Case History: In September 2021, Wisconsin Department of Natural Resources reported a mortality event involving two eastern gray squirrels (Sciurus carolinensis) and three eastern cottontail rabbits (Sylvilagus floridanus) in a residential neighborhood in Wisconsin, U.S.
Pathology Case of the Month - Eastern Gray Squirrel
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.
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.
- Brain: Encephalitis, lymphoplasmacytic, subacute to chronic, moderate, with gliosis and neuronal necrosis
- Heart: Myocarditis, lymphoplasmacytic, subacute to chronic, mild to moderate
- 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).
- Byas AD, Ebel GD. 2020. Comparative pathology of West Nile virus in humans and non-human animals. Pathogens. 9(1):48. https://doi.org/10.3390/pathogens9010048
- Gamino V, Höfle U. 2013. Pathology and tissue tropism of natural West Nile virus infection in birds: a review. Vet Res. 44(1):39. https://doi.org/10.1186/1297-9716-44-39
- Heinz-Taheny KM, Andrews JJ, Kinsel MJ, Pessier AP, Pinkerton ME, Lemberger KY, Novak RJ, Dizikes GJ, Edwards E, Komar N. 2004. West Nile virus infection in free-ranging squirrels in Illinois. J Vet Diagn Invest. 16(3):186-90. https://doi.org/10.1177/104063870401600302
- Johnston BL, Conly JM. 2000. West Nile virus - where did it come from and where might it go?. Can J Infect Dis. 11(4):175-178. https://doi.org/10.1155/2000/856598
- Marra PP, Griffing S, Caffrey C, Kilpatrick MA, McLean R, Brand C, Saito E, Dupuis AP, Kramer L, Novak N. 2004. West Nile virus and wildlife. BioScience. 54(5):393–402 https://doi.org/10.1641/0006-3568(2004)054[0393:WNVAW]2.0.CO;2
- Nevarez JG, Mitchell MA, Morgan T, Roy A, Johnson A. 2008. Association of West Nile virus with lymphohistiocytic proliferative cutaneous lesions in American alligators (Alligator mississippiensis) detected by RT-PCR. J Zoo Wildl Med. 39(4):562-6. https://doi.org/10.1638/2007-0133.1
- Padgett KA, Reisen WK, Kahl-Purcell N, Fang Y, Cahoon-Young B, Carney R, Anderson N, Zucca L, Woods L, Husted S, Kramer VL. 2007. West Nile virus infection in tree squirrels (Rodentia: Sciuridae) in California, 2004-2005. Am J Trop Med Hyg. 76(5):810-3. https://doi.org/10.4269/ajtmh.2007.76.810
- Root JJ, Bosco-Lauth AM. 2019. West Nile virus associations in wild mammals: An update. Viruses. May 21;11(5):459 https://doi.org/10.3390/v11050459
- Suen WW, Uddin MJ, Wang W, Brown V, Adney DR, Broad N, Prow NA, Bowen RA, Hall RA, Bielefeldt-Ohmann H. 2015. Experimental West Nile virus infection in rabbits: An alternative model for studying induction of disease and virus control. Pathogens. 4(3):529-58. https://doi.org/10.3390/pathogens4030529
- Tiawsirisup S, Platt KB, Tucker BJ, Rowley WA. 2005. Eastern cottontail rabbits (Sylvilagus floridanus) develop West Nile virus viremias sufficient for infecting select mosquito species. Vector Borne Zoonotic Dis. 5(4):342-50. https://doi.org/10.1089/vbz.2005.5.342