JPC SYSTEMIC PATHOLOGY

NERVOUS SYSTEM

April 2026

N-V11

Signalment (JPC #2320162): 9-month-old male sheepdog

HISTORY: This dog presented with progressive signs of CNS disease including ataxia, incoordination, and hyperreflexia.

HISTOPATHOLOGIC DESCRIPTION: Cerebellum and brainstem: Affecting over 50% of the cerebellar white matter, myelin sheaths are often either lost with replacement by gitter cells (demyelination), or are markedly dilated (spongiosis) and contain either swollen hypereosinophilic axons (spheroids) or gitter cells and cellular debris (digestion chambers or ellipsoids) (Wallerian degeneration). Within the most severely affected white matter, primarily at the tips of folia, but occasionally extending into the overlying gray matter, there is rarefaction and replacement by numerous gitter cells and fewer gemistocytic astrocytes, lymphocytes, plasma cells, occasional syncytia with up to 5 nuclei, and small amounts of karyorrhectic debris and edema fluid (liquefactive necrosis). Multifocally, Purkinje cells are degenerate, with markedly swollen, hypereosinophilic, fusiform proximal axons in the granule cell layer (torpedoes), or are lost, leaving circular clear spaces in the Purkinje cell layer (empty baskets), and the adjacent granular cell layer is often paucicellular. Numerous inflammatory cells, astrocytes, Purkinje cells, and small neurons in the granular cell and molecular layers contain intranuclear and fewer intracytoplasmic, 3-5 µm diameter, irregularly ovoid, eosinophilic viral inclusions that, when intranuclear, marginate the chromatin. There is mild focally extensive gliosis in the brainstem. Multifocally, vessels within the affected gray matter, white matter, and leptomeninges are mildly congested and lined by reactive endothelium, and Virchow-Robin spaces are expanded by many lymphocytes and plasma cells and fewer macrophages (perivascular cuffing).

MORPHOLOGIC DIAGNOSIS: Cerebellum and brainstem: Demyelination and necrosis, multifocal, severe, with Purkinje cell degeneration and loss, spongiosis, lymphoplasmacytic perivascular meningoencephalitis, viral syncytia, and eosinophilic intranuclear and intracytoplasmic viral inclusions, sheepdog, canine.

ETIOLOGIC DIAGNOSIS: Morbilliviral meningoencephalitis

CAUSE: Canine distemper virus (canine morbillivirus)

CONDITION: Canine distemper; Carre's disease

GENERAL DISCUSSION:

• Canine distemper, caused by canine distemper virus (CDV), is an important, ubiquitous infectious disease of dogs, other canids, wild felids, mustelids (ferrets, mink), procyonids (raccoons), pinnipeds, and other species worldwide
• Causes systemic disease often with respiratory, gastrointestinal and central nervous system (CNS) involvement
• CDV is a pantropic, negative-sense, single-stranded, enveloped RNA virus, 150-300 nm diameter, genus Morbillivirus, family Paramyxoviridae
• One recognized serotype; variable strain pathogenicity and tissue tropism
• Four distinct forms of CNS disease: Classic CDV encephalitis; multifocal distemper encephalomyelitis in mature dogs; post-vaccinal canine distemper encephalitis; “old dog” encephalitis (ODE)

PATHOGENESIS:

