Type host:Cynomys ludovicianus

Synonyms: Eimeria cynomysis; E. marmotae; E. os.

Other hosts: Cynomys ludovicianus; Marmota baibacina; M. bobak; M. monax.

Type locality: North America: USA, Colorado, near Fort Collins.

Other localities: Asia: Kazakhstan; North America: USA, Illinois, New York, Pennsylvania.

Description of oocyst: (from Vetterling, 1964 [primarily] and Dorney, 1965): Oocyst shape: subspheroid to ovoid; wall thickness: 1.4; layers: 2; outer layer proportion of total thickness: not given; outer layer colour: yellow-green; outer layer texture: smooth; inner wall characteristics: not given; micropyle: present; micropyle width: 5; MC: absent; OR: absent; PG: sometimes present; number of PGs: 0-1; PG shape: not given; PG L x W: 1-2.5; size: 32.5 x 28.4 (30-37 x 23-31); L/W ratio: 1.2 (1.1-1.3). Distinctive features of oocyst: oöcyst tapers toward micropyle.

Description of sporocysts and sporozoites: Sporocyst shape: lemon-shaped; size: 15-18 x 9-12; SB: present; SB L x W: not given; SB characteristics: prominent; SSB: absent; PSB: absent; SR: present; SR characteristics: compact, composed of many small granules (in drawing); SR size: not given; SP: vermiform. Distinctive features of sporocyst: none.

Prevalence: 1 of 86 (1%).

Site of infection: Unknown. Oocysts collected from faeces.

Material deposited: Host symbiotype in the Colorado State University Mammal Collections # 10184 (Vetterling, 1964). There is a suitable drawing (Figure 1) in Vetterling (1964) and a suitable photomicrograph (Figure 3) in Dorney (1965).

Etymology: The trivial name is derived from gigas- (Gk, a giant) and os, because Dorney (1965) originally named the form he saw as E. os.

Remarks: Vetterling (1964) found oocysts similar in size to those of E. cynomysis (30-37 x 25-31 [Vetterling, 1964] vs. 25-37 x 22-32 [Andrews, 1928]) and, thus, emended the original description. However, Vetterling's (1964) oocysts were smooth-walled and had a prominent SB while those of Andrews (1928) were rough-walled with an inconspicuous SB. Thus, we renamed those of Vetterling (1964) to E. giganteos. Dorney (1965) described and photographed what he called E. os from M. monax, but the sporulated oocysts he described were so much larger (34-37 x 23-26) than those of E. os (20-26 x 18-22) of Crouch & Becker (1931), that we believe the form Dorney saw was E. giganteos not E. os. The sporocysts measured by Dorney (1965) also were proportionally smaller than those of Crouch & Becker (1931) to further support our decision to separate these species. Nukerbaeva & Abenov (1979) described a species from M. bobak they called E. marmotae, (described by Galli-Valerio [1923], but designated a species inquirenda by us because the description was based on unsporulated oocysts). The few descriptive characters (by Nukerbaeva and Abenov, 1979) are indistinguishable from the description by Vetterling (1964) for E. cynomysis (now E. giganteos). Fleming et al. (1979) and McQuistion & Wright (1984) reported E. os from M. monax. However, they followed Dorney (1965); therefore, it is likely they all actually saw E. giganteos.

References: Dorney (1965); Fleming, et al. (1979); McQuistion & Wright (1984); Nukerbaeva & Abenov (1979); Vetterling (1964).