JPC SYSTEMIC PATHOLOGY
NERVOUS SYSTEM
January 2026
N-M07
Signalment (JPC #2287965): Three-year-old Welsh cob mare
HISTORY: This mare had a one-year history of ataxia. No abnormalities were seen on cervical radiographs taken 3-months before euthanasia.
HISTOPATHOLOGIC DESCRIPTION: Brainstem: Bilaterally and symmetrically within the dorsal brainstem, primarily within the lateral and medial (accessory) cuneate nuclei, there are moderate numbers of swollen eosinophilic axons, up to 60 µm in diameter, with homogenous granular axoplasm (spheroids) and occasional dilated myelin sheaths. Multifocally, neurons within affected areas are shrunken and hypereosinophilic with loss of Nissl substance and nuclear pyknosis or karyolysis (neuronal necrosis) or are swollen with central dispersion of Nissl substance and peripheralized nuclei (central chromatolysis). Within the affected areas, there are mildly increased numbers of astrocytes (astrocytosis) that are often hypertrophied (reactive; astrogliosis), and there are occasional scattered macrophages that contain a yellow-brown, lipofuscin-like, granular intracytoplasmic pigment.
MORPHOLOGIC DIAGNOSIS: Brainstem, lateral and medial cuneate nuclei: Axonal degeneration (spheroids), bilaterally symmetrical, moderate, with astrogliosis, neuronal necrosis and myelin sheath dilation, Welsh cob, equine.
CONDITION: Equine degenerative myeloencephalopathy (EDM)
SYNONYMS: Neuroaxonal dystrophy
GENERAL DISCUSSION:
- Chronically progressive, irreversible syndrome of symmetrical ataxia in young horses (6-12 months of age)
- Wallerian degeneration in white matter of spinal cord funiculi and brainstem
- Simultaneous involvement in multiple cord segments of ascending and descending tracts
- Reported in a variety of purebred and mixed breed horses, and a similar disease exists in zebras.
- May be a familial predisposition in Appaloosa, Standardbred, Paso Fino, and Lusitano horses. Also seen in Morgan and Quarter horse breeds
- Considered an advanced form of neuroaxonal dystrophy
PATHOGENESIS:
- Unknown; appears to be a combination of genetic predisposition and environmental risk factors (specifically diets low in α-tocopherol (vitamin E)
- Vitamin E deficiency may play a role in the disease process since supplementation reduces the severity of clinical deficits as well as decreased incidence at farms with high prevalence of EDM
- Oxidative damage and lipid peroxidation of cell membranes > accumulation of lipopigment
- Other risk factors that may play a role: Copper deficiency, toxins (insecticides or wood preservatives), inadequate access to green pasture
- Disruption of axonal transport likely plays a crucial role in the pathogenesis of dystrophic axons in EDM.
