JPC SYSTEMIC PATHOLOGY

NERVOUS SYSTEM

January 2023

N-M08

 

Slide A: Signalment (JPC #1945671): 4-year-old Suffolk ewe

 

HISTORY: This ewe presented with loss of wool around the tailhead, hyperesthesia, mild hypermetria and some difficulty in prehension and swallowing. The ewe was euthanized.

 

HISTOPATHOLOGIC DESCRIPTION: Brainstem, medulla: Diffusely within the gray matter, there is vacuolation of neuronal perikarya and neural processes with variably sized, intracytoplasmic, clear vacuoles up to 30 µm diameter. The associated neuropil often contains variably sized clear spaces up to 25 µm in diameter (spongiform change).  Other neurons are occasionally shrunken and angular with hypereosinophilic cytoplasm and pyknotic nuclei (necrosis) or have rounded perikarya and peripheralized Nissl substance with eccentrically placed nuclei (central chromatolysis, degeneration). There are increased numbers of astrocytes that are frequently reactive (astrocytosis) as well as increased microglial cells (gliosis) that occasionally surround neuronal perikarya (satellitosis).

 

MORPHOLOGIC DIAGNOSIS: Brainstem, gray matter: Neuronal vacuolation, degeneration, and necrosis, diffuse, moderate, with spongiform change and astrocytosis, Suffolk, ovine.

 

ETIOLOGIC DIAGNOSIS: Prion spongiform encephalopathy

 

CONDITION: Scrapie (Ovine spongiform encephalopathy)

 

Slide B: Signalment (JPC #1232805): Tissue from a mink.

 

HISTORY: This mink was inoculated intracerebrally with an infectious agent. Seven months later, the animal began to display nervous signs, went progressively “downhill”, and was euthanized.

 

HISTOPATHOLOGIC DESCRIPTION: Cerebrum at the level of the hippocampus, midbrain: Diffusely, there is bilaterally symmetric, severe vacuolation of the gray matter neuropil, characterized by variably sized, round, discrete, clear spaces up to 25 µm in diameter (spongiform change).  Multifocally, there is expansion of neuronal perikarya and neuritic processes by variably sized, up to 20µm diameter, discrete, intracytoplasmic, clear vacuoles. There is multifocal neuronal necrosis and astrocytosis as previously described. 

 

MORPHOLOGIC DIAGNOSIS: Cerebrum and midbrain, gray matter: Spongiform change, diffuse, moderate, with multifocal neuronal vacuolation, degeneration, and necrosis, and astrocytosis, breed not specified, mink.

 

ETIOLOGIC DIAGNOSIS: Prion spongiform encephalopathy

 

CONDITION: Transmissible mink encephalopathy (TME)

 

GENERAL DISCUSSION:  

• Scrapie: Natural disease of sheep (and rarely goats); prototype prion-induced transmissible spongiform encephalopathy (TSE) 
• TME:  Affects ranch mink in North America and northern Europe; contamination of feed by the scrapie agent suggested as source of infection
• Other TSEs: Bovine spongiform encephalopathy (BSE), chronic wasting disease (CWD) of cervids, feline spongiform encephalopathy (FSE), and several human spongiform encephalopathies
• Prion diseases are characterized by chronic progressive fatal neurologic manifestations, with both sensory and motor deficits and spongiform encephalopathy, and result from accumulation of an abnormal isoform PrP^sc of the host prion protein PrP^c
• Transmissible prion diseases result from infection with an external PrP^sc and accumulation of host-produced PrP^sc
• Prions are resistant to physical and chemical agents

 

PATHOGENESIS:

• TSEs in general:  

• Most widely-accepted is the protein-only hypothesis:

1. PrP^sc is the etiologic agent of TSEs
2. Transformation from alpha-helical PrP^c to beta-pleated PrP^sc requires either a genetic predisposition or unknown protein cofactors (protein X) that serve as chaperones for this conversion
3. PrP^sc triggers the PrP gene (PRNP) to produce more PrP^sc (self-replication); PrP^sc acts as a template and catalyst for the abnormal folding and polymerization of PrP^c in target cells such as neurons
4. PrP^sc itself can be infectious; there is an incomplete species barrier to transmission of TSE agents between species; some TSEs are attributed to ingestion of feedstuffs contaminated with TSE agents of other species

