318 



SARCOCYSTIS, TOXOPLASMA AND RELATED PROTOZOA 



perhaps also in the young cysts by schi- 

 zogony. There is a single genus, Sarco- 

 cystis. The group has been reviewed by 

 Babudieri (1932), Scott (1930, 1943), 

 Barreto (1940), Erickson (1940), Grasse 

 (1953) and Eisenstein and Innes (1956). 



Genus SARCOCYSTIS 

 Lankester, 1882 



In this genus the cysts are found in 

 the striated and heart muscles, and are 

 usually divided into compartments by in- 

 ternal septa. Synonyms of this name are 

 Miesclieria Blanchard, 1885 and Balbiania 

 Blanchard, 1885. 



Sarcocystis is common in many spe- 

 cies of animals. It is found in the great 

 majority of sheep, cattle, horses and 

 swine, and is often seen in wild ducks. 

 It is extremely rare, however, in car- 

 nivores such as the dog and cat, and the 

 reports of its presence in these animals 

 require verification. Dogs cannot be in- 

 fected experimentally (Pfeiffer, 1891). 



More than 50 species of Sarcocystis 

 have been named, but it is not at all cer- 

 tain that they are all valid. They are 

 differentiated on the basis of the host in 

 which they occur, the structure of the 

 cyst wall and the size of the trophozoites. 

 However, Sarcocystis is not very host- 

 specific. The rat and guinea pig can be 

 infected with the form from the mouse, 

 the mouse, guinea pig, chicken and duck 

 with that from the sheep, and the rat and 

 mouse with that from the pig. In addition, 

 the same species does not look the same 

 in all hosts. For example, in the guinea 

 pig the trophozoites of the form from the 

 mouse are only half their former size and 

 the cysts do not have alveoli. Finally, 

 the structure of the cyst wall may vary 

 with its age even in the same host. The 

 specific names below are therefore used 

 more as a matter of convenience and 

 custom than from any conviction that they 

 are all necessarily valid. 



Morphology : The cysts are known 

 as Miescher's tubules and are easily vis- 

 ible to the naked eye. They are usually 

 cylindroid or spindle-shaped, running 



lengthwise in the muscles, but they may 

 also be ellipsoidal or rather irregular. 

 They vary in size depending in part on the 

 host. The ellipsoidal cysts in the sheep 

 may reach 1 cm in diameter, but consid- 

 erably smaller ones are the rule. Those 

 in the duck are 1 or 2 mm in diameter and 

 1 cm or more long. 



The cyst wall varies in appearance 

 with the species. There are 3 types. In 

 one, e. g. , Sarcocystis nuiris of the mouse, 

 it is smooth. In another, e. g. , S. platy- 

 dactyli of the gecko, it has an outer layer 

 of radial spines, villi or fibrils called 

 cytophaneres. In a third, e.g., S. lenella 

 of the sheep, the wall is smooth in the 

 young cysts, acquires a layer of cyto- 

 phaneres as the cysts develop, and then 

 loses it when they become old. 



The cyst wall of S. tenella is com- 

 posed of 2 thick layers (Ludvik, 1958). 

 The inner one is homogeneous and contains 

 nuclei. Extensions from it form septa be- 

 tween the compartments in the cyst. The 

 outer layer contains no nuclei and appears 

 spongy in electron micrographs. It forms 

 the cytophaneres. The inner layer con- 

 tains RNA and the outer a polysaccharide. 

 The cyst wall is essentially negative to the 

 periodic acid-Schiff test, altho the cyto- 

 phaneres stain slightly according to Fren- 

 kel (1956a). 



The cyst wall of S. »iiesclieriaiia dif- 

 fers from that of S. teiielta in being com- 

 posed of only a single layer with a com- 

 plicated surface structure (Ludvik, 1960). 

 The cytoplasm of the wall is granulated, 

 and fine septa project from its inner sur- 

 face to divide the interior of the cyst into 

 small compartments. The outer surface 

 of the cyst wall is spongy, with a fine 

 honeycomb structure. It sends numerous 

 parallel, hollow, finger-like projections 

 or villi into the surrounding muscle tissue. 

 These villi may be as much as 8 to 10 /^ 

 long, and are circular or ellipsoidal in 

 cross section and about 0. 7 to 0. 8 /i in 

 diameter. They contain slender, long 

 double fibrils 100 A thick. 



There is a difference of opinion as to 

 whether the cyst wall is formed by the 