TYPICAL CLINICAL FINDINGS:
- Slowly progressive course with proprioceptive and upper motor neuron deficits in a young horse
- Spastic gait, knuckling, stumbling, circumduction, abduction, hypermetria
- All limbs may be equally affected, or may be more pronounced in the rear limbs
TYPICAL GROSS FINDINGS:
- No gross lesions
TYPICAL LIGHT MICROSCOPIC FINDINGS:
- Bilateral axonal degeneration affecting all funiculi throughout the spinal cord and brain stem (ongoing Wallerian degeneration and post-Wallerian astrogliosis); degenerate and necrotic neurons
- Loss of myelinated axons and dense reactive gliosis
- Spheroids (swellings filled with amorphous debris, membranous profiles, and effete organelles)
- Astrocytosis
- Accumulation of lipofuscin-like pigment in macrophages, endothelial cells, and neurons; degenerate and necrotic neurons
- Lesions predominate in the sensory axons and are most dramatic in the mid-thoracic and anterior cervical segments in the ascending dorsolateral (spinocerebellar) and descending ventromedial (motor) tracts
- Chromatolytic and necrotic neurons in Clarke’s column in Clarke’s column (the nucleus of the dorsal spinocerebellar tract)
- Spheroids have been described in the nucleus gracilis, medial and lateral cuneate nuclei of the terminal brainstem, and thoracic nucleus of the spinal cord
- Similar but mild changes are noted in myelinated tracts of the caudal medulla and caudal cerebellar peduncle
ADDITIONAL DIAGNOSTIC TESTS:
- Bloodwork: Plasma alpha-tocopherol levels
- Calretinin IHC
DIFFERENTIAL DIAGNOSIS:
- Equine motor neuron disease- clinically similar syndrome to EDM with degenerative changes in the brainstem; older horses; slightly different anatomic locations affected; vitamin E deficiency may play a role in the disease
- Cervical stenotic myelopathy (N-M02, Wobbler syndrome): Axonal degeneration and demyelination (Wallerian degeneration); more common in older horses (1-4 years); histologic findings vary based on location in relation to site of compression
- Equine protozoal myeloencephalitis (N-P01, Sarcocystis neurona): Necrotizing, nonsuppurative myeloencephalitis, axonal degeneration, schizonts and merozoites
- Cauda equina syndrome (N-M18, idiopathic polyradiculoneuritis): Granulomatous inflammation around cauda equine
- Equine herpesvirus-1 (EHV-1, alphaherpesvirus) myeloencephalopathy: Vasculitis and necrosis in spinal cord and brain; often accompanies or follows respiratory disease and abortions
- Perinneal rye grass staggers (N-T12, Lolium perenne): No specific pathologic changes; can have axonal swelling, especially in Purkinje cells (torpedoes)
- Parasite migration: Strongylus spp., Hypoderma spp.
COMPARATIVE PATHOLOGY:
- Neuroaxonal dystrophy (including EDM) are disorders associated with axonal swellings (axonal spheroids) and have a species/breed association and usually early onset (<1 year of age (4 weeks-3 years))
- Described in many species, most common in dogs and horses
- Characterized by prominent axonal swellings in various nuclei (often sensory) in the brainstem, cerebellum, and spinal cord
- Loss of cerebellar Purkinje and granule cells reported in Rottweilers and cats, loss of brainstem neurons in cats
- Other neuropathies in which Vitamin E plays a role:
- Equine motor neuron disease (EMND): Clinically similar syndrome to EDM with degenerative changes in the brainstem; older horses; associated with a vitamin E deficiency but lacks genetic component; affects ventral and intermediate gray columns of the spinal cord; a lower motor neuron disease
- Zebras and Mongolian wild horses (Equus przewalskii): Disease similar to EDM, associated with hypovitaminosis E
- Spontaneous hemorrhagic necrosis (SHN) in fetal hamsters, with symmetrical, subependymal vascular degeneration and intraventricular hemorrhage
- Encephalomalacia in Guinea Pigs from sows deficient in vit E
- Encephalomalacia (Crazy Chick Disease) in poultry: Ataxia, loss of balance, prostration; cerebellum often contains yellow or congested, hemorrhagic, or necrotic areas visible on the surface, usually within the first month of life
- Avians with hypovitaminosis E may have malacia, edema, white matter astrogliosis,capillary thrombi, hemorrhage, and neuronal necrosis
- Young turkeys lesions are seen in the spinal cord
- Prominent myocardial and skeletal muscle lesions are also present in these birds
- Hypovitaminosis E/ Vitamin E deficiency leads to:
- Yellow fat disease/nutritional panniculitis, in multiple mammalian and reptilian species
- Nutritional myopathy (M-M11) or cardiomyopathy (C-M06) in a number of mammals, avian species, and fish
- Intestinal ceroidosis (AKA leiomyometaplasia, brown dog gut) in dogs
- Neuropathies
References:
- Barthold SW, Imai DM. Pathology of Laboratory Rodents and Rabbits. 5th ed. Hoboken, NJ: Wiley Blackwell; 2026:252, 253, 315.
