

 sidered to be pseudocysts and the latter 

 true cysts. 



The trophozoites in the cysts differ 

 slightly from the proliferative ones in the 

 pseudocysts. They contain large glycogen 

 granules, are more resistant to external 

 agents, and multiply slowly. Dasgupta 

 and Kulasiri (1959) found that PAS-positive 

 granules were abundant in the stages in the 

 "pseudocysts" from the brains of mice, 

 but that they were not universally present 

 in the intracellular and extracellular 

 trophozoites at all days of infection. 



Life Cycle : Reproduction in Toxo- 

 plasma has generally been considered to 

 take place by binary fission. However, 

 Goldman, Carver and Sulzer (1958) re- 

 ported on the basis of silver protein stain- 

 ing that T. gondii reproduces by a process 

 of internal budding which they named endo- 

 dyogeny. In this process, 2 daughter cells 

 are formed within the parent cell. They 

 are small at first, but grow until they des- 

 troy the parent cell and are released. 



The natural mode of infection is un- 

 known except in congenital toxoplasmosis, 

 but experimental infections can be estab- 

 lished by intravenous, intraperitoneal or 

 any other type of parenteral inoculation or 

 even by feeding. Weinman and Chandler 

 (1954) transmitted toxoplasmosis to swine 

 and rodents, and Makstenieks and Verlinde 

 (1957) transmitted it to mice and a cyno- 

 molgus monkey by feeding infected tissue 

 or peritoneal fluid. However, Schmidtke 

 (1956) and van Thiel and van der Waaij 

 (1956) considered that infection by feeding 

 can occur only when there are epithelial 

 lesions in the mouth or esophagus. 



Jacobs, Remington and Melton (1960) 

 found that the cysts of T. gondii are not 

 able to survive freezing and drying, but 

 they survive as long as 68 days at 4° C. 

 Proliferative forms are destroyed within 

 a few minutes by artificial gastric juice. 



