JPC SYSTEMIC PATHOLOGY

NERVOUS SYSTEM

April 2026

N-V09

Signalment (JPC #2841678): 4-year-old Tennessee walking horse gelding

HISTORY: This horse exhibited severe lethargy that rapidly progressed to recumbency.

HISTOPATHOLOGIC DESCRIPTION: Brainstem: Multifocally, Virchow-Robins space is moderately expanded by lymphocytes, plasma cells, and fewer neutrophils admixed with pale eosinophilic fluid (edema). This inflammatory infiltrate often extends into the surrounding neuroparenchyma and is admixed with abundant glial cells (gliosis). The leptomeninges are expanded up to 3 times normal by a similar inflammatory infiltrate admixed with scant eosinophilic cellular and karyorrhectic debris, eosinophilic fibrillar material (fibrin), mild hemorrhage, and increased clear space (edema). Inflammatory cells often surround individual neurons (satellitosis) which are either degenerate with vacuolation of the perikaryon, swollen with an eccentric nucleus, peripheralized Nissl substance and central eosinophilic glassy cytoplasm (central chromatolysis), or are necrotic with shrunken, angular, eosinophilic cytoplasm and a pyknotic nucleus. Rarely, necrotic neurons are phagocytized by gitter cells (neuronophagia). Within the white matter, there is mild spongiosis and rare swollen axons within dilated myelin sheaths (spheroids). Multifocally endothelium is hypertrophied (reactive) and there is margination of neutrophils in postcapillary venules.

MORPHOLOGIC DIAGNOSIS: Brainstem: Meningoencephalitis, perivascular, lymphoplasmacytic and neutrophilic, multifocal, moderate, with neuronal necrosis, satellitosis, and neuronophagia, Tennessee walking horse, equine.

ETIOLOGIC DIAGNOSIS: Alphaviral meningoencephalitis

CAUSE: Eastern equine encephalitis virus

CONDITION: Eastern equine encephalitis

GENERAL DISCUSSION:

• Venezuelan, eastern, and western equine encephalitis viruses (VEE, EEE, WEE) are in the virus family Togaviridae, genus Alphavirus, and are transmitted via arthropod vectors “arboviruses” (usually mosquitoes) and reservoir hosts (birds, rodents) and occasionally to incidental, dead end hosts that may develop encephalitis (humans, horses); reservoir hosts: water birds (EEE), wild birds (primarily passerines, WEE), and rodents (VEE)
• Not all birds are capable of acting as reservoir hosts:
  • Red-winged blackbirds, cardinals, sparrows, cedar waxwings, and the captive Chinese pheasant are highly susceptible to infection and nearly always die
  • Other species, including adult domestic fowl and turkeys, are not sickened by the infection, although fatalities can be produced in the young of these species (see comp path)
• Outbreaks may involve humans only, horses only, or both and may be limited to inapparent systemic infection or lethal encephalitis
• Generally, WEE is less virulent for horses while EEE is often acutely fatal; VEE commonly occurs in epizootics that reoccur at approximately 10 year intervals
• Target cell in the CNS is the neuron, VEE virus is proinflammatory within the CNS

PATHOGENESIS:

• EEE: North America Atlantic course in Caribbean, Central America and northeast course of South America: Mosquito Culiseta melanura (ornithophilic species i.e. feeds on birds only) transmits virus to waterfowl; mosquitoes Aedes spp., Coquillettidia sp., Ochlerotatus sp.(bridge vectors) > horse and human transmission; seasonal with mosquito
• WEE: Only in valleys of weastern North American and some South America: Mosquito Culex tarsalis (indiscriminate feeder) > primarily passerine birds, but also humans and other mammals; seasonal with mosquito
• VEE:
  • Avirulent (enzootic ID, IE, subtypes II-VI) endemic form that cycles between Culex sp. mosquitoes and small rodents in the Caribbean
  • Virulent (epizootic IAB and IC) – circulate through multiple mosquito species and horses > high titer viremia in horses -> infects mosquitoes; Venezuela, Colombia, Peru
  • Outbreaks occur cyclically every 10 years
• Pathogenesis: Transmission via mosquito bite > initial viral replication in regional lymph nodes and blood vessels > primary viremia > replication in muscle and lymph nodes > secondary viremia > high viral titer and hematogenous dissemination > neuronal invasion, neural necrosis and apoptosis, in CNS, viral replication in neurons, glia, and vascular endothelium
• Mosquitoes are infected for life and are capable of permitting viral replication
• The virus has been found in chicken mites (Dermanyssus gallinae), chicken lice (Menopon pallidum, Menocanthus stramineus), and assassin bugs (Triatoma sanguisuga).
• The spotted-fever tick, Dermacentor andersoni, is capable of transmitting the infection stage to stage and hereditarily.
• Transmission by aerosolization is reported only in humans; laboratory workers are at high risk of infection by aerosols.