• Virulence factors: Hemagglutinin glycoproteins on viral envelope mediate attachment to host cells; fusion glycoproteins allow penetration of host cells and fusion of infected cells with uninfected cells
• Receptors: Signaling lymphocyte activation molecule (SLAM, CD150; leukocyte-restricted, mediates entry into cells), Nectin-4 on endothelial cells, keratinocytes, and epithelial cells at adherens junction complexes.
• Overall pathogenesis: Inhalation > virus infects macrophages in upper respiratory tract or lungs > leukocytic trafficking to regional lymph nodes > viral replication in tonsils and lymph nodes in first 24 hours > cell-associated viremia by 2 days PI > spread to all lymphoid tissues and blood lymphocytes by 2-5 days PI > lymphocytolysis and leukopenia > immunosupression > clinical course depends on host immune response:
  • Adequate humoral/cellular immunity: Neutralize virus by 14d PI; may not shed virus
  • Delayed / intermediate humoral / cellular immunity: Viral infection / persistence in mucosal epithelium and brain > may develop neurologic disease +/- disease associated with epithelial infection
  • Failure to develop neutralizing antibody by 8 or 9 days PI: Virus disseminates to respiratory, GI, urogenital, and central nervous systems; integumentary, exocrine and endocrine systems also affected > virus shed in all excretions during the systemic phase of infection > secondary infections common
    • Common secondary infections: Adenovirus, Bordetella sp., Clostridium piliforme, Cryptosporidium sp., E. coli, Encephalitozoon sp., Pneumocystis sp., Sarcocystis sp., and Toxoplasma sp.
• Pathogenesis of CNS lesions: CNS signs develop 1-3 weeks after systemic signs or may seemingly occur de novo after a subclinical infection
  • Virus spread through leukocyte trafficking and cell-free viremia hematogenously to brain and choroid plexus > infected cells migrate through endothelium and virus replicates in endothelial cells 🡪 perivascular lymphocytic cuffing > virus replicates in pericytes, microglial cells, astrocytic foot processes, choroid plexus epithelial cells > characteristic white matter vacuolation (intramyelinic edema) thought to result from a direct effect of the virus on oligodendrocytes
• ODE may be a consequence of long-term, subclinical, persistent infection of CDV in a replication-defective state
• Post-vaccinal CDV: Vaccination with modified live vaccines (MLV) is often fatal in exotic carnivores (e.g., ferret, mink, lesser panda, grey fox); rarely causes fatal encephalitis in young dogs, possibly due to immune stimulation by other canine viruses (e.g., canine parvovirus) at time of vaccination; vaccination of pregnant dogs can cause disease in puppies or abortion

TYPICAL CLINICAL FINDINGS:

• Classic canine distemper:
• Disease most common in 12-16 week-old puppies (waning passive immunity)
  • Early: Fever, conjunctivitis, cough, vomiting, diarrhea, depression, anorexia, serous to mucopurulent oculonasal discharge, death
  • Clinical pathology: Lymphopenia, thrombocytopenia (virus-antibody immune complexes on platelets and direct infection of megakaryocytes), regenerative anemia, hypoalbuminemia, hypergamma- and alpha-globulinemia
  • Later (1-4 weeks and if animal did not die in early stage): Neurologic disease (seizures, cerebellar or vestibular ataxia, paraparesis, myoclonus); may see minimal or no signs of epithelial infection and only neurologic signs in some dogs; hyperkeratosis of footpads and nose, and enamel hypoplasia are late manifestations
  • 50-70% of infections are subclinical
• Multifocal distemper encephalomyelitis in mature dogs:
  • Occurs when CDV infects dogs at 4-8 years of age
  • Rare, chronic, progressive disease
  • Not preceded by signs of classic distemper
  • Slow, progressive course of weakness and incoordination, but no seizures
• “Old dog” encephalitis (ODE):
  • Extremely rare; most cases occur in dogs past middle age
  • Insidious onset of depression, circling, head pressing, visual deficits, seizures and muscle fasciculations
  • No paralysis or convulsions
  • Progression over 3-4 months to coma or death
• Post-vaccinal canine distemper encephalitis:
  • Idiopathic
  • Affects young dogs 1-3 weeks post-vaccination with MLV CDV vaccines
  • Acute/subacute (1-5 days) clinical course
  • Signs reminiscent of furious form rabies; aggressive/attack behavior

TYPICAL GROSS FINDINGS:

• CNS: Lesions are uncommon; white matter softening with brown discoloration +/- hemorrhage; brain reduced in size with moderately dilated ventricles in ODE
• Lungs: Patchy red-tan, rubbery subpleural and marginal lesions (bronchointerstitial pneumonia); may be edematous and consolidated (secondary bronchopneumonia)
• Eyes/conjunctiva: Conjunctivitis, keratitis
• Integument: Hyperkeratosis of footpads and nose; +/- secondary pyoderma (pustular dermatitis, especially ventral abdomen)
• Lymphoid tissues: Tonsillar enlargement, thymic atrophy
• Teeth: Enamel hypoplasia
• Long bones: Metaphyseal osteosclerosis (young growing dogs)

TYPICAL LIGHT MICROSCOPIC FINDINGS:

• Eosinophilic intracytoplasmic (IC) and/or intranuclear (IN) inclusion bodies:
  • Most numerous 10-14 days post-infection
  • Most obvious in brain (often INIBs) and epithelium (usually ICIBs), especially in urinary bladder (U-V07); less obvious in lymphoid tissues
• Central nervous system:
  • Classic canine distemper:
    • Lesions can involve both gray and white matter, one may predominate
    • White matter: Demyelination (with early axon sparing) and status spongiosis of white matter with axonal degeneration and necrosis (most severe in cerebellar peduncles, rostral medullary velum, optic tracts, hippocampal fornix, spinal cord, surrounding the fourth ventricle); nonsuppurative encephalitis, gitter cells, astrocytosis, syncytia
      • Demyelinating leucoencephalomyelitis (DLEM) is one presentation of CDV; spinal cord lesions consistently affect the white matter, most commonly in the lumbosacral region; demyelination, astrocytosis, microgliosis, gemistocytes, and non-suppurative inflammation most common white matter lesions; no correlation between pathologic changes and reported clinical signs (Areco, J Comp Path 2021)
    • Gray matter (lesions less common than white matter lesions): INIBs +/- ICIBs in neurons > neuronal necrosis > mononuclear infiltrate surrounding necrotic neurons > nonsuppurative mild perivascular encephalitis and meningitis; rare cases with severe seizure activity feature bilateral polioencephalomalacia involving pyriform lobes and hippocampus
• Multifocal distemper encephalomyelitis in mature dogs:
  • Lesions are restricted to CNS and most prevalent in cerebellum and white matter of spinal cord; cerebral cortex often spared (in contrast to ODE)
  • Multifocal necrotizing nonsuppurative encephalitis with rare intra-astrocytic INIBs; demyelination in internal capsule and corona radiata
• “Old dog” encephalitis:
  • Cerebral cortex, thalamus, and brainstem are consistently affected (in contrast to multifocal distemper encephalomyelitis in mature dogs)
  • Most obvious change is large lymphocytic perivascular cuffing at grey-white matter junction
  • Demyelination of white matter with microgliosis, astrogliosis, and variable leptomeningitis and neuronal degeneration
  • Characteristic feature is widespread nonsuppurative perivascular encephalitis; IN& ICIBs in astrocytes and neurons; viral antigen detectable by IHC, but cannot isolate virus from the brain (viral antigen not in cerebellum)
• Post-vaccinal canine distemper encephalitis:
  • Lesions always restricted to CNS
  • Similar to classic CDV lesions, but with relative sparing of white matter
• Eyes: Acute lymphoplasmacytic chorioretinitis and optic neuritis, perivascular cuffing and edema, exudative retinal separation, hypertrophy of RPE, eosinophilic INIBs in ganglion cells and astrocytes; multiple random foci of retinal degeneration (usually full thickness) and scarring are more prevalent lesions; optic nerve may show nonsuppurative neuritis, astrocytic scarring, and demyelination similar to that seen in the brain
• Lungs (common; P-V01): Bronchointerstitial pneumonia
  • Patchy bronchiolar necrosis/attenuation
  • IBs are most obvious in airway epithelium (persist longer than in CNS)
  • Alveoli: Edema, fibrin, mononuclear cells, necrosis; type II pneumocyte hyperplasia, IBs common in type II pneumocytes and alveolar macrophages, alveolar epithelial syncytia
• Integument (I-V12): Orthokeratotic and/or parakeratotic hyperkeratosis of footpad, nose, rarely haired skin
  • +/- Epidermal syncytia; INIBs / ICIBs in epithelial cells and syncytia
  • +/- Secondary pustular dermatitis (pyoderma)
  • Orthokeratotic hyperkeratosis predominant histologic feature, accompanied by inclusion bodies, epidermal hyperplasia, hydropic degeneration of keratinocytes; viral antigen expression most common in sweat glands, epidermis, and vascular endothelial cells or pericytes (Areco, J Comp Path 2022)
• Lymphoid tissues: Early lymphoid depletion and lymphocytic necrosis > syncytia +/- IBs > histiocytic cell hyperplasia; thymic atrophy particularly common in puppies
• Teeth: Cystic degeneration of ameloblastic epithelium, syncytia and ICIBs > enamel defects / hypoplasia
• Other epithelia (urinary bladder [U-V07]; renal pelvis; gastric surface, chief, and parietal cells; cholangiolar epithelium; pancreatic ductular epithelium; epididymis; testis): Degeneration, mononuclear infiltrates, ICIBs +/- INIBs
• Heart: Myocardial necrosis and mineralization
  • More recent report of myocarditis with myocardial necrosis and fibrosis in four dogs with confirmed naturally acquired CDV infection (Kim, JVDI 2021)
• Bone: Growth retardation lattices; Osteoclast necrosis 🡪 impaired osteoclastic resorption of primary spongiosa

