JPC SYSTEMIC PATHOLOGY

NERVOUS SYSTEM

April 2026

N-V06D

Slide A: Signalment (JPC #4129627): A 4-year-old sheep

HISTORY: This animal was euthanized for suspected rabies infection based on loud vocalization, staggering, and erratic behavior such as mouthing the pen wires.

HISTOPATHOLOGIC DESCRIPTION: Cerebrum, cortex: Multifocally blood vessels within both the gray and white matter are surrounded by a cuff of lymphocytes and fewer plasma cells and macrophages admixed with edema, fibrin, and scant hemorrhage. Rarely, neurons are swollen with loss or peripheralization of Nissl substance (chromatolysis, degeneration) or neurons have shrunken, angulated, hypereosinophilic cytoplasm with pyknotic nuclei (necrosis), and/or contain one or multiple, 2-4 µm diameter, round to oval, eosinophilic, intracytoplasmic viral inclusions (Negri bodies). There is diffuse, moderate gliosis and multifocal glial nodules.

MORPHOLOGIC DIAGNOSIS: Cerebrum: Meningoencephalitis, lymphoplasmacytic, multifocal, moderate, with neuronal eosinophilic intracytoplasmic viral inclusion bodies (Negri bodies) and gliosis, Mixed breed sheep, ovine.

Slide B: Signalment (JPC #2377756): A 2-year-old female mink

HISTORY: This animal lived outdoors in a wire cage. Raccoons occasionally visited the cage at night. The animal became depressed and unresponsive. It failed to respond to supportive care and was found dead 36 hours later.

HISTOPATHOLOGIC DESCRIPTION: Cerebrum at the level of the hippocampus: Rarely Virchow-Robin space and the leptomeninges are expanded by low numbers of lymphocytes and perivascular hemorrhage. There is multifocal mild gliosis. Hippocampal and cerebrocortical neurons occasionally contain one or multiple, 2-4 µm diameter, round to oval, eosinophilic intracytoplasmic viral inclusions (Negri bodies). Multifocally, the neuropil is either diffusely thinned with overall increased pallor (rarefaction) or is vacuolated and contains variably sized clear spaces (spongiosis).

Cerebellum: Multifocally Purkinje cells are either lost with remnant clear space (empty baskets), contain several small clear cytoplasmic vacuoles (degeneration), or are shrunken with intensely eosinophilic cytoplasm and pyknotic nuclei (necrosis). Purkinje cells also occasionally contain one or multiple, 2-5 µm diameter, round to oval, eosinophilic intracytoplasmic viral inclusions (Negri bodies) or granular brown intracytoplasmic pigment (lipofuscin). There is multifocal spongiosis and gliosis in white matter tracts and the granular cell layer especially in association with affected Purkinje cells. Multifocally, there are low to moderate numbers of lymphocytes and plasma cells within the leptomeninges.

Brainstem: Multifocally, neurons contain one or multiple, 2-5 µm diameter, round to oval, eosinophilic intracytoplasmic viral inclusions (Negri bodies). Multifocally there is rare perivascular lymphocytic cuffing, mild gliosis, and few small glial nodules.

Multifocally in all tissues, there are vessels with large colonies of intraluminal 1-2 µm basophilic cocci (postmortem overgrowth).

MORPHOLOGIC DIAGNOSIS: Cerebrum; cerebellum; brainstem: Meningoencephalitis, perivascular, lymphoplasmacytic, multifocal, mild, with gliosis, spongiosis, and moderate numbers of neuronal eosinophilic intracytoplasmic viral inclusions (Negri bodies), mink, mustelid.

Slide C: Signalment (JPC #2198366): Skunk; age and gender unspecified

HISTORY: This skunk was found dead.

HISTOPATHOLOGIC DESCRIPTION: Cerebrum, level of the hippocampus: The Virchow-Robin space and leptomeninges are diffusely mildly to moderately expanded by a dense cuff of lymphocytes and rare plasma cells. There is diffuse, moderate gliosis with multifocal glial nodules (Babès nodules). Neurons, especially pyramidal and hippocampal neurons, occasionally contain one or multiple, 2-7 µm diameter, round to oval, eosinophilic, intracytoplasmic viral inclusions (Negri bodies). Multifocally neurons are shrunken with angular cell borders, hypereosinophilic cytoplasm, and a pyknotic nucleus (necrosis). There are multifocal to coalescing regions of spongiosis of the cortical gray matter.