TYPICAL CLINICAL FINDINGS:

• Infections are more common in late summer and fall (mosquito season)
• Young horses are more susceptible than old
• Infection may be subclinical or consist of fever and mild depression
• Neurologic disease is predominantly referable to the cerebral cortex; it occurs suddenly after a 1–3-week incubation period with rapid progression to profound depression, central blindness, circling, head pressing, recumbency, facial paralysis, occasionally seizures, animal can stay in this “dummy stage or if fatal death within 2-4 days

TYPICAL GROSS FINDINGS:

• There are typically no gross lesions; when present, gross lesions are asymmetrical, most commonly in the gray matter of the spinal cord, and include congestion, hemorrhage, and often malacic
• Cerebral hyperemia, edema, petechiation, focal necrosis, increased CSF in the subarachnoid space may be seen

TYPICAL LIGHT MICROSCOPIC FINDINGS:

• Lesions are confined to gray matter and are more severe in the cerebral cortex (principle site), especially in the frontal, rhinencephalic, and occipital areas with lesions of lesser intensity in the pyriform lobes; severe lesions may also be present in the thalamus and hypothalamus; lesions are typically less severe caudal to the thalamus
• Microscopic lesions: Perivascular cuffing with lymphocytes, macrophages, and neutrophils; neuronal degeneration, necrosis, neuronophagia; microgliosis; perivascular edema and hemorrhage
• In early stages (2 days) of EEE there is a prominent neutrophilic perivascular infiltration with neutrophils found in scattered malacic foci; neutrophils may also be present in cases of VEE
• Necrotizing vasculitis, thrombosis, and cerebrocortical necrosis are particularly evident in VEE but also reported in EEE, sparing the trigeminal ganglion
• Rare intranuclear inclusions have been described
• In horses, EEE may cause small intestinal lesions that include multifocal myonecrosis and lymphomonocytic infiltration in the muscular layer and focal mild perivascular lymphocytic infiltration in the submucosa
• Horses with VEE may develop myeloid bone marrow depletion and lymphocytolysis in the spleen and lymph nodes

ULTRASTRUCTURAL FINDINGS:

• Virions are spherical, enveloped, with an icosahedral nucleocapsid
• Replication occurs in the cytoplasm with budding from the plasma membrane

ADDITIONAL DIAGNOSTIC TESTS:

• Serology is used for antemortem diagnosis
• RT-PCR and immunohistochemistry of CNS tissue are used for postmortem diagnosis (IHC and in situ hybridization will demonstrate intracytoplasmic neuronal and glial antigen and RNA)
• Isolation and identification by neutralization or partial sequencing is especially important in VEE, which occurs in avirulent endemic and virulent epidemic types

DIFFERENTIAL DIAGNOSIS:

Microscopic differentials:

• West Nile virus (N-V19): Mosquito transmitted flavivirus, is maintained in a mosquito to bird cycle; horses and humans are aberrant, dead-end hosts; severe polioencephalomyelitis with lymphocytic and histiocytic perivascular infiltrate, multifocal microgliosis, and neuronophagia; primarily affects gray matter of lower brainstem and thoracolumbar spinal cord
• Japanese encephalitis virus: Mosquito transmitted flavivirus confined to Asia, produces lesions indistinguishable from EEE and similarly affect humans; pigs serve as a major amplification host
• Equine herpes virus-1 (P-V10): Alphaherpesvirus causing sporadic outbreaks of neurologic disease, respiratory disease, or abortion; virus causes endothelial cell damage leading to necrotizing vasculitis, thrombosis, and infarction; affects both white and gray matter
• Borna disease virus: Genus Bornavirus, family Bornaviridae, causes viral encephalitis in horses in Europe; severe encephalomyelitis with lymphoplasmacytic perivascular cuffing without neuronal necrosis; eosinophilic intranuclear inclusions
• Equine protozoal myelitis (EPM, N-P01): Apicomplexan coccidian parasite (Sarcocystis neurona) causes multifocal, asymmetric hemorrhages, malacia, and discoloration in white matter of the spinal cord; brainstem is more frequently affected than other parts of the brain, but less affected than cervical and lumbar spinal cord; affects both white matter (WM) and gray matter (GM)

Gross differentials:(GM: grey matter; WM: white matter)

• EPM- both/either GM and WM
• Post anesthetic hemorrhagic myelopathy-GM entire length of spinal cord
• Rabies- GM
• EHV1- GM or WM
• WNV- GM thoracolumbar

COMPARATIVE PATHOLOGY:

Other Alphavirus Encephalitides:

• Highlands J virus of America’s east coast, Getah virus of southeast Asia, Ross River virus of Australia, and Semliki Forest virus of the Americas—are all equine pathogens that are able to induce at least a febrile disease, maintained via the mosquito bird lifecycle, Getah virus does not induce encephalitis rather clinical signs and lesions similar to equine viral arteritis
• Dogs, cats and sheep are considered refractory although there has been rare reports of EEE in puppies.
• Humans: Alphaviruses are an important cause of epidemic encephalitis in humans; lesions are similar to those of the horse; EEE has been researched as potential biological weapon capable of aerosolization (humans are the only species susceptible to EEE aerosol transmission)
• Pigs: Readily develop asymptomatic infections, small viremia (cannot propagate infection) and may develop myocarditis from EEE
• Laboratory animals: Guinea pigs and white mice are highly susceptible, rabbits somewhat susceptible
• Camels: Fever, seizures, ataxia, lethargy, recumbency, and a high mortality rate are characteristic; no gross lesions; histologic CNS lesions very similar to the horse
• Birds: Some bird species are highly susceptible to EEE and have high mortality (commercial pheasant flocks, red-winged blackbirds, cardinals, sparrows, and cedar waxwings), vasculitis is frequent finding and non-heterophilic meningoencephalomyelitis especially in the rostral portion of the telencephalic complex
• Emus: Frequently acute death in all age groups; depression, bloody vomitus and diarrhea; grossly, hemorrhagic enterocolitis and systemic serosal petechiation/ecchymoses; lymphoid necrosis in spleen and intestine, hepatocellular and intestinal lamina propria necrosis; horizontal transmission without insect vector
• Cassowary: Sudden death, ataxia; vasculitis of the spleen and liver (not hemorrhagic enteritis)
• Penguins: Anorexia, lethargy, intermittent vomiting; severe, chronic, multifocal, lymphoplasmacytic encephalitis with prominent perivascular cuffing; cerebellar dendrite mineralization, focal gliosis, Purkinje cell loss, demyelination and axonal degeneration; mild necrotizing and histiocytic myocarditis
• Great egret and whooping crane: Reportable; mortality; grossly myocardial pallor, pericardial effusion, hepatosplenomegaly; necrotizing splenitis, hepatitis, pneumonitis
• Ring-necked pheasants, chukar partridges, and turkeys: High mortality, young birds; lymphocytic meningoencephalitis, lymphocytic myocarditis and myonecrosis
• Turkeys: Lymphoid depletion and necrosis in the bursa of Fabricius and thymus
• Passerine birds: Reservoir hosts in the southern US; leukopenia (immune suppressive); necrosis in splenic ellipsoids, liver, myocardium, systemic vasculature
• Psittacine birds: Avian viral serositis; neonates and juvenile psittacines; nonsuppurative meningitis and encephalitis, fibrinous epicarditis and multifocal lymphocytic infiltrates
• House Sparrow (Passer domesticus) nestlings infected with BCV Fort Morgan virus (Buggy Creek virus (BCV) commonly had perivascular lymphoplasmacytic meningoencephalomyelitis admixed with few heterophils, variable neuronal necrosis, and gliosis that predominated in the cerebrum and optic lobe gray matter.
• EEE has also been reported in cattle (calves), sheep, cats, and white-tailed deer

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