April 2017


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 the horse was unable to swallow. Due to poor prognosis, the horse was euthanized.


HISTOPATHOLOGIC DESCRIPTION:  Brainstem:  Multifocally affecting the meninges,  and both the gray and white matter (though more severely within the gray 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 neuropil.  Within gray matter, neurons are often markedly swollen with eosinophilic cytoplasm (degeneration), have clearing of the cytoplasm with peripheralization of chromatin (central chromatolysis), or are shrunken and angular with hypereosinophilic cytoplasm (necrosis). Affected neurons are occasionally surrounded by glial cells (satellitosis), with multifocal neuronophagia.  Multifocally there are increased numbers of microglia and astrocytes (gliosis) and glial nodules with mild necrosis and spongiosis, mild perivascular edema, multiple small areas of hemorrhage, and occasional swollen axons (spheroids) within the white matter. 


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


ETIOLOGIC DIAGNOSIS:  Flaviviral encephalitis


CAUSE:  West Nilevirus



·       West Nile virus (WNV), an arbovirus of the family Flaviviridae, genus Flavivirus, is an increasingly widespread cause of fatal disease in humans, horses, birds, a harbor seal, and other mammalian and reptilian species in the US since the first appearance in New York in 1999; (originally discovered in 1937 in Uganda)

·       Birds are the reservoir for sylvatic transmission (bird – mosquito – bird cycle with crows being the amplifying host), with humans and horses (especially susceptible) becoming infected during the urban transmission cycle and considered dead end hosts

o   Virus distributed in almost every organ in birds

o   Virus mainly limited to CNS in horses

·       Transmitted primarily via mosquito bites; also by ingesting infected prey (bird), and direct contact through open cuts (zoonotic potential)

·       Mosquitoes are considered the primary vectors in the US thus infections are seasonal in summer and fall; ticks are capable of harboring the virus,

·       Several bird species, including corvids (crows, magpies, blue jays), other passerine species (house sparrows, grackles), shorebirds (gulls), raptors (hawks, owls), and flamingoes, are highly susceptible to infection; domestic poultry and psittacines are generally resistant to fatal infections



·       Mammalian / dead end hosts: Virus injected by mosquito > propogates in keratinocytes, cutaneous dendritic cells/Langerhans cells, regional endothelial cells and fibroblasts > Spread to lymph nodes then viscera > Viremia > Virus enters brain hematogenously (crosses the BBB) or potentially through retrograde axonal transport > encephalitis with targeting of neurons and glial cells (endothelial targeting not reported in horses as it occurs in birds)

·       Monocytes and macrophages are important cellular targets that contribute to systemic spread

·       Viral envelope contains two membrane-anchored glycoproteins, E2 use to attach to target cell and E1 used to enter cell via endocytosis (glycoprotein E may confer invasiveness)

·       Pronounced, if not exclusive CNS tropism in the horse   

·       Macrophage dysfunction and infection of connective tissues may contribute to triggering mediators of the coagulation system, resulting in hemorrhages; there is no apparent targeting of endothelial cells by the virus   

·       Toll-like receptor 3 (TLR3)-dependent inflammatory response to WNV infection is involved in blood-brain barrier breakdown and subsequent viral infection of the brain and neuronal necrosis

·       Caspase 3 dependent apoptosis of target cells may play a role in pathogenesis in some species (i.e. experimentally infected mice and hamsters)



·       Weakness, recumbency, ataxia, anorexia, tremors, abnormal head posture, and circling

·       Severity of clinical signs does not correlate with histologic findings



·       Usually absent

·       Occasionally acute areas of hemorrhage or malacia affecting the thoracic and/or lumbar spinal cord (mostly grey matter); few other gross lesions in the horse



·       Lesions primarily in brainstem and thoracolumbar spinal cord and less often in the cerebral cortex and cervical spinal cord

·       Lymphoplasmacytic meningoencephalitis / encephalomyelitis (few neutrophils may be present), perivascular cuffing, usually nonsuppurative, with gliosis, glial nodule formation, neuronal degeneration and necrosis, ring hemorrhages, axonal degeneration and spheroid formation; lesions more pronounced in gray matter

·       Lesion severity variable and may only consist of mild perivascular cuffing in the brainstem

·       May only have very small accumulations of viral antigen in CNS tissues in horses (unlike birds)

·       Polioencephalomyelitis and hemorrhage that involves the brain stem, and ventral and lateral horns throughout the spinal cord, especially thoracic

·       Extraneural lesions do not occur in horses



·       35-45 nm diameter virions, with dense core surrounded by thin, diffuse outer layer

·       Virions within cytoplasmic vacuoles, less frequently in rough endoplasmic reticulum (rER)