- 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; 2026:325-326. Duncan M. Perissodactyls. In: Terio KA, McAloose D, St. Leger J eds. Pathology of Wildlife and Zoo Animals. San Diego, CA: Elsevier; 2018:437-438.
- Mauldin EA, Welle MM. Integumentary System. In: Maxie MG, ed. Jubb, Kennedy & Palmer's Pathology of Domestic Animals. Vol 1. 7th ed. St. Louis, MO: Elsevier; 2026:583.
- Miller AD, Porter BF. Nervous System. In: Zachary JF, McGavin MD, eds. Pathologic Basis of Veterinary Disease. 7th ed. St. Louis, MO: Elsevier Mosby; 2022:963-964.
- Struthers JD, Schmidt RE. Nervous system. In: Schmidt R, Struthers JD, Phalen DN, eds. Pathology of Pet and Aviary Birds. 3rd ed. Hoboken, NJ: John Wiley & Sons, Inc.; 2024:394.
- Shivaprasad HL. Nutritional Diseases. In: Boulianne M, ed. Avian Disease Manual. 8th ed. Jacksonville, FL: American Association of Avian Pathologists, Inc; 2019:151-152.
- Swayne DE, Barnes HJ, Abdul-Aziz T, Fletcher OJ. Chapter 10: Nervous System. In: Abdul-Aziz T, Fletcher OJ, Barns HJ, eds. Avian Histopathology. 4th ed. Madison, WI: Omnipress; 2016:470.
- Terio K, McAloose, St. Leger J. Pathology of Wildlife and Zoo Animals. 1st ed. London, United Kingdom. Elsevier. 2018:287, 612, 655, 748, 855.
- Uchida K, Kobayashi Y. Muscle and Tendon. In: Maxie MG, ed. Jubb, Kennedy & Palmer's Pathology of Domestic Animals. Vol 1. 7th ed. St. Louis, MO: Elsevier; 2026:215.
Signalment (JPC #2287965): Three-year-old Welsh cob mare
HISTORY: This mare had a one-year history of ataxia. No abnormalities were seen on cervical radiographs taken 3-months before euthanasia.
HISTOPATHOLOGIC DESCRIPTION: Brainstem: Bilaterally and symmetrically within the dorsal brainstem, primarily within the lateral and medial (accessory) cuneate nuclei, there are moderate numbers of swollen eosinophilic axons, up to 60 µm in diameter, with homogenous granular axoplasm (spheroids) and occasional dilated myelin sheaths. Multifocally, neurons within affected areas are shrunken and hypereosinophilic with loss of Nissl substance and nuclear pyknosis or karyolysis (neuronal necrosis) or are swollen with central dispersion of Nissl substance and peripheralized nuclei (central chromatolysis). Within the affected areas, there are mildly increased numbers of astrocytes (astrocytosis) that are often hypertrophied (reactive; astrogliosis), and there are occasional scattered macrophages that contain a yellow-brown, lipofuscin-like, granular intracytoplasmic pigment.
MORPHOLOGIC DIAGNOSIS: Brainstem, lateral and medial cuneate nuclei: Axonal degeneration (spheroids), bilaterally symmetrical, moderate, with astrogliosis, neuronal necrosis and myelin sheath dilation, Welsh cob, equine.
CONDITION: Equine degenerative myeloencephalopathy (EDM)
SYNONYMS: Neuroaxonal dystrophy
GENERAL DISCUSSION:
- Chronically progressive, irreversible syndrome of symmetrical ataxia in young horses (6-12 months of age)
- Wallerian degeneration in white matter of spinal cord funiculi and brainstem
- Simultaneous involvement in multiple cord segments of ascending and descending tracts
- Reported in a variety of purebred and mixed breed horses, and a similar disease exists in zebras.