MORPHOLOGIC DIAGNOSIS: Cerebrum: Meningoencephalitis, lymphoplasmacytic, multifocal, moderate, with gliosis, neuronal necrosis, spongiosis, and neuronal eosinophilic intracytoplasmic viral inclusions (Negri bodies), skunk, mustelid.

Slide D: Signalment (JPC #4072879): Ox; age, breed and gender unspecified

HISTORY: None

HISTOPATHOLOGIC DESCRIPTION: Ganglion, trigeminal (per contributor): Infiltrating and expanding 90% of the endoneurium and perineurium and extending into the surrounding epineurium are abundant lymphocytes and plasma cells, fewer histiocytes and a marked proliferation of satellite cells. Occasionally, neurons contain one or multiple, 2-6 µm diameter, round to oval, eosinophilic intracytoplasmic viral inclusions (Negri bodies). Occasional neurons are swollen with loss or peripheralization of Nissl substance (chromatolysis, degeneration), are shrunken with angular cell borders, hypereosinophilic cytoplasm, and a pyknotic nucleus (necrosis), or have mildly vacuolated cytoplasm (degeneration). Multifocally, neurons occasionally undergo neuronophagia or are replaced by nodules of microglia.

MORPHOLOGIC DIAGNOSIS: Ganglion, trigeminal (per contributor): Ganglioneuritis, lymphoplasmacytic, moderate, with glial nodules and neuronal degeneration, necrosis, and eosinophilic intracytoplasmic viral inclusions (Negri bodies), ox, bovine.

ETIOLOGIC DIAGNOSIS: Lyssaviral meningoencephalitis

CAUSE: Rabies virus (lyssavirus)

CONDITION: Rabies

GENERAL DISCUSSION:

• Rabies virus (family Rhabdoviridae, genus Lyssavirus) causes a lethal nonsuppurative encephalomyelitis and ganglioneuritis of mammals (zoonotic)
• Viral tropism for the neural and salivary tissues
• Once neurologic disease presents clinically, the disease is almost invariably fatal
• Reservoir hosts may vary temporally and regionally; among the most common are foxes, skunks, raccoons, feral dogs, wolves, jackals, and mongoose
  • Fructivorous, insectivorous, and vampire bats are also capable of transmitting rabies virus
• Herbivores are typically dead-end hosts

PATHOGENESIS:

• Transmitted through infected saliva in a bite wound; occasionally by aerosol
• Inoculation of virus (via saliva) into a bite wound > rabies virus glycoprotein (RVG) binds to the acetylcholine receptor site at the neuromuscular junction > primary viral replication in myocytes > virus enters sensory axon terminal > moves centripetally by retrograde axoplasmic flow in the motor neuron to the spinal cord ventral horn or to brain stem nuclei > virus enters the central nervous system > multiplies in neurons > virus migrates to salivary glands via axonal transport using parasympathetic nerves in the facial (VII) and glossopharyngeal (IX) cranial nerves and sympathetic nerves in thoracic segments of T1-T3 spinal cord segments > virus replicates in salivary acinar epithelial cells and is released directly into ducts
  • Recent data suggests that rabies virus phosphoprotein interacts with a microtubule motor protein used in retrograde axonal transport (dynein LC8)
  • Viral infection of peripheral nerves also results in increased salivary secretions by directly stimulating beta-adrenergic receptors on salivary acinar/ductal cells and indirectly by stimulating the nerves innervating the salivary gland blood vessels
• Virus often concentrates in the limbic system, thereby causing the behavior abnormalities that favor its spread to other hosts
  • The furious form is likely due to infection of hippocampal neurons while the dumb form is probably due to infection of neurons within the cerebellum
• Virus incites minimal immunologic response
• Rabies viral neurotropism is due to a viral coat protein known as rabies virus glycoprotein (RVG), which binds several neural tissue receptors, including neuronal cell adhesion molecule (NCAM) and the p75 neurotrophin receptor (p75NTR)

