April 2017



Signalment (JPC #2328791) (Case A): 4-year-old Santa Gertrudis bull


HISTORY: This animal had lost weight over the last 2 weeks and recently had become depressed and dehydrated. Two days prior to presentation, the bull became ataxic and disoriented, developed aggressive behavior and abnormal vocalization. Euthanasia was elected.


HISTOPATHOLOGIC DESCRIPTION: Brainstem: Multifocally blood vessels are surrounded by a dense cuff of lymphocytes, with rare plasma cells and macrophages. Occasionally neurons contain one or multiple, 2-4 um diameter, round to oval, eosinophilic cytoplasmic viral inclusions (Negri bodies) or are swollen with loss or peripheralization of Nissl substance (chromatolysis). There are rare glial nodules and multifocal perivascular hemorrhage.


MORPHOLOGIC DIAGNOSIS: Brainstem: Encephalitis, perivascular, lymphohistiocytic, multifocal, mild, with few neuronal eosinophilic intracytoplasmic viral inclusions (Negri bodies), Santa Gertrudis, bovine.


Signalment (JPC #2377756) (Case B): 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 hrs later.


HISTOPATHOLOGIC DESCRIPTION: Cerebrum at the level of the hippocampus: Rarely Virchow-Robin space and the meninges are expanded by low numbers of lymphocytes. There is multifocal, mild gliosis, and mild perivascular hemorrhage. Often hippocampal and cerebrocortical neurons contain one or multiple, 2-4 um diameter, round to oval, eosinophilic intracytoplasmic viral inclusions (Negri bodies).


Cerebellum: Multifocally Purkinje cells are either lost, contain several small clear vacuoles (degeneration), or are shrunken, intensely eosinophilic with pyknotic nuclei (necrosis), and/or contain one or multiple, 2-5 um diameter, round to oval, eosinophilic intracytoplasmic viral inclusions (Negri bodies). Multifocally, disrupting the white matter tracts and granular cell layers are nodular areas of spongiosis and gliosis. Multifocally, there are low to moderate numbers of lymphocytes and plasma cells within the leptomeninges. Focally, there is an artery with a large colony of intraluminal 1-2 micron basophilic cocci (postmortem overgrowth).


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


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


Signalment (JPC #2198366) (Case C): A skunk


HISTORY: This skunk was found dead.


HISTOPATHOLOGIC DESCRIPTION: Cerebrum: The Virchow-Robin space and meninges are expanded by a dense cuff of lymphocytes and rare plasma cells. There is diffuse, moderate gliosis, with multifocal glial nodules. Pyramidal and hippocampal neurons often contain one or multiple, 2-7 um diameter, round to oval, eosinophilic, intracytoplasmic viral inclusions (Negri bodies). Multifocally neurons have shrunken, angulated, hypereosinophilic cytoplasm with pyknotic nuclei (necrosis). There is a focally extensive area of spongiosis of the gray matter.


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


ETIOLOGIC DIAGNOSIS: Lyssaviral meningoencephalitis


CAUSE: Rabies virus (lyssavirus)





·      Rabies virus belongs to the family Rhabdoviridae, genus Lyssavirus that causes a lethal nonsuppurative encephalomyelitis and ganglionitis of mammals

·      Viral tropism for the neural and salivary tissues

·      Zoonotic disease

·      Reservoir hosts may vary temporally and regionally; among the most common are foxes, skunks, raccoons, feral dogs, wolves, jackals and mongoose

o   Fructivorous, insectivorous and vampire bats are also capable of transmitting rabies virus

·      Herbivores are typically dead end hosts



·      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

o   Recent data suggests that rabies virus phosphoprotein interacts with a microtubule motor protein used in retrograde axonal transport (dynein LC8)

o   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

o   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)



·      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: Dumb, furious, and paralytic

o   Furious form: Aggressive, destructive, ataxia, hypersalivation

o   Dumb form: Stuporous, lethargic, paralysis, abnormal vocalization


·      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



·      Nonsuppurative polioencephalomyelitis with ganglionitis

o   Lesions most severe from the pons to the hypothalamus, and the cervical spinal cord; medulla generally spared

o   Perivascular lymphocytic cuffing; perivascular hemorrhage; leptomeningitis

o   Glial nodules (“Babe’s nodules”): composed mostly of microglia and occur in both white and gray matter

o   Neuronal degeneration is most severe in carnivores and mild in pigs and herbivores

o   Negri bodies are round to oval, 2-8 um, eosinophilic cytoplasmic viral inclusions

·      Aggregations of strands of viral nucleocapsid; transforms into a granular


·      Negri bodies are most common in the hippocampal neurons in carnivores and the Purkinje cells in herbivores and are the hallmark of rabies infection

·      Negri bodies are rarely found in the ganglion cells of the adrenal medulla,

salivary gland and retina

·      Early infections and 15-30% of rabies infections fail to incite Negri bodies

·      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

o   Intracytoplasmic membrane-bound vacuoles primarily within dendrites

·      Parotid adenitis

o   Epithelial cell degeneration

o   Lymphoplasmacytic inflammation



·      Negri body: Central amorphous matrix with 120 nm bullet-shaped particles within the matrix or budding from cell membranes



·      Fluorescent antibody test on fresh brain tissue; Immunohistochemistry; RT-PCR

·      Mouse inoculation

·      The best sites for rabies virus detection in various species include:

o   Dog/cat: Hippocampus

o   Cattle: Brainstem, cerebellum

o   Horse: Cervical spinal cord, brainstem

§  NOT in hippocampus, cerebellum or cerebrum

o   Swine: Brainstem

o   Raccoon, skunk, llama: Wide dispersal

o   Red/grey fox: Cerebral cortex

·      If composite sampling cannot be performed, the thalamus is the area of choice



·      Australian bat lyssavirus is a virus closely related to rabies that was discovered in Australia in 1996; it occurs in various species of flying foxes and occasionally in other species of bats and is responsible for a few human fatalities; lesions are similar to rabies (nonsuppurative meningoencephalomyelitis and ganglioneuritis with Negri bodies)

·      Pseudo-Negri bodies:

o   Cats, skunks, dogs: Nonspecific, 1.5 um, homogenous inclusions in the pyramidal cells of the hippocampus

o   Cats: Nonspecific inclusions in the lateral geniculate neurons

o   Dogs: Cytoplasmic lamellar bodies in the thalamic neurons and Purkinje cells

o   Old sheep and cattle: Nonspecific, 1 um, brightly eosinophilic, angulated inclusions in the large neurons of the medulla and spinal cord

o   Japanese brown beef cattle: Eosinophilic bodies

o   Mouse: Hippocampal inclusions

o   Woodchucks: Inclusions in brainstem



·      Skunks, wild canids, raccoons, bats and cattle are most susceptible

·      Dogs, cats, horses, sheep, goats and nonhuman primates are of intermediate susceptibility

·      Horses: Lesions are often limited to the spinal cord with clinical signs associated with spinal cord injury, such as pelvic limb lameness, proprioceptive defects, ataxia, paralysis, and colic

·      Negri bodies are only identified in 30% to 50% of affected animals

·      Rabbits: Myocardial necrosis of the ventricular wall in rabbits with experimentally induced rabies




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