JPC SYSTEMIC PATHOLOGY

NERVOUS SYSTEM

April 2026

N-V19

Signalment (JPC #2839005): 13-year-old female thoroughbred, equine

HISTORY: The mare was vaccinated for West Nile Virus when 4-months pregnant. She developed sudden onset of neurologic disease with flaccid paralysis of the tongue, lips, and jaws, and was unable to swallow. Due to poor prognosis, the horse was euthanized.

HISTOPATHOLOGIC DESCRIPTION: Brainstem: Multifocally affecting the meninges, the gray matter, and to a lesser extent the white matter, there is perivascular cuffing with low to moderate numbers of lymphocytes, plasma cells, macrophages, and fewer neutrophils that expand Virchow-Robin space up to 10 times normal and often extend into the adjacent neuroparenchyma. Within gray matter, neurons are often markedly swollen with eosinophilic cytoplasm (degeneration), have clearing of the cytoplasm with peripheralization of Nissl substance (central chromatolysis), or are shrunken and angular with hypereosinophilic cytoplasm and nuclear pyknosis (necrosis). Affected neurons are occasionally surrounded by glial cells (satellitosis). Multifocally, there are increased numbers of microglia and astrocytes (gliosis) and glial nodules with mild necrosis and spongiosis, mild perivascular edema, and multiple small areas of hemorrhage.

MORPHOLOGIC DIAGNOSIS: Brainstem: Meningoencephalitis, perivascular, lymphohistiocytic, multifocal, moderate, with neuronal degeneration and necrosis, gliosis, and mild hemorrhage, thoroughbred, equine.

ETIOLOGIC DIAGNOSIS: Flaviviral encephalitis

CAUSE: West Nile virus

GENERAL DISCUSSION:

• Arbovirus of the family Flaviviridae, genus Flavivirus; two lineages: lineage 1 found in North America, lineage 2 restricted to enzootic areas in Africa
• Originally discovered in 1937 in Uganda; first appearance in New York in 1999
• Increasingly widespread cause of fatal disease in humans, horses, and birds
• Corvids (crows), American robins, and other wild birds are the main amplifying hosts

PATHOGENESIS:

• Transmitted primarily via mosquitos (Culex) in the US; ticks are capable of harboring the virus; other methods of transmission: ingesting infected prey (bird, cannibalism among birds), direct contact through open cuts (zoonotic potential), mechanical transmission by other vectors (deerflies, etc)
• Birds are the reservoir for sylvatic transmission (bird – mosquito – bird cycle)
• Humans and equids/horses (dead end hosts) are infected during the urban transmission cycle
• Dead-end host (mammal): mosquito bite > virus propagates in keratinocytes, cutaneous dendritic cells/Langerhans cells, regional endothelial cells, and fibroblasts > viremia > crosses the BBB 🡪 hematogenous spread to brain; may also reach brain via retrograde axonal transport
• Birds: Virus distributed systemically
• Virus targets neurons and microglial cells

TYPICAL CLINICAL FINDINGS:

• Seasonal: Summer and fall (transmitted by mosquitos)
• Mortality in horses up to 50% in naïve areas; lower mortality in subsequent years
• Weakness, recumbency, ataxia, anorexia, tremors, abnormal head posture, circling
• Severity of clinical signs does not correlate with degree of histologic lesions

TYPICAL GROSS FINDINGS:

• Usually absent
• In acute cases, hemorrhage/malacia of brain stem and thoracolumbar spinal cord, particularly the gray matter, may occasionally occur

TYPICAL LIGHT MICROSCOPIC FINDINGS:

• Primarily in brainstem and thoracolumbar spinal cord; less often in the cerebral cortex and cervical spinal cord; cerebellum spared
• Mononuclear polioencephalomyelitis (nonsuppurative encephalomyelitis) with gliosis, glial nodule formation, neuronal degeneration and necrosis; more pronounced in gray matter; axonal swelling and spheroid formation; histology lesions typically mild (thin perivascular cuffing) and don’t correlate to clinical signs
• Extraneural lesions: Hepatitis, myocarditis in birds; do not occur in horses