ULTRASTRUCTURAL FINDINGS:

• Tubular CDV nucleocapsids are seen in non-membrane bound intracytoplasmic aggregates; similar tubular structures may be seen in the nucleus (despite lack of viral replication in the nucleus)
• Destruction of ensheathing myelin envelope

ADDITIONAL DIAGNOSTIC TESTS:

• Virus isolation, immunohistochemistry (IHC), fluorescent antibody test
• Cytology (blood smear): Inclusions appear as monomorphic or pleomorphic, red to purplish red or pale blue cytoplasmic inclusions found (rarely) in neutrophils, monocytes, lymphocytes, and erythrocytes, typically in the early viremic stage and before clinical illness; stain better with quick stains, such as Diff-Quick, than with Romanowsky stains, which typically produce pale blue inclusions.
• Recent report states that, even with grossly and histologically normal skin samples, immunohistochemistry of skin was useful in the diagnosis of CDV infection, which was found in both epithelium and endothelium. Nectin-4, the known CDV receptor in epithelium and endothelium, was co-distributed in cells with positive immunoreactivity for CDV (Garcia, Vet Path 2022)

DIFFERENTIAL DIAGNOSIS:

For CNS lesions:

• Toxoplasma gondii (N-P02) / Neospora caninum (N-P03): Random, multifocal, necrotizing foci in the grey and white matter; protozoa at the margins of the lesions
• Rabies (N-V06, Rhabdoviridae; lyssa virus): ICIBs (Negri bodies) in hippocampal neurons and Purkinje cells; occasional lymphocytic perivascular cuffing
• Pseudorabies (N-V07, Alphaherpesvirus, suid herpesvirus 1): Polioencephalomyelitis +/- ganglionitis, INIBs in neurons of spinal ganglia, spinal cord, medulla, pons
• Granulomatous meningoencephalitis (N-M26): Progressive neurological disease of adult dogs with granulomatous inflammation
• Canine adenovirus type I (Infectious canine hepatitis): Brainstem hemorrhages with mild vasculitis and intranuclear inclusions in endothelial cells
• Canine herpesvirus: Usually puppies less than 21 days old; multifocal necrotizing lesions in many organs including lung, liver, kidneys and CNS
• Case report of several neonatal pups with CDV developing respiratory tract lesions without central nervous signs; distemper should be considered a differential for neonatal canine mortality / fading puppy syndrome

Other viruses causing IBs in bronchiolar or airway epithelium:

• Canine adenovirus-2
• Canine herpesvirus-1
• Canine minute virus (canine parvovirus 1)

COMPARATIVE PATHOLOGY:

Other species affected with CDV:

• Mustelidae: One of the most important viral diseases in ferrets, also weasels, mink, skunk, badger, marten, otter
• Canidae: Dingo, fox, coyote, jackal, wolf
  • Recent report of Clostridium piliforme and CDV coinfection in domestic dog littermates and a gray fox kit; cases indicate importance of considering antecedent CDV infection in any canid species diagnosed with Tyzzer disease (Jacobson, JVDI 2022)
  • Mortality in gray foxes approaches 100%
• Procyonidae: Raccoon, coati
• Ailuridae: Red panda
• Suidae: Encephalitis in collared peccaries.
• Ursidae: Bears, panda
• Viverridae: Civet, mongoose
• Felidae: Lions, leopards, cheetahs, tigers
• Phocids (e.g. Baikal seals) susceptible to CDV and phocine distemper virus (N-V12)
• Rodents: Marmots
• NHP: Rhesus monkeys
• Xenartha: CDV infection reported in a single family group of anteaters

Other morbilliviruses:

• Rinderpest (D-V28): Primarily large ruminants – oral mucosal erosions, diarrhea; worldwide eradication declared by OIE in 2011
• Peste-des-petits-ruminants: Small ruminants – respiratory signs, oral erosions, diarrhea
• Cetacean morbillivirus (dolphin morbillivirus and porpoise morbilliviruses are two strains of the same species): Closely related to RP and PPRV
• Phocine distemper virus (PDV, N-V12): Closely related to CDV
• Measles virus (P-V02): Humans and non-human primates

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