Type host: Pseudacris triseriata (Wied-Nuweid, 1838), Western chorus frog.

Other hosts: None reported to date.

Type locality: NORTH AMERICA: U.S.A., Nebraska, Lancaster County, Pawnee Lake (40º 51' 10.8" N, 96º 53' 6.6" W).

Geographic distribution: NORTH AMERICA: U.S.A.

Description of oocyst: Oocyst shape: ovoidal, rarely subspheroidal; number of walls: 1; wall thickness: ~0.5; wall characteristics: smooth, colorless; L x W: 19.3 x 15.1 (18-23 x 11-20); L/W ratio: 1.3 (1.1-1.6); M: absent; OR: absent; PG: absent. Distinctive features of oocyst: wall frequently ruptures after sporulation, releasing free sporocysts.

Description of sporocysts and sporozoite: Sporocyst shape: ovoidal with a thin colorless wall, ~0.4; L x W: 13.3 x 9.9 (11-15 x 9-13); L/W ratio: 1.3 (1.0-1.6); SB: absent; SSB: absent; PSB: absent; SR: present; SR characteristics: spheroidal, 5.5 x 5.3 (4-7 x 4-7) with numerous coarse granules bound by a membrane; SZ: elongate, 12.8 c 3.2 (10-15 x 2.5-4), arranged so that 2 SZ lie in one direction and the other 2 lie in the opposite direction; each with 2 RBs--anterior-central RB spheroidal to ovoidal, 2.3 x 2.4 (2-3 x 2-3); posterior RB spheroidal to ovoidal, 2.6 x 2.4 (2-3 x 2-3); N indistinct, located between RBs. Distinctive features of sporocysts: SR bound by a distinct membrane.

Prevalence: 23 of 30 (70%) of adult frogs; 4 of 16 (25%) of tadpoles.

Sporulation: Exogenous. Oocysts passed unsporulated or semi-sporulated in the sporoblast stage and became fully sporulated in 12-24 h ar room temperature.

Prepatent and patent periods: Unknown.

Site of infection: Supra-nuclear in luminal epithelial cells of the posterior small intestine, although in heavily infected frogs, endogenous stages are found throughout the small intestine.

Endogenous development: Mature meronts were 14.8 x 8.8 (12–19 x 7–10) and contained 8–9 banana-shaped merozoites, 6.1 x 1.4 (5–7 x 1–2). Developing microgamonts with peripherally located N were 11.6 x 9.3 (10– 16 x 7–12), mature microgamonts with numerous microgametes were 18.2 x 14.8 (16–22 x 13–17), and mature macrogamonts with numerous granules and vacuoles were 11.9 x 10.3 (10–14 x 8–13). Freshly released oocysts with basophilic sporoplasm and numerous granules were 14.1 x 11.7 (12–15 x 10–13); these began sporulating in the gut lumen.

Materials deposited: Sporulated oocysts preserved in 70% ethanol in the HWML (HWML 16978) as are photosyntypes of sporulated oocysts (HWML 16979), histological sections of adult frog small intestine (HWML 16980) and histological section of tadpole small intestine (HWML 16981); the symbiotype host is in the University of Nebraska State Museum (ZM-23844).

Remarks: Of the 22 isosporans reported from anuran hosts, this species most closely resembles I. cruzi Pinto and Vallim, 1926, from Scinax spp. (Hylidae) from South America (Pinto & Vallim 1926; Carini 1936; Walton 1947). Sporulated oocysts of this species differ from those of I. cruzi in being a little smaller (19.3 x 15.1 vs. 20.7 x 17.2) and more ovoidal in shape (1.3 vs. 1.2). Additionally, the SR is bound by a distinct membrane not mentioned in I. cruzi. Based on these subtle differences as well as differences in hosts and geographic location, Bolek et al. (2003) were justified in naming this species from Pseudacris as distinct.