Case of the Month - January 2026

Signalment and History

A male, adult, big brown bat (Eptesicus fuscus) was found inside a private residence in Minnesota and submitted to the Minnesota Veterinary Diagnostic Laboratory (MVDL) for rabies testing. The animal was euthanized and subsequently necropsied.

Gross Findings

The cortex of each kidney had a slightly raised, approximately 1 to 2 mm in diameter, well-demarcated, white cyst.

Figure 1: Kidneys of a big brown bat with multiple beige, raised, well-demarcated cortical cysts (white arrows). 

Histopathology

Kidney: Expanding and compressing the renal cortex are protozoal cysts, which vary in size from 0.5 to 1 mm in diameter (Figure 2 A), and are surrounded by a 1–5-micron thickened hyaline basement membrane. The inner surface of the cyst is lined by 2-3 layers of hypertrophied and hyperplastic renal tubular epithelial cells. The epithelial cells contain developmental stages of a protozoan parasite, namely meronts, macrogametocytes, and microgametocytes (Figure 2 B). Oocysts, some of which are sporulated, and sporocysts with sporozoites are present in the lumen of the cystic dilated tubule (Figure 2 B).

Figure 2: (A) Two cortical tubular cysts (arrows) in the kidney of a big brown bat. (B) The tubular epithelial cells contain various developmental stages including meronts with merozoites (arrow head), macrogametocytes (blue arrows), and microgametocytes (red arrow). Oocysts, some of which are sporulated, and sporocysts with sporozoites (orange arrows) are present in the lumen of the cystic dilated tubule.

Ancillary Testing

• Brain samples were negative for rabies virus antigen by direct fluorescent antibody (DFA) test (performed at the Minnesota Department of Health).

Morphologic Diagnosis

• Kidney: Tubular protozoal cysts, multifocal, with numerous intraepithelial microgamonts, macrogamonts, schizonts and sporulating intraluminal oocytes. 

Etiology

Nephroisospora eptesicii

Comments

Renal coccidiosis in big brown bats (Eptesicus fuscus) is an unusual and well-adapted parasitic condition characterized by focal cystic dilation of renal tubules containing coccidian developmental stages, including oocysts. Notably, this entity holds particular regional significance, as the causative organism since Nephroisospora eptesici was first described from bats submitted for rabies testing to the Minnesota Veterinary Diagnostic Laboratory (MVDL).

Grossly, lesions are typically small (<1 to 2 mm in diameter), well-demarcated, raised, white cortical cysts that are considered to be incidental findings. Histologically, these correspond to cystic, dilated tubules lined by hypertrophied epithelial cells, which contain multiple stages of the parasite, including meronts, gametocytes, and oocysts within the lumen. The presence of sporulated oocysts (with sporocysts containing sporozoites) within the kidney, as seen in this case, is particularly noteworthy because it indicates that the parasite can complete its life cycle within a single host and a single organ, a feature that distinguishes Nephroisospora from most other coccidia.

Phylogenetically, Nephroisospora eptesici belongs to the Sarcocystidae and is closely related to organisms such as Toxoplasma, Neospora, Hammondia, and Besnoitia. However, unlike many of these parasites, which require two hosts to complete their life cycle, Nephroisospora appears to have a direct life cycle, with sporogony occurring within the renal tubules rather than in the external environment. 

The apparent restriction to the big brown bat may reflect true host specificity, although sampling bias is also possible, as this species is by far the most commonly species submitted for rabies testing at the Minnesota Veterinary Diagnostic Laboratory. Behavioral factors such as colony living and grooming may facilitate vertical transmission of Nephroisospora eptesicii, potentially through exposure of individual bats to urine containing oocysts or sporocysts. Although the cysts ultimately may collapse and elicit a focal eosinophilic and granulomatous inflammation, renal coccidiosis in big brown bats is generally considered a clinically insignificant finding.

References

Wünschmann A, Wellehan JF Jr, Armien A, Bemrick WJ, Barnes D, Averbeck GA, Roback R, Schwabenlander M, D'Almeida E, Joki R, Childress AL, Cortinas R, Gardiner CH, Greiner EC. Renal infection by a new coccidian genus in big brown bats (Eptesicus fuscus). J. Parasitol. 2010;96(1):178-183

Funding provided by the Minnesota Environment and Natural Resources Trust Fund.