ULTRASTRUCTURAL FINDINGS:

• Spherical, enveloped, 40-60 nm in diameter; single stranded RNA

ADDITIONAL DIAGNOSTIC TESTS:

• PCR, IHC, ISH, virus isolation (BSL-3, zoonotic)
• In horses, antigen is scarce in CNS lesions (unlike birds); no extraneural antigen often limited to sparse axonal immunostaining using IHC
• A real-time PCR panel for multiple vector-borne infections, including West Nile virus, has been developed for epidemiologic studies (Warang, J Vet Diagn Invest, 2021)

DIFFERENTIAL DIAGNOSIS:

Microscopic differentials for brain and spinal cord lesions in horses:

• Eastern equine encephalitis (EEE) (Togaviridae; Alphavirus; N-V09)
• Rabies (N-V06)
• Equine protozoal meningoencephalomyelitis (N-P01)
• Equine herpesvirus-1 (alphaherpesvirus)

COMPARATIVE PATHOLOGY:

WNV in Birds: Affected species include crows/corvid, passerines, birds of prey (hawks, owls, falcons, eagles), psittacines, flamingoes, penguins, pelicans, cormorants, geese, ducks; turkeys appear to be resistant

• Lesions/susceptibility variable between species and age groups: Crows, flamingoes, penguins experience high mortality
• Hawks: Vision impairment and neurologic disease due to WNV infection commonly leads to trauma; histologic triad: Myocarditis, endopthalmitis, meningoencephalomyelitis
• Owls: Species with a more northern range have acute death with no or short duration of clinical signs; species with more southern range (i.e. great horned owl) demonstrate progressive neurologic clinical signs
• Clinical signs: Sudden death, especially in crows
• Gross lesions:
  • Meningeal and brain congestion
  • Hemorrhage and necrosis in multiple organs, including myocardium, intestine, spleen, pancreas, lung, kidney
  • Enlarged, flaccid heart with pale streaking of myocardium
• Microscopic findings:
  • Lymphoplasmacytic meningoencephalitis with perivascular cuffing, gliosis, and astrocytosis in the cerebellum, brain stem, and ventral horns of the thoracolumbar spinal cord; cerebellar malacia and Purkinje cell necrosis/loss
  • Hemorrhage, lymphoplasmacytic inflammation, and necrosis in multiple organs, including myocardium, intestine, spleen, pancreas, lung, kidney
  • Peripheral neuritis
  • Raptors: intraocular hemorrhage, retinal necrosis, lymphoplasmacytic retinitis and optic neuritis, atrophy; pectinitis
  • Bald eagles hydrocephalus ex vacuo following cerebral pan necrosis.
• In most bird species, high levels of antigen found in molecular layer and Purkinje cells of cerebellum and neurons of brainstem; in crows, high levels found in tissue macrophages in spleen, Kupffer cells in liver, and blood monocytes
• Differentials for WNV infection in birds include:
  • Exotic Newcastle’s disease (Paramyxoviridae, N-V10)
  • Highly pathogenic avian influenza (Orthomyxoviridae, D-V25)
  • Lead intoxication (vascular necrosis without inflammation)
  • Avian bornavirus/proventricular dilatation disease (both can cause mononuclear encephalitis of brainstem/spinal cord and peripheral neuritis)

WNV in Other Species:

• Ruminants, dogs, cats, and pigs: Less susceptible than horses; may develop subclinical disease and brief viremia; in sheep WNV may cause abortion, stillbirth, and neonatal death in sheep
• New world camelids susceptible
• Primates: Majority of infections are subclinical; natural infection resulting in encephalitis has been documented in a barbary macaque
• Farmed American alligators: Hatchlings and juveniles more severely affected; acute death +/- neurologic symptoms gross lesions include coelomic effusion, oropharyngeal fibrinonecrotic inflammation, intestinal hemorrhage; heterophilic to lymphoplasmacytic meningoencephalitis +/- spinal cord involvement, variable inflammation in multiple organs; also associated with lymphohistiocytic proliferative syndrome of alligators

References:

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