Case of the Month - June 2026

Signalment and History

An adult, male big brown bat (Eptesicus fuscus) was found alive on a sidewalk behind a house. The owner of the house reported the animal to “Animal Control”. It was captured by an animal control officer and euthanized. Human exposure to the animal (e.g. by bit) did not occur. In accordance with Minnesota statutes, the animal was submitted to the Minnesota Veterinary Diagnostic Laboratory where the brain was removed and forwarded to the Minnesota Department of Health for testing for rabies virus infection. 

Gross Findings

A necropsy was performed. Significant macroscopic lesions were not detected in the brain and other organs.

Histopathology

Brain: Since a fairly large portion of the brain was being used for the rabies virus antigen-specific direct fluoresence antibody test (performed at the Minnesota Department of Health), the official rabies test throughout the United States of America, only an olfactory lobe and a small portion of the cerebral cortex were avialable for the microscopic examination) and spinal cord: Multiple perivascular infiltrates were present in the neuopil of the olfactory lobe of the brain and gray matter of the spinal cord. The perivascular infiltrates were 1 to 3 cell layers thick and consisted of lymphocytes and plasma cells. Intracytoplasmic inclusion bodies were not detected in neurons. 

The small intestine contained numerous cestodes. There were no significant microscopic lesions in heart, liver, lungs, kidneys, adrenal gland, testis, accessort sex glands, stomach, and spleen. 

Virology

Brain samples tested positive for rabies virus antigen by direct fluoresence antibody test (performed at the Minnesota Department of Health).

Morphologic Diagnosis

1. Brain, encephalitis, lymphoplasmacytic, mild to moderate.
2. (Small intestine, cestodiasis, marked)

Etiologic Diagnosis

Rabies virus

Discussion

Rabies is one of the oldest and deadliest diseases known to man. It is caused by a variety of lyssavirus species that all invariably target neurons of the central nervous system. The prototype lyssavirus species is “rabies virus” (RABV = lyssavirus rabies) which exists worldwide in many phylogenetic lineages and is maintained in animal reservoirs, belonging to the orders Carnivora (carnivores) and Chiroptera (bats). Bat related rabies in humans is more common in the United States than rabies due to exposure to non bat infected mammals (e.g. raccoon and skunk). Bats in North America are infected with various variants (or lineages) of RABV. Once animals and humans show clinical signs of rabies virus infection, they invariably die from the disease (or infected animals are euthanized). Even with the most advanced treatment regimen, rabies virus infection in humans also is almost always fatal. 

Two outbreaks of rabies in captive populations of big brown bats have been reported. Nevertheless, little is known about the central nervous system lesions in big brown bats with RABV infection. The small size of the brain of big brown bats (less than 0.8cm long and less than 0.5g heavy) and the need for unfixed brain samples for the mandatory direct fluoresence antibody assay explains why histologic studies using formalin fixed brain samples from big brown bats are extremely rare. While the direct fluoresence antibody assay is the preferred test in the United States to diagnose rabies, alternate diagnostic modalities are being employed based on the circumstances of the case. The histologic diagnosis of rabies in mammals is usually based on the detection of a mononuclear polioencephalitis or polioencephalomeningitis and the presence of intracytoplasmic inclusion bodies in brain neurons (Negri bodies). In this rabid bat, intracytoplasmic inclusion bodies were not detected. 

The Minnesota Department of Health provides information regarding rabies including the animal testing in Minnesota under the following link: https://www.health.state.mn.us/diseases/rabies/index.html

References

1. Abbott RA, Saindon L, Falendysz EA, Greenberg L, Orclan L, Sateshkumar PS, and Rocke TE. 2020. Rabies outbreak in captive big brown bats (Eptesicus fuscus) used in a white-nose syndrome vaccine trial. Journal of Wildlife Diseases, 56: 197-202.

2. Begeman L, GeurtsvanKessel C, Finke S, Freuling CM, Koopmans M, Mueller T, Ruigrok TJH and Kuiken T. 2018. Comparative pathogenesis of rabies in bats and carnivores, and implications for spillover to humans. The Lancet Infectious Diseases 18:e147-e159. http://dx.doi.org/10.1016/S1473-3099(17)30574-1

3. Boutelle C, Bonaparte S, Orciari LA, et al. (2025). Rabies surveillance in the United States during 2023. Journal of the American Veterinary Medical Association (JAVMA) 263(10): 1310-1317.

4. Caraballo DA, Vico ML, Piccirilli MG, et al. (2024). Bat rabies in the Americas: Is Myotis the main ancestral spreader? Viruses 16: https://doi.org/10.3390/v16081302

4. Gilbert AT, van Pelt LI, Hastings LA, et al. (2024). Reemergence of a big brown bat lyssavirus rabies variant in striped skunks in Flagstaff, Arizona, USA 2021-2023. Vector-Borne and Zoonotic Diseases 24: 552-562. https://doi.org/10.1089/vbz.2023.0126

5. Shankar V., Bowen RA, Davis AD, Rupprecht CE, and O’Shea TJ. (2004). Rabies in a captive colony of big brown bats (Eptesicus fuscus). Journal of Wildlife Diseases, 40: 403-413.