- May be a familial predisposition in Appaloosa, Standardbred, Paso Fino, and Lusitano horses. Also seen in Morgan and Quarter horse breeds
- Considered an advanced form of neuroaxonal dystrophy
PATHOGENESIS:
- Unknown; appears to be a combination of genetic predisposition and environmental risk factors (specifically diets low in α-tocopherol (vitamin E)
- Vitamin E deficiency may play a role in the disease process since supplementation reduces the severity of clinical deficits as well as decreased incidence at farms with high prevalence of EDM
- Oxidative damage and lipid peroxidation of cell membranes > accumulation of lipopigment
- Other risk factors that may play a role: Copper deficiency, toxins (insecticides or wood preservatives), inadequate access to green pasture
- Disruption of axonal transport likely plays a crucial role in the pathogenesis of dystrophic axons in EDM.
TYPICAL CLINICAL FINDINGS:
- Slowly progressive course with proprioceptive and upper motor neuron deficits in a young horse
- Spastic gait, knuckling, stumbling, circumduction, abduction, hypermetria
- All limbs may be equally affected, or may be more pronounced in the rear limbs
TYPICAL GROSS FINDINGS:
- No gross lesions
TYPICAL LIGHT MICROSCOPIC FINDINGS:
- Bilateral axonal degeneration affecting all funiculi throughout the spinal cord and brain stem (ongoing Wallerian degeneration and post-Wallerian astrogliosis); degenerate and necrotic neurons
- Loss of myelinated axons and dense reactive gliosis
- Spheroids (swellings filled with amorphous debris, membranous profiles, and effete organelles)
- Astrocytosis
- Accumulation of lipofuscin-like pigment in macrophages, endothelial cells, and neurons; degenerate and necrotic neurons
- Lesions predominate in the sensory axons and are most dramatic in the mid-thoracic and anterior cervical segments in the ascending dorsolateral (spinocerebellar) and descending ventromedial (motor) tracts
- Chromatolytic and necrotic neurons in Clarke’s column in Clarke’s column (the nucleus of the dorsal spinocerebellar tract)
- Spheroids have been described in the nucleus gracilis, medial and lateral cuneate nuclei of the terminal brainstem, and thoracic nucleus of the spinal cord
- Similar but mild changes are noted in myelinated tracts of the caudal medulla and caudal cerebellar peduncle
ADDITIONAL DIAGNOSTIC TESTS:
- Bloodwork: Plasma alpha-tocopherol levels
- Calretinin IHC
DIFFERENTIAL DIAGNOSIS:
- Equine motor neuron disease- clinically similar syndrome to EDM with degenerative changes in the brainstem; older horses; slightly different anatomic locations affected; vitamin E deficiency may play a role in the disease
- Cervical stenotic myelopathy (N-M02, Wobbler syndrome): Axonal degeneration and demyelination (Wallerian degeneration); more common in older horses (1-4 years); histologic findings vary based on location in relation to site of compression
- Equine protozoal myeloencephalitis (N-P01, Sarcocystis neurona): Necrotizing, nonsuppurative myeloencephalitis, axonal degeneration, schizonts and merozoites
- Cauda equina syndrome (N-M18, idiopathic polyradiculoneuritis): Granulomatous inflammation around cauda equine
- Equine herpesvirus-1 (EHV-1, alphaherpesvirus) myeloencephalopathy: Vasculitis and necrosis in spinal cord and brain; often accompanies or follows respiratory disease and abortions
- Perinneal rye grass staggers (N-T12, Lolium perenne): No specific pathologic changes; can have axonal swelling, especially in Purkinje cells (torpedoes)
- Parasite migration: Strongylus spp., Hypoderma spp.
