The animals were described as ataxic and unable to stand. NWHC received one squirrel and one rabbit found dead in the neighborhood for cause-of-death determination. The cause of death of the rabbit was undetermined. A summary of the diagnostic investigation for the squirrel is provided below.

Gross Findings: The squirrel was a juvenile male in fair body condition. There were no significant gross abnormalities to indicate a cause of death.

Histopathological Findings:

There is widespread mild to moderate lymphoplasmacytic perivascular cuffing within the brain and meninges (Fig. 1A, B). Rare neutrophils are admixed with the perivascular infiltrate or are within adjacent neuropil (Fig. 1C). There is frequent gliosis (Fig. 1A, B) and occasional neuronal necrosis. In the heart, there is mild multifocal lymphoplasmacytic interstitial inflammation associated with areas of fibrosis and myocyte loss; atria are most affected (Fig. 1D). In the spleen, there is moderate lymphoid depletion.

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Morphologic Diagnoses:

1. Brain: Encephalitis, lymphoplasmacytic, subacute to chronic, moderate, with gliosis and neuronal necrosis
2. Heart: Myocarditis, lymphoplasmacytic, subacute to chronic, mild to moderate
3. Spleen: Lymphoid depletion, moderate

Ancillary testing: The squirrel tested positive for West Nile virus by PCR and virus isolation on pooled kidney and spleen.

Disease: West Nile virus infection.

Etiology: West Nile virus (WNV)

Distribution: Worldwide; WNV is the most widely distributed arbovirus in the world

Seasonality: In North America, cases occur most often in the summer and fall, corresponding with periods of highest mosquito abundance.

Host range: Host range is broad and includes many avian species, as well as domestic and wild mammals. Tree squirrels, including eastern gray squirrels, are reported to be among the mammals most susceptible to WNV infection and disease. Squirrels may shed WNV in oral secretions and, unlike most mammals, can even infect mosquitoes due to high viremia.

In North America, other wild mammals documented with WNV infection or exposure include the white-tailed deer (Odocoileus virginianus), Virginia opossum (Didelphis virginiana), raccoon (Procyon lotor), striped skunk (Mephitis mephitis), and black bear (Ursus americanus). Eastern cottontail rabbits have proven to be susceptible to WNV in laboratory settings and can also develop viremias capable of infecting mosquitoes, but are not known to develop clinical disease. American alligators (Alligator mississippiensis) are also susceptible to WNV infection.

Transmission: WNV is typically transmitted through a mosquito vector and is maintained through a mosquito-to-bird transmission cycle. Most mammals, including humans, are considered dead-end hosts, lacking levels of viremia sufficient to re-infect mosquitoes.

Clinical signs: Neurologic signs are typical of clinical cases, although many avian, mammalian, and human cases are asymptomatic.

Pathology: In birds, pathologic findings can include lymphoplasmacytic meningoencephalitis, myocarditis, and multisystemic inflammation, including in the kidneys and spleen. Ocular involvement is reported in hawks, eagles, and owls. In contrast, reports of WNV infection in mammals, including humans, horses, sheep, and most wild mammals, are typically associated with meningoencephalitis and myelitis, with other systems less often impacted. However, microscopic findings of both meningoencephalitis and myocarditis in this gray squirrel from Wisconsin are similar to those reported in WNV-positive gray and fox squirrels (Sciurus niger) from Illinois. Lymphohistiocytic dermatitis is reported in WNV-infected American alligators. 

Diagnosis: Diagnosis can be made through a combination of the presence of neurologic clinical signs, characteristic pathologic lesions, and identification of the virus by PCR or virus isolation. Serology can be performed to document the production of antibodies, indicating exposure.

Public health concerns: Due to its zoonotic nature, WNV is a public health concern. Human infection is typically through a mosquito vector rather than through direct or indirect contact with infected wildlife.

Wildlife population impacts: Local and regional population impacts have been documented in highly susceptible bird species, including American Crows (Corvus brachyrhynchos) and Great-horned Owls (Bubo virginianus).

References:

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