• Less accepted theories are that prion diseases are immune-mediated and could result from ingestion of feed contaminated with certain bacteria (e.g. Acinetobacter spp. and Spiroplasma mirum) which show molecular mimicry to CNS tissue
• Proposed mechanisms of nerve cell degeneration:

• Mechanical destruction of nerve cell membranes from excessive accumulation
• Lysosomal accumulation of PrP^sc may trigger apoptosis by release of cathepsin D
• PrP^sc may be a direct neurotoxin

• Scrapie:

• Infection probably around the time of birth probably via ingestion
• Goats sharing pasture with affected sheep may be infected; not reported in other species sharing pasture with infected sheep
• Proliferates in lymphoid tissues and lower intestine > long incubation period (up to 2 years) > retrograde axonal transport via splanchnic nerves to thoracic spinal cord to brain > gradual accumulation of PrP^sc in neurons (medulla oblongata and diencephalon) > neurodegeneration and loss (likely via astrocyte and microglial activation and apoptosis) > neurologic signs up to several years after PrP^sc reaches CNS
• The agent may proliferate in the lymphoid tissues up to 2 years before it reaches the CNS; a further 2 years may lapse before clinical signs appear
• In the CNS, PrP^sc first accumulates in astroglial cells, which may be the primary site of replication in the brain
• Genetic variation in the PrP gene affects susceptibility to disease:

• Most susceptible = Homozygous VRQ/VRQ 
• Most resistant = ARR

• TME: Not determined but contamination of feed by scrapie agent is suggested 

 

TYPICAL CLINICAL FINDINGS:

• Pruritus, causing sheep to rub and excoriate fleece (“scrapie”), is thought to be due to cutaneous paresthesia
• Alert but excitable, trembles, progression to ataxia, head tremors, emaciation, seizures, paralysis, and death from 10 days to several months after clinical manifestation

 

TYPICAL GROSS FINDINGS:  

• No gross abnormalities in brain 
• Self-trauma secondary to pruritus; emaciation

 

TYPICAL LIGHT MICROSCOPIC FINDINGS:

• TSEs in general:  

• Distribution, nature, and severity of CNS lesions depend on agent strain and host genotype; medulla oblongata (particularly at obex) is most consistently affected
• Hallmarks of TSEs are 1) neuronal vacuolation, 2) spongiform change, and 3) astrocytic hypertrophy and hyperplasia; PrP^Sc accumulation precedes these findings 

• Neuronal vacuolation:  Single, multiple, or chains of vacuoles in soma, perikaryon, or neurites +/- spongiform change; neuronophagia is absent
• Spongiform change:  Diffuse or clustered to confluent small round vacuoles in the cerebral gray matter neuropil +/- extension into neurites; extracellular space is normal
• Astrocytosis:  Usually involves gray matter affected by spongiform change; may spread to adjacent white matter

• No inflammatory reaction
• Amyloid plaques, thought to contain prion protein, are rare in most animal TSEs but common in human variant Creutzfeldt-Jakob disease (vCJD) 

• Scrapie:   Most characteristic finding is large intraneuronal vacuoles in medullary reticular, medial vestibular, lateral cuneate, and papilliform nuclei (may be present throughout brainstem and spinal cord); vacuoles generally appear empty and displace organelles

• +/- shrunken, degenerate neurons in mesencephalic, medullary, and deep cerebellar nuclei and intermediolateral nucleus of spinal cord
• +/- neuronal loss in late stages
• +/- fibrillary astrogliosis in mesencephalon and molecular layer of cerebellum
• Spongiform vacuolation of neuropil is rarely found in scrapie; generally seen with long incubation periods
• Lesions are bilaterally symmetrical and cortex is rarely involved
• Atrophy of the inner and out nuclear layers with atrophy of the outer plexiform layer of the eye; prions can be detected in the retina of most affected sheep; can also see optic nerve degeneration with vacuolation, Wallerian degeneration, gliosis, and infiltration of Gitter cells

• TME: In contrast to scrapie, neuronal vacuolation is sparse and generally confined to the brainstem; spongiform degeneration and astrocytosis predominate and are most prominent in the neocortex and amygdala, corpus striatum, caudodorsal thalamus, and medial geniculate bodies

 

ULTRASTRUCTURAL FINDINGS:  

• Scrapie-associated fibrils (SAF) consist of two (sometimes four) twisted filaments, each 4-6 nm in diameter (resemble amyloid fibrils)
• Vacuoles bounded by single or double membranes within neuronal perikarya, dendrites, and axons

