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November 1, 2022

Case History: After a winter storm, over a hundred Brazilian Free-tailed Bats (Tadarida brasiliensis) were found dead in Texas, USA and several bats were submitted for necropsy.  Hypothermia was the suspected cause of death.

Histopathological findings: Moderate numbers of oocysts, macrogamonts, and microgamonts are observed in the lamina propria with no associated inflammatory response.

Photomicrographs from the intestine of a Brazilian Free-tailed Bat showing coccidia.
Figure 1. Photomicrographs from the intestine of a Brazilian Free-tailed Bat (Tadarida brasiliensis) found dead in Texas, USA. H&E.  (A) Within the lamina propria of the intestine are various coccidial stages.  (B) An unsporulated oocyst with a thick wall, granular cytoplasm, and indistinct nuclear material is present in the lamina propria.  (C) A macrogamete with a peripheral ring of eosinophilic granules is present in the lamina propria.  (D) A microgamont filled with basophilic microgametes is present in the lamina propria.

Etiologic diagnosis: Intestinal coccidiosis

Etiology: Suspect Eimeria spp.  Eimeria spp. are single-celled obligate intracellular parasites of the phylum Apicomplexa.  Eimeria spp. are not well-studied in bats and only 40 species have been described worldwide. 

Distribution: Worldwide.

Host range: Eimeria spp. infect a wide variety of birds, mammals, reptiles, amphibians, and fish with most species being host-specific.  Eimeria spp. have been described in 30 species of bats.

Life cycle: The life cycle is direct.  An infected host sheds unsporulated oocysts in their feces.  Oocysts sporulate in the environment becoming infectious and if ingested by a new host can cause intestinal coccidiosis.  In the gastrointestinal tract, sporozoites are released from the oocyst and invade intestinal epithelial cells to form trophozoites, and eventually undergo several cycles of asexual multiplication (merogony, schizogony).  Merozoites released from asexual stages within the host cell will infect new enterocytes to form the female sexual stage, a macrogamete, or a male sexual stage, a microgamete, which unite to form oocysts.

Clinical signs: Diarrhea and decreased growth are the main clinical signs of coccidiosis and typically occur in young, infected animals.  In severe cases, dysentery, dehydration, and tenesmus can occur.  Some infections are asymptomatic and self-limiting.

Pathology: Destruction of infected intestinal cells and surrounding tissue can result in necrosis, hemorrhage, and inflammation.

Diagnosis: Diagnosis can be made via oocyst identification in feces using salt or sugar flotation methods, direct intestinal smears, or a McMaster counting chamber, histopathology, and PCR.

Public health concerns: None.

Wildlife population impacts: While mortality can occur in severely infected animals, infections are usually self-limiting.  Coccidia observed in this case were an incidental finding. 


  • de Santana Miglionico MT, Costa LM, Mota EM, Bergallo HG, Dias D. 2020. A new species of Eimeria Schneider, 1875 (Apicomplexa: Eimeriidae) from Myotis riparius Handley, 1960 (Chiroptera: Vespertilionidae) in the Atlantic Forest of Brazil, with a checklist of Eimeria spp. reported from bats. Acta parasitologica, 65(2):496–503,
  • Gardiner, CH, Fayer, R, Dubey, JP. 1998. An Atlas of Protozoan Parasites in Animal Tissues, 2nd ed. Registry of Veterinary Pathology, Armed Forces Institute of Pathology, American Registry of Pathology, Washington, DC. ISBN 1–81041–48–1. 84 pp.
  • Merck Veterinary Manual. 2016. Overview of Coccidiosis in Animals, Accessed October 2022.

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