COMPARATIVE PATHOLOGY:
- Neuroaxonal dystrophy (including EDM) are disorders associated with axonal swellings (axonal spheroids) and have a species/breed association and usually early onset (<1 year of age (4 weeks-3 years))
- Described in many species, most common in dogs and horses
- Characterized by prominent axonal swellings in various nuclei (often sensory) in the brainstem, cerebellum, and spinal cord
- Loss of cerebellar Purkinje and granule cells reported in Rottweilers and cats, loss of brainstem neurons in cats
- Other neuropathies in which Vitamin E plays a role:
- Equine motor neuron disease (EMND): Clinically similar syndrome to EDM with degenerative changes in the brainstem; older horses; associated with a vitamin E deficiency but lacks genetic component; affects ventral and intermediate gray columns of the spinal cord; a lower motor neuron disease
- Zebras and Mongolian wild horses (Equus przewalskii): Disease similar to EDM, associated with hypovitaminosis E
- Spontaneous hemorrhagic necrosis (SHN) in fetal hamsters, with symmetrical, subependymal vascular degeneration and intraventricular hemorrhage
- Encephalomalacia in Guinea Pigs from sows deficient in vit E
- Encephalomalacia (Crazy Chick Disease) in poultry: Ataxia, loss of balance, prostration; cerebellum often contains yellow or congested, hemorrhagic, or necrotic areas visible on the surface, usually within the first month of life
- Avians with hypovitaminosis E may have malacia, edema, white matter astrogliosis,capillary thrombi, hemorrhage, and neuronal necrosis
- Young turkeys lesions are seen in the spinal cord
- Prominent myocardial and skeletal muscle lesions are also present in these birds
- Hypovitaminosis E/ Vitamin E deficiency leads to:
- Yellow fat disease/nutritional panniculitis, in multiple mammalian and reptilian species
- Nutritional myopathy (M-M11) or cardiomyopathy (C-M06) in a number of mammals, avian species, and fish
- Intestinal ceroidosis (AKA leiomyometaplasia, brown dog gut) in dogs
- Neuropathies
References:
- Barthold SW, Imai DM. Pathology of Laboratory Rodents and Rabbits. 5th ed. Hoboken, NJ: Wiley Blackwell; 2026:252, 253, 315.
- 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; 2026:325-326. Duncan M. Perissodactyls. In: Terio KA, McAloose D, St. Leger J eds. Pathology of Wildlife and Zoo Animals. San Diego, CA: Elsevier; 2018:437-438.
- Mauldin EA, Welle MM. Integumentary System. In: Maxie MG, ed. Jubb, Kennedy & Palmer's Pathology of Domestic Animals. Vol 1. 7th ed. St. Louis, MO: Elsevier; 2026:583.
- Miller AD, Porter BF. Nervous System. In: Zachary JF, McGavin MD, eds. Pathologic Basis of Veterinary Disease. 7th ed. St. Louis, MO: Elsevier Mosby; 2022:963-964.
- Struthers JD, Schmidt RE. Nervous system. In: Schmidt R, Struthers JD, Phalen DN, eds. Pathology of Pet and Aviary Birds. 3rd ed. Hoboken, NJ: John Wiley & Sons, Inc.; 2024:394.
- Shivaprasad HL. Nutritional Diseases. In: Boulianne M, ed. Avian Disease Manual. 8th ed. Jacksonville, FL: American Association of Avian Pathologists, Inc; 2019:151-152.
- Swayne DE, Barnes HJ, Abdul-Aziz T, Fletcher OJ. Chapter 10: Nervous System. In: Abdul-Aziz T, Fletcher OJ, Barns HJ, eds. Avian Histopathology. 4th ed. Madison, WI: Omnipress; 2016:470.
- Terio K, McAloose, St. Leger J. Pathology of Wildlife and Zoo Animals. 1st ed. London, United Kingdom. Elsevier. 2018:287, 612, 655, 748, 855.
- Uchida K, Kobayashi Y. Muscle and Tendon. In: Maxie MG, ed. Jubb, Kennedy & Palmer's Pathology of Domestic Animals. Vol 1. 7th ed. St. Louis, MO: Elsevier; 2026:215.
