JPC SYSTEMIC PATHOLOGY

NERVOUS SYSTEM

January 2023

N-M09

 

Signalment JPC #2292125: 5 1/2-year-old Simmental cow

 

HISTORY: The first signs of a neurological disorder were observed about one month before euthanasia, including anxiety, falling over, and difficulty rising from a recumbent position. Clinical examination revealed tremor of the head, tooth grinding, hyperesthesia to sound and touch, and hypermetric movements of the forelimbs.

 

HISTOPATHOLOGIC DESCRIPTION: Brainstem: Within the solitary tract nucleus, and to a lesser extent the dorsal vagal motor nucleus, there are few scattered neurons in which the perikaryon is expanded with one or more clear, round, distinct vacuoles up to 50 µm in diameter. Multifocally, adjacent to the solitary tract nucleus, there is mild spongiosis characterized by clear vacuoles up to 60 µm in diameter within the neuropil (spongiform change). Multifocally, astrocytes are increased in number (astrocytosis) and size with enlarged, pale nuclei containing peripheralized chromatin and there is mild gliosis in the surrounding neuroparenchyma.

 

MORPHOLOGIC DIAGNOSIS: Brain stem, solitary tract nucleus: Neuronal degeneration and vacuolation, multifocal, mild, with spongiosis and gliosis, Simmental, bovine.

 

ETIOLOGIC DIAGNOSIS: Bovine prion spongiform encephalopathy

 

CAUSE: Bovine prion, PrP^Sc

 

CONDITION: Bovine spongiform encephalopathy (BSE)

 

SYNONYMS: Mad cow disease

 

GENERAL DISCUSSION:

• Fatal, symmetrical, chronic, progressive, non-inflammatory, neurodegenerative disease caused by a prion; BSE affects the central nervous system (CNS) of adult cattle, usually 3 to 6 years of age
• Caused by a conformational change of a normal prion α-helix-containing protein isoform, PrP^c ("c" for cell) into an abnormal β-pleated sheet isoform, which results in the accumulation of a protease-resistant prion isoform (PrP^Sc)

• PrP^c   is expressed in most cells of the body, with the highest concentration in the nervous system as a neuronal membrane glycoprotein; PrP^c function is unknown

• Spongiform encephalopathies of domestic animals and humans can be transmitted to other animals under experimental conditions, called transmissible spongiform encephalopathies (TSEs)
• For most transmissible spongiform encephalopathies (TSEs), PrP^Sc normally accumulates in particularly high concentrations within lymphoreticular and nervous tissues; however, in cattle with BSE, over 95% of PrP^Sc is in the nervous system with minimal accumulation in lymphoreticular tissues
• BSE is the only TSE transmissible to humans, associated with variant Creutzfeldt-Jakob disease (vCJD) in humans
• BSE:

• First reported in the UK in the 1980s; the origin of the first outbreak is unknown; however, the recycling of ruminant tissues into ruminant feed amplified the BSE prions to cause an epidemic

• In addition to classic BSE, there are two atypical strains of BSE: atypical “H-type” and atypical “L-type”; since 2005, all BSE-positive animals in the US have been of the atypical variant 

• “L” and “H” denotes characteristic lighter and heavier molecular masses compared to classic BSE

 

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

• BSE:

• Ingestion of PrP^Sc > colonization of palatine tonsillar mucosal associated lymphoid tissue (MALT) > replication in MALT and the Peyer’s patches of the ileum (in BSE, amplification mainly occurs in monocyte-macrophage systems before entering nerve endings) > infects vagus, sympathetic, and/or sensory nerves that innervate lymphoid tissues > retrograde axonal transport to CNS > PrP^Sc serves as a translation template for the conformational change of PrP^c to PrP^Sc > PRP^Sc is resistant to proteases > accumulates as amyloid in neurons and neuropil > degeneration of neurons and the neuropil; thus, the infectious prion isoform replicates and propagates the infection
• Prion protein translation occurs in a stepwise fashion: (1) initiation, (2) propagation and conversion, (3) aggregation to form insoluble amyloid and fibrous plaques
• PrP^Sc that is not aggregated into the misfolded isoform following propagation and conversion is “recycled” and interacts with PrP^c (a self-amplifying process) 

 