TYPICAL CLINICAL FINDINGS:

• It is not possible to detect rabies infection during the incubation period, which is generally 1 to 8 weeks but varies with anatomic location of inoculation, virus strain, quantity of virus, host age, and host immune status
• Three clinical manifestations of rabies are described in carnivorous species: Dumb, furious, and paralytic
  • Furious form: Aggressive, destructive, ataxia, hypersalivation
  • Dumb form: Stuporous, lethargic, paralysis, abnormal vocalization

TYPICAL GROSS FINDINGS:

• Often no gross lesions
• Cattle and horses may have hemorrhage in the spinal cord
• Evidence of injury or self-mutilation, or alimentary foreign bodies raises suspicion of rabies

TYPICAL LIGHT MICROSCOPIC FINDINGS:

Nervous system lesions:

• Nonsuppurative encephalomyelitis with ganglionitis; perivascular lymphocytic cuffing and focal gliosis; perivascular hemorrhage; leptomeningitis
  • Ganglioneuritis in the paravertebral ganglia may or may not be due to rabies
  • Glial nodules (“Babès nodules”): composed mostly of microglia and occur in both white and gray matter; especially ruminants
• Lesions most severe from the pons to the hypothalamus, and the cervical spinal cord; medulla generally spared in domestic species
• Neuronal degeneration: most severe in carnivores; mild in pigs and herbivores
• Negri bodies: are round to oval, 2-8 µm, eosinophilic intracytoplasmic viral inclusions
  • Negri bodies are most common in the hippocampal neurons in carnivores and the Purkinje cells in herbivores and are the hallmark of rabies infection; can also be seen in salivary gland and adrenal medulla
  • No Negri bodies in early infections and 15-30% of all rabies infections
• A spongiform lesion with vacuolation of gray matter neuropil, similar to scrapie, was originally described in foxes and skunks with rare reports in the horse, cow, cat and sheep
• Intracytoplasmic membrane-bound vacuoles primarily within dendrites

Non-Nervous System Lesions:

• Parotid adenitis: Epithelial cell degeneration, lymphoplasmacytic inflammation

ULTRASTRUCTURAL FINDINGS:

• Negri body: Aggregations of strands of nucleocapsid that transforms into a central amorphous matrix; 120 nm bullet-shaped particles within the matrix or budding from cell membranes

ADDITIONAL DIAGNOSTIC TESTS:

• Fluorescent antibody test on fresh brain tissue; fixed tissue sometimes used
• Immunohistochemistry, ISH, electron microscopy, mouse inoculation, and PCR-based testing may be useful
• The best sites for rabies virus detection in various species include:
  • Dog/cat: Hippocampus
  • Cattle: Brainstem, cerebellum
  • Horse: Cervical spinal cord, brainstem
    • NOT in hippocampus, cerebellum or cerebrum
  • Swine: Brainstem
  • Raccoon, skunk, llama: Wide dispersal
  • Red/gray fox: Cerebral cortex
• If composite sampling cannot be performed, the thalamus is the area of choice
• Viral PCR on saliva or CSF
• CSF analysis reveals lymphocytic pleocytosis

DIFFERENTIAL DIAGNOSIS:

• Australian bat lyssavirus: closely related to rabies; discovered in Australia in 1996; occurs in various species of flying foxes and occasionally in other bat species but has caused a few human fatalities; lesions are similar to rabies (nonsuppurative meningoencephalomyelitis and ganglioneuritis with Negri bodies)
  • Pseudo-Negri bodies:
  • Cats, skunks, dogs: Nonspecific, 1.5 µm, homogenous inclusions in the pyramidal cells of the hippocampus
  • Cats: Nonspecific inclusions in the lateral geniculate neurons
  • Dogs: Cytoplasmic lamellar bodies in the thalamic neurons and Purkinje cells
  • Old sheep and cattle: Nonspecific, 1 µm, brightly eosinophilic, angulated inclusions in the large neurons of the medulla and spinal cord
  • Japanese brown beef cattle: Eosinophilic bodies
  • Mouse: Hippocampal inclusions
  • Woodchucks: Inclusions in brainstem

