A new Eimeria sp. is described as the cause of an outbreak of renal coccidiosis in double-crested cormorants (Phalacrocorax auritus) in Georgia. Sporulated oocysts were spherical to subspherical and measured 16.1 × 16.5 (13.8–18 × 14–19) μm, with an average length–width ratio of 1:1. Oocyst wall was thin (1 μm), greenish, and pitted on the outer surface. Micropyle, micropylar cap, Stieda body, and polar bodies were absent. Small oocyst residuum (4–8 granules) was usually absent but occasionally present. Sporocysts were oval and measured 6.6 × 9.3 (6–7 × 8–10.5) μm, with an average length–width ratio of 1:1.4. A sporocyst residuum was present, located in between sporozoites, and was composed of numerous granules of unequal size. A small refractile body was present in each sporozoite. Collecting duct and distal renal tubular epithelial cells were distended by large oocysts in their cytoplasm, and many oocysts were present in the lumen of dilated tubules. Various stages of meronts, gamonts, and developing oocysts were present in other renal tubular epithelial cells. Multiple infections of parasitized cells were frequently observed, with cells containing up to 12 gamonts or developing oocysts. The importance of this Eimeria sp. on double-crested cormorant populations is not known. But in this case, significant lesions and mortality were associated with infection.

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