·       100 nm smooth membrane vesicles (SMV) in dilated rER and vacuoles

·       Marked vesiculation and vacuolation of the cytoplasm, with disorganization of the rER and Golgi apparatus



·       Antigen scarce in brain lesions; no extraneural antigen

·       Immunohistochemistry

o   Labeling may be sparse

o   Cytoplasm of neurons, nerve fibers, glial cells

·       In situ hybridization

·       Polymerase chain reaction on brainstem

o   Preferred method of diagnosis in combination with histo findings

·       IgM capture ELISA (MAC-ELISA)

·       Virus isolation (requires Biosafety level 3 lab)



Microscopically for brain and spinal cord lesions:

·       Eastern equine encephalitis (Togaviridae; Alphavirus)

·       Rabies

·       Equine protozoal meningoencephalomyelitis

·       Equine Herpesvirus-1



Birds (lesions/susceptibility variable between species and age groups):  (water fowl, raptors, passerines, psittacines)

·       Sudden death without premonitory signs, especially in crows 

·       Gross lesions include:

o   Lymphoplasmacytic enterocolitis

o   Cerebral necrosis with hydrocephalus ex vacuo

o   Splenitis, pancreatitis, adrenalitis, nephritis, and hepatitis

o   Diffuse lymphoid necrosis

o   Endophthalmitis and optic neuritis

o   Splenomegaly

o   Myocardial hemorrhage and pale streaks/necrosis

o   Intraosseous calvarial hemorrhage

o   Intestinal mucosal hemorrhage

o   Renal and pulmonary congestion or hemorrhage

o   One report of hemorrhagic tracheitis

·       Microscopic findings include:

o   Hemorrhage of the brain and meninges, especially cerebellum and lower brain stem, and ventral horns of the thoracolumbar spinal cord;

o   Lymphoplasmacytic encephalitis with gliosis, astrocytosis especially in the cerebrum

o   Lymphoplasmcytic pectenitis and choroiditis 

o   Necrotizing myocarditis

o   Lymphoplasmacytic hepatitis and lymphoplasmacytic and histiocytic inflammation at other sites

o   Splenic and bursal lymphoid depletion

o   Pulmonary perivascular edema and hemorrhage

o   Ocular lesions reported include (red tailed hawks and related species): Pectenitis, choroidal/retinal lymphoplasmacytic inflammation, retinal degeneration and necrosis; more pronounced choroiditis with retinal collapse, atrophy and scarring (chronic)

·       In the bird, kidney, heart, brain, and spleen most consistently harbor WNV antigen but there is species variability; another study reported the heart, brain and spinal cord as being the most effected tissues in birds (the heart is one of the main target organs in birds)

o   Neurons, glial cells AND endothelium may contain viral antigen in the CNS (also endothelium in heart)

·       Differentials include: Exotic Newcastle’s disease (Paramyxoviridae; Rubulavirus); highly pathogenic avian influenza (Orthomyxoviridae; Influenza virus A)


Other species:

·       Eastern fox squirrels:  Lymphoplasmacytic inflammation in kidney, brain, heart, and liver

·       Arctic wolf:  Severe renal lymphoplasmacytic vasculitis and multifocal cerebral cortical necrosis and gliosis

·       American alligators: Heterophilic to lymphoplasmacytic meningoencephalitis, necrotizing hepatitis, splenitis, pancreatitis, myocarditis, stomatitis, and glossitis

o   May serve as an amplifying host; develop high viremia and shed virus in feces

·       Natural infections with subsequent encephalitis have also been reported in bats, a chipmunk, a skunk, a domestic rabbit, reindeer, a white-tailed deer, gray squirrels, an alpaca, a Suffolk ewe, a Barbary macaque, a cat, and several dogs

·       SheepLymphoplasmacytic meningoencephalitis with severe perivascular cuffing, and myelitis, with brain tissue positive for WNV in neurons and axons and less in glial cells

o   High viral antigenic load in some cases (unlike horses)

o   Differential diagnosis for viral lymphocytic encephalitis in sheep includes: bluetongue, SRLV, rabies, pseudorabies, Borna disease and Nipah virus infection

·       Mice, hamsters, and Rhesus macaques have been infected experimentally

o   Gastric and small intestinal lesions (distension) reported in mice, in addition to CNS – neuron lesions



·       Alphavirus (Togaviridae):  EEE, WEE, VEE

·       Flavivirus (Flaviviridae):  WNV, Japanese encephalitis virus, St. Louis encephalitis virus, Dengue, Yellow fever, Tick-borne encephalitis virus

·       Bunyavirus (Bunyaviridae):  California encephalitis virus, LaCrosse virus

·       Phlebovirus (Bunyaviridae):  Rift Valley fever virus, Sandfly fever virus



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