 

ADDITIONAL DIAGNOSTIC TESTS:

• Immunohistochemical staining of suspect brain and lymphoid tissue (medial retropharyngeal lymph node, tonsil, nictitating membrane)
• Detection of glycoprotein component of SAF in lymphoid tissue allows a preclinical diagnosis of scrapie
• Western blot assays of solubilized brain extracts and cerebrospinal fluid detects prion protein
• ELISA

 

DIFFERENTIAL DIAGNOSIS:

• Atypical scrapie (Nor98):

• First identified in Norway and described in 2003
• Cause is a newly identified TSE agent with a western blot signature distinct from the classical scrapie TSE agent
• Affects older animals (5 to >10 years) than classical scrapie (3-5 years)
• Affects sheep genetically resistant to classical scrapie
• Salient clinical sign is ataxia; pruritus and wool-loss absent
• Neuroanatomical distribution of PrP^sc differs from classical scrapie (cerebral and cerebellar cortices in atypical scrapie vs. medulla oblongata); neuronal vacuolation and spongiform change are largely absent
• PrP^sc has not been detected in lymphoid tissues of sheep with atypical scrapie in contrast to classical scrapie

• BSE - experimentally transmitted in sheep
• Neuronal vacuolar degeneration of Angora goats - scrapie-like neuronopathy in young (3 months-of-age) Angora goats in Australia; vacuolation in brainstem nuclei and spinal motor neurons
• Lysosomal or other storage diseases (N-M11, N-M13, N-M14) - vacuolation of neurons
• Rabies (N-V06) - occasionally spongiform change

 

COMPARATIVE PATHOLOGY:  

• BSE (N-M09):  Mainly in 3- to 6-year-old cattle; SAF demonstrated from BSE brain, most frequently in basal nuclei and brainstem; an atypical form has recently been described as for scrapie

• Differential diagnosis:  Bovine citrullinemia - Newborn Holstein-Friesian calves - autosomal recessive defect; deficiency of arginosuccinate synthetase resulting from a blockade of the urea cycle leading to hyperammonemia and citrullinemia; spongy vacuolation of deep lamina of the cerebral cortex due to astrocytic swelling

• Chronic wasting disease (CWD):  Mule deer, white-tailed and black-tailed deer, Rocky Mountain elk, reindeer, wapiti, and moose; usually 3- to 5-year-old; spongiform change, neuronal vacuolation, gliosis, and amyloid deposition prominent in olfactory tubercle, cortex hypothalamus, and vagal nuclei
• Feline Spongiform Encephalopathy (FSE):  Domestic cats and captive felids; likely transmitted by ingestion of BSE-contaminated food; primarily spongiform change in deep cerebral laminae, corpus striatum, thalamus, medial geniculate body, and cerebellar cortex; neuronal vacuolation in medulla is less prominent
• Exotic ungulate encephalopathy (EUE):  Captive nyala, greater kudu, oryx; associated with ingestion of BSE-contaminated feed
• Spongiform encephalopathy in nonhuman primates:  Associated with ingestion of primate food containing BSE-contaminated beef
• Dogs:   Focal spongiform encephalopathies (not TSEs): 

• Bull mastiffs, salukis:  Familial cerebellar ataxia, hydrocephalus, spongiform change in gray matter (deep cerebellar nuclei, posterior colliculi, lateral ventricular nuclei) with axonal spheroids; nerve cell bodies appear normal
• Rottweilers:  Unknown etiology; progressive laryngeal paralysis, tetraparesis, cerebellar ataxia beginning at 8 weeks of age; neuronal vacuolation in cerebellar roof nuclei, extrapyramidal nuclei, dorsal nerve root ganglia, myenteric plexus, other ganglia of autonomic nervous system
• Australian cattle dog:  Autosomal recessive defect in astrocytic mitochondria; neuronal, neuropilar, and astrocytic vacuolation especially in ventral horn of spinal cord, cerebellar, and brainstem nuclei
• Alaskan husky:  Hereditary familial encephalopathy; clinical signs begin before 1 year of age; bilateral symmetrical malacia, spongiosis, gemistocytic astrocytosis, marked intracytoplasmic vacuolation, gliosis, vascular hyperplasia; relative neuronal sparing

 

REFERENCES:  

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