COMPARATIVE PATHOLOGY:

• Skunks, wild canids, raccoons, bats, and cattle are most susceptible
  • Rabies accounted for 8.2% of the deaths of 1,277 beef cattle that died between 2008-2018 in southern Brazil
• Dogs, cats, horses, sheep, goats, and nonhuman primates are of intermediate susceptibility
• Horses: Lesions are often limited to the spinal cord; clinical signs are associated with spinal cord injury, such as pelvic limb lameness, proprioceptive defects, ataxia, paralysis, and colic
  • Gross lesions: bladder distension, rectal distension, leptomeningeal hyperemia, gray matter hemorrhage in spinal cord
  • Negri bodies present in 28-64% of confirmed cases (Carvalho, 2025)
• Case report of rabies in a captive lowland tapir (Tapirus terrestris)

References:

1. Cantile C, Miller AD, Youssef S. Nervous System. In: Maxie MG, ed. Jubb, Kennedy & Palmer's Pathology of Domestic Animals. Vol 1. 7th ed. St. Louis, MO: Elsevier; 2026:365-367.
2. Carvalho KS, de Barros CSL, Mendonça FS, Machado M, Riet-Correa F. Diseases of the nervous system of equids in Brazil: a review. J Vet Diagn Invest. 2025;37(5):729-752.Countrymann K, Ruby R, Miller AD. A retrospective study of 171 cases of equine meningoencephalomyelitis in the United States, 1996-2023. J Vet Diagn Invest. 2026;38(1):100-111.
3. De Lorenzi D, Mandara MT. Nervous System. In: Raskin RE, Meyer DJ, eds. Canine and Feline Cytopathology: A Color Atlas and Interpretation Guide. 4th ed. St. Louis, MO: Elsevier; 2023:531-532.
4. Farina LL, Lankton JS. Chiroptera. In: Terio KA, McAloose D, St. Leger J eds. Pathology of Wildlife and Zoo Animals. San Diego, CA: Elsevier. 2018:618-619.
5. Ilha MRS, Dawson KA, Atkinson EL, et al. Retrospective study of laboratory-based surveillance of rabies in wild and domestic animals in the southern United States, 2010-2021. J Vet Diagn Invest. 2024;36(4):554-559.
6. Keel MK, Terio KA, McAloose D. Canidae, Ursidae, and Ailuridae. In: Terio KA, McAloose D, St. Leger J eds. Pathology of Wildlife and Zoo Animals. San Diego, CA: Elsevier. 2018:241-242.
7. Levine GJ, Cook JR. Cerebrospinal Fluid and Central Nervous System Cytology. In: Valenciano AC, Cowell RL, eds. Diagnostic Cytology and Hematology of the Dog and Cat. 5th ed. St. Louis, MO: Elsevier Mosby; 2020:210-228.
8. Miller AD, Zachary JF. Nervous system. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 7th ed. St. Louis, MO: Elsevier; 2022:925-927.
9. Molossi FA, de Cecco BS, Pohl CB, Borges RB, Sonne L, Pavarini SP, Driemeier D. Causes of death in beef cattle in southern Brazil. J Vet Diagn Invest. 2021;33(4):677-683.
10. Pereira FMAM, Oliveira AR, Soares-Neto LL, Munhoz L, Fernandes LS, Santos DOD, Carvalho TP, Langoni H, Santos RL. Rabies in a Captive Lowland Tapir (Tapirus terrestris). J Comp Pathol. 2022;198:29-32.
11. Wachtman L, Mansfield K. Viral diseases of nonhuman primates. In: Abee CR, Mansfield K, Tardif S, Morris T. Nonhuman Primates in Biomedical Research: Volume 2: Diseases. 2^nd ed. San Diego, CA: Elsevier; 2012:36.
12. Williams BH, Burek Huntington KA, Miller M. Mustelids. In: Terio KA, McAloose D, St. Leger J eds. Pathology of Wildlife and Zoo Animals. San Diego, CA: Elsevier. 2018:295-296.
13. Zachary JF. Mechanisms of microbial infections. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 7th ed. St. Louis, MO: Elsevier; 2022:268-269.

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