Gross Findings: Three out of five American Crows were examined at necropsy. The birds were subadults of undetermined sex that were in fair body condition and poor postmortem condition. The major gross finding in all three birds was mild small intestinal dilation with segmental to diffuse red-purple discoloration of the small intestinal serosa (Fig. 1A). Small intestinal mucosa was red and the lumen contained red-brown, soft, granular necrotic material (necrotizing enteritis) (Fig. 1B).

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Histopathological Findings: Microscopic changes were similar for all three birds. There was segmental necrosis of the small intestinal mucosa characterized by superficial to mild to moderate villar necrosis and degenerate inflammatory cells (identification was prevented by autolysis) effacing the mucosa with eosinophilic necrotic cellular debris and mineral in the lumen (Fig. 2A). Multifocally in primarily the red pulp spleen there were foci of eosinophilic cellular debris (necrosis) and degenerate inflammatory cells (identification was prevented by autolysis) (Fig. 2B). Lymphoid depletion in the bursa of Fabricius was present in two birds, and one bird had minimal random hepatocellular necrosis in the liver. Additionally, all three birds had microfilariae in pulmonary blood vessels, and one bird had Filariid nematodes within the great vessels adjacent the thyroid gland (parasites considered an incidental finding in this case). Also, all birds had marked vacuolization of the tracheal epithelial cells, the cause and significance of which is unknown.

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Ancillary Tests: Reovirus was detected by virus isolation in Vero cell culture with subsequent PCR on small intestine from two of three birds tested. Tracheal and cloacal swabs from all five birds were negative for avian influenza by matrix RT-PCR.

Morphologic Diagnoses:

1. Intestines: Moderate to marked segmental to diffuse acute necrotizing enteritis
2. Spleen: Marked multifocal splenic necrosis
3. Bursa of Fabricius: Marked lymphoid depletion

Disease: Winter Mortality of Crows (WMC)

Etiology: A novel Orthoreovirus sp. is the cause of WMC. Based on genetic analyses, a new genus of Corvid orthoreovirus