TYPICAL CLINICAL FINDINGS:

• Apprehension, nervousness, truncal ataxia, aggression, hypermetria, progressive weakness, emaciation, and ultimately death
• Clinical signs are apparent when infection of target nuclei in the CNS causes progressive neuronal dysfunction

• Long incubation period, usually 2 to 8 years 
• Long clinical course, usually 2 weeks to 6 months

 

TYPICAL GROSS FINDINGS:

• There are typically no gross lesions associated with prionosis; gross lesions are often associated with older age in affected animals
• Brain atrophy may occur in chronic cases

 

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

• 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) 

 

BSE: (pathologic features are similar to scrapie)

• Scrapie-associated fibrils and immunoreactive amyloid
• Vacuolation of neuronal cell bodies and processes prominent in dorsal vagal, medullary reticular, vestibular, solitary, spinal trigeminal, red nuclei; accompanied by moderate neuropil spongiform change

• Neuronal and neuropil vacuolation varies between affected regions; affected neurons may exhibit ceroid-lipofuscin granules which may be single to multiple, and are not often associated with neuronal degeneration; they may be clear or contain amorphous inclusions

• +/- necrotic neurons, axonal spheroids, mild astrogliosis
• Retina: Nuclear displacement of inner and outer nuclear layers into photoreceptor and inner plexiform layers

Atypical BSE:  

• Relatively few atypical BSE prions are found in the obex; more commonly located in the thalamus and olfactory bulb
• L-type atypical BSE is characterized by the presence of PrP^Sc – positive amyloid plaques = Bovine amyloidotic spongiform encephalopathy (BASE)

 

 

ULTRASTRUCTURAL FINDINGS:

• Scrapie-associated fibrils (SAF):  2 to 4 twisted filaments that are 4-6 nm in diameter; accumulate in astrocytes
• Neurons are enlarged, usually with a single, membrane-bound cytoplasmic vacuole that may contain granular material

 

ADDITIONAL DIAGNOSTIC TESTS:

• Rapid screening testing of fresh CNS tissue

• ELISA
• Lateral flow immunoassay (dipstick)

• If test results are positive using a rapid assay, then BSE is confirmed with additional testing, including immunohistochemical (IHC) stains or Western blot
• Antemortem testing via IHC may be performed on tonsil, rectal lymphoid tissue, or lymphoid tissue of the third eyelid

 

DIFFERENTIAL DIAGNOSIS:

• Spongy degeneration, brain edema, and postmortem degeneration exhibit similar histologic features 
• Vacuolation within the white matter, primarily in the substantia nigra, and the red and vestibular nuclei in cattle may be an incidental finding
• Microscopic lesions

• Bovine citrullinemia

• Autosomal recessive defect in Holstein-Friesian calves
• Deficiency of arginosuccinate synthetase interferes with urea cycle metabolism leading to hyperammonemia and citrullinemia
• Astrocyte swelling leads to spongy vacuolation of the deep laminae of the cerebral cortex

• Congenital brain edema 

• Horned Hereford calves in New Zealand; diffuse vacuolation of myelin extending into the gray matter, and hydropic degeneration of astrocytes (apparent on EM)
• A similar condition in polled Hereford calves in Britain, though diffuse myelination confined to white matter; cases are considered a hereditary neuraxial edema (in the absence of findings consistent with BKAD, see below)

• Hereditary neuraxial edema (maple syrup urine disease)(see N-M05)

• Two neurologic syndromes:

• Branched-chain ketoacid dehydrogenase deficiency (BKAD):  Status spongiosis (severe vacuolar change of white matter myelin), most pronounced in large myelinated tracts, cerebellar folia and nuclei, reticular formation, pons, medulla, and basal nuclei, corona radiata, deep cerebral cortex; some extension into gray matter; spinal white matter usually not affected 
• Inherited congenital myoclonus:  No CNS lesions

 

COMPARATIVE PATHOLOGY:

BSE in other species:

Other TSEs:

• Sheep and goats:  Scrapie (see N-M08A); 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; spongiform vacuolation of neuropil is rarely found in scrapie; generally seen with long incubation periods; EU rapid tests developed for ovine use exhibited significantly less sensitivity in goats, particularly in goats with polymorphic PRNP gene with serine or aspartic acid at codon 146 (Simmons, Jour Vet Diagn Invest 2020)
• Felids (domestic and exotic):  Feline spongiform encephalopathy (FSE); neuropil vacuolation most pronounced in deep cerebral laminae, corpus striatum, thalamus, medial geniculate body, and cerebellar cortex; neuronal vacuoles are not a prominent feature but are mostly found in the nuclei of caudal brainstem, red and vestibular nuclei
• Mink:  Transmissible mink encephalopathy (TME) (N-M08B); North American and Northern European mink; high morbidity and mortality; source unknown (scrapie agent contamination of feed suggested); spongy degeneration and astrocytosis of parallel intensity; neuronal vacuolation predominantly in brainstem; experimentally pathogenic to numerous wild carnivores, sheep, cattle, and primates
• Cervids: Chronic wasting disease (CWD) of cervids; White-tailed deer, mule deer, moose, and Rocky Mountain elk; United States and western Canada; only TSE that occurs in free-ranging animals; CWD PrP^Sc widespread in CNS and lymphoid tissues, including gut-associated lymphoid tissue (GALT); horizontal spread via fecal-oral transmission suspected; histologic lesions typically found in olfactory tubercule, cortex hypothalamus, and vagal nuclei
• Exotic ruminants (Greater kudu, Arabian oryx, nyala, gemsbok, eland, and a bison):  Exotic ungulate encephalopathy of captive wild ruminants; associated with ingestion of BSE-contaminated feed products during BSE epidemic in the UK
• Non-human primates: Lemurs and one rhesus macaque in France; consumption of BSE-contaminated primate food suspected 
• Humans:  Kuru (ritualistic cannibalism); four types of Creutzfeldt-Jakob disease (sporadic, familial, iatrogenic, vCJD); Gerstmann-Straussler-Scheinker disease; fatal familial insomnia; all heritable human prion diseases are associated with PRNP gene mutation and exhibit a dominant inheritance pattern
• Raccoons:   Experimentally (intracranial inoculation), raccoons are highly susceptible to raccoon- and bovine-passaged TME agents, while CWD isolates from white-tailed deer or elk and hamster-adapted TME transmit poorly (Moore, Jour Vet Diagn Invest 2019)
• Mice experimentally infected with a TSE (mouse-adapted Rocky Mountain Lab Scrapie) developed progressive upregulation of iNOS in microglia and a mixed astrocytic profile featuring iNOS expression in white matter tracts, and detection of Arg1-positive populations throughout the brain; in mice with clinical disease, iNOS was expressed in high levels within the microenvironment, leading to a M1/classical microglial phenotype (Michael, Vet Pathol 2020)

 

REFERENCES:

1. Cantile C, Youssef S. Nervous system. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 1. 6th ed. Philadelphia, PA: Elsevier Ltd; 2016: 343, 347, 349.
2. Labelle P. The Eye. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 7th ed. St. Louis, MO: Elsevier; 2022:1379-1433.
3. Michael AV, et al. In situ temporospatial characterization of the glial response to prion infection. Vet Pathol. 2020;57(1): 90-107.
4. Miller AD, Porter, BF. Nervous System. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 7th ed. St. Louis, MO: Elsevier; 2022:932, 970-971.
5. Moore SJ, Smith JD, Richt JA, Greenlee JJ. Raccoons accumulate PrPSc after intracranial inoculation of the agents of chronic wasting disease or transmissible mink encephalopathy but not typical scrapie. J Vet Diagn Invest. 2019;31(2): 200-209.
6. Simmons MM, Thorne L, Ortiz-Pelaez A, et al. Transmissible spongiform encephalopathy in goats: is PrP rapid test sensitivity affected by genotype?. J Vet Diagn Invest. 2020;32(1):87-93.
7. Stanton JB, Zachary JF. Mechanisms of Microbial Infections. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 7th ed. St. Louis, MO: Elsevier; 2022:293. 
8. Tewari D, Steward D, Fasnacht M, Livengood J. Detection by real-time quaking-induced conversion (RT-QuIC), ELISA, and IHC of chronic wasting disease prion in lymph nodes from Pennsylvania white-tailed deer with specific PRNP genotypes. J Vet Diagn Invest. 2021;33(5):943-948.

 

 

 

 

 

 

 

 

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