Synonyms: Cryptosporidium parvum Tyzzer, 1912 pro parte.
Location: All stages occur in the striated border (cuticular layer) of the surface epithelial cells of the tubular part of the ceca.
Geographic Distribution: North America (Massachusetts).
Morphology: Tyzzer (1929) did not describe this form in detail, but illustrated it and said that it appeared morphologically identical with C. parvum of the mouse. The following description is based primarily on that given by Tyzzer (1912) for C. parvum. The oocyst is ovoid or spherical, 4 to 5 by 3 u. The oocyst wall is smooth, composed of a single layer, with a small, knob-like attachment organ. A micropyle is absent. An oocyst residuum is present. The sporozoites are slender, bow- or boomerang-shaped, 5.5 to 6 u long, with a rod-shaped, slender nucleus near the anterior end.
Life Cycle: The following description is based primarily on that given by Tyzzer (1912) for C. parvum, which is morphologically identical with C. tyzzeri. The schizonts are 3 to 5 u in diameter when mature and have an attachment organ. They are attached to the cell surface or embedded in its striated border. They form 8 falciform merozoites 2.5 to 5 by 0.5 to 0.7 u, with a nucleus near the thicker end, and a small residual mass. The microgametocytes are smaller than the schizonts and also have an attachment organ. They give rise to 16 tiny microgametes and a spherical mass of residual material. The microgametes are chromatin rods about 1 u long and not more than 0.4 u wide, without visible flagella. The macrogametes are larger than the schizonts and microgametocytes, and contain tiny, refractile granules. They have a thin, dense limiting membrane and an attachment organ.
Pathogenesis: Apparently non-pathogenic.
Remarks: Tyzzer (1929) thought that this was the same species he had previously found in mice, but he attempted no cross-infection experiments. He said that even if such experiments failed, the morphological agreement was such that the chicken and mouse forms could only be regarded as biological varieties of the same species. However, such a narrow species concept is no longer held, and it seems best to draw attention to the chicken form by giving it a name of its own. Consequently it is named Cryptosporidium tyzzeri.
In this connection, too, it might be mentioned that Tyzzer (1910) was unable to infect the laboratory rat with the closely related C. muris from the laboratory mouse.