Adult female bald eagle, Haliaeetus leukocephalusThis animal was admitted to The Raptor Center of the University of Minnesota on September 19, 2013. The animal was underweight and unable to fly. It had neurologic signs including nystagmus and muscle tremors. Blood lead levels were low. The animal was euthanatized one day after admission due to a grave prognosis for survival and rehabilitation.
The animal was moderately underweight.Â There was marked bilateral symmetrical pan-necrosis of the caudal third of the cerebral hemispheres with collapse of the parenchyma and increased quantity of cerebrospinal fluid (hydrocephalus ex vacuo) when compared to a control brain of a bald eagle.Â
Both slides (C and D) are cross sections of the caudal aspects of cerebrum at around the level of the optic chiasm (with the more rostral aspect of the thalamus) and had similar histologic features.Â The lateral ventricles were dilated.
There was bilateral symmetric pan-necrosis of the gray matter of the dorsal aspect of the cerebral hemispheres (pallium) along the lateral ventricles.Â The neuroparenchyma was collapsed and cavitated around what appeared to be a remaining scaffold of vasculature.Â The neuroparenchyma was largely infiltrated and replaced by numerous macrophages with gitter cell morphology in the pallium.Â In addition, there was a widespread massive lymphoplasmacytic perivascular infiltration.Â The endothelial cells were hypertrophied.Â Numerous cells contained basophilic granular material (interpreted to be calcified mitochondria) and occasional neurons were entirely calcified.
The gray matter adjacent and subjacent to the necrotic parenchyma was hypercellular.Â The hypercellularity was due to infiltration with lymphocytes and macrophages and due to infiltration by reactive astrocytes.Â These astrocytes were plump, had an increased amount of a faint eosinophilic cytoplasm and one and occasionally two enlarged vacuolar (euchromatic) nuclei.Â Occasionally distinctly eosinophilic globules were present in the gray matter (interpreted to be spheroids).
White matter tracts such as the occipitomesencephalic tracts and optic tracts had a bilateral symmetric spongiform change characterized by the presence of numerous optically empty spaces (interpreted as myelin sheath edema).Â In addition, mild to moderate infiltrates of lymphocytes and plasma cells were present around capillaries of the white matter.
Brain (cerebrum and thalamus), polioencephalitis, lymphoplasmacytic and histiocytic, chronic, marked with cerebral pan-necrosis and dilation of lateral ventricles (hydrocephalus ex vacuo)
Cerebrospinal fluid and brain samples were positive for West Nile virus (WNV) and negative for Saint Louis encephalitis virus by PCR (performed at the Animal Health Diagnostic Center of Cornell University).Â West Nile virus antigen was detected in the cerebrum and retina by immunohistochemistry using a monoclonal antibody specific for the E protein of WNV (clone 7H2, Bioreliance).
West Nile Virus
The lesions were highly suggestive of an ischemic injury or massive viral encephalitis.Â A protozoal encephalitis (e.g.Â Sarcocystis falcatula) seemed to be unlikely based on the absence of protozoal organisms in the HE-stained sections although protozoal encephalitis cannot be ruled out entirely.(7) West Nile virus (WNV) is a member of the Japanese encephalitis and Saint Louis encephalitis antigenic complex (family: Flaviviridae).Â The virus is primarily transmitted by mosquitoes, but direct contact with infected animals and via oral uptake of virus infected tissues have also been reported.Â Raptors including bald eagles are frequently infected by WNV and succumb to West Nile disease.(1,4)
Gross lesions of WN disease in bald eagles may include macroscopically appreciable cerebral pan-necrosis as in the presented case in approximately half the cases and in individual cases may include myocarditis and endophthalmitis.(10) The presence of a triad of histopathological changes including encephalitis, endophthalmitis, and myocarditis are a hallmark of WN disease in bald eagles and other Accipitridae including Coopers hawks (Accipiter cooperi), red tailed hawks (Buteo jamaicensis), and goshawks (Accipiter gentilis).(5,8-10) The pathogenesis of the cerebral pan-necrosis may include a vasogenic component due to damage of endothelial cells by the virus during an early phase of the infection or due to damage of the capillary integrity in the wake of the inflammatory response by extravasating inflammatory cells.Â Overt WNV-associated arterial necrosis has been described in kestrels and other falcons but does not appear to be a prominent feature of WN disease in hawks and eagles.(4,12) Alternatively or concurrently, the pathogenesis of the pan-necrosis may include direct cytolysis by the virus targeting neurons and possibly glial cells and/or cytolysis of neurons and glial cells as a result of bystander injury in the context of the antiviral immune response.Â High WNV antigen concentrations are present in the cerebrum (and cerebellum) of bald eagles with fairly acute WNV-associated encephalitis.(10) The cerebral necrosis was similar to a lesion described in one pigeon that was experimentally infected with highly pathogenic avian influenza virus.(2)
Based on immunohistochemical analysis, brain (cerebrum and cerebellum), eyes (retina) and to a lesser extent heart and kidney harbor viral antigen similar to findings in other Accipitridae.Â PCR of brain tissue commonly yields a positive result in bald eagles with West Nile disease except for cases that had a significantly prolonged disease, e.g.Â because they were kept and cared for at a rehabilitation facility for months.Â In these animals, detection of WNV-specific antibodies in the CSF may be helpful to confirm WN encephalitis.(10)
Brain: Encephalitis, necrotizing, bilateral, severe, with lymphoplasmacytic perivascular cuffing, white matter spongiosis, mineralization and hydrocephalus ex vacuo.
This is an exceptional example of West Nile virus (WNV), the arthropod-borne Flavivirus which utilizes many avian species as a natural reservoir.Â WNV is found throughout the world and is spread among the migratory bird population.Â Passeriformes are considered most susceptible to disease, to include the American crow, the blue jay, and the American robin.Â WNV is maintained in a silent mosquito-bird cycle in natural habitats until introduced into humanized areas where it cycles through humans and horses.Â This cycle accompanies a seasonal cycle, as the earliest infections arise in late spring and taper off in the fall.Â Human and horse cases are usually preceded by a few weeks of avian mortalities.(1)
WNV has a wide range of tissue tropism, thus there are no pathognomonic macroscopic lesions.Â Multiorgan hemorrhages are most characteristically found, and specifically to raptors, cerebral atrophy and malacia is often observed.Â Microscopically, lymphoplasmacytic and histiocytic inflammation, necrosis, and hemorrhage occur most commonly in the CNS, heart, kidney, spleen and liver.Â Paradoxically, the most susceptible species have the least amount of inflammation, and raptors which contract more chronic disease develop the triad of lesions described by the contributor.(1)
WNV contains two envelope glycoproteins, E1 and E2, which facilitate tropism for specific tissues.(11) Neuroinvasion requires crossing the blood-brain barrier and may be assisted by the release of proinflammatory cytokines.(11) The chemoreceptor CCR5 contributes to neuroinvasive resistance in humans, as those with CCR5 mutations have an increased rate of symptomatic infection, though this link has not been identified in animals.(3)
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