In a goldfish stock held in a pet fish pond, heavy coccidian infection, caused by an epicellularly developing Goussia species, appeared in April of 3 consecutive years (2014 to 2016). The shape and size of the oocysts resembled those of an inadequately described species, Goussia carassiusaurati (Romero-Rodriguez, 1978). In histological sections, gamogonic and sporogonic stages infested mostly the second fifth of the intestine, where almost all epithelial cells were infected. Both gamonts and young oocysts occurred intracellularly but in an extracytoplasmal position, seemingly outside the cells. Oocysts were shed non-sporulated. Spheroid to ellipsoidal non-sporulated oocysts measured 12.4 × 13.5 µm on average, but after 48 h sporulation in tap water they reached a size of 16 × 13 µm. The 4 elliptical sporocysts located loosely within the sporulated oocysts measured 13 × 5.4 µm. The oocysts and sporocysts were smaller than those of the better-known Goussia species G. aurati (Hoffman, 1965).
Keywords: Coccidiosis; Epicellular location; Goldfish; Goussia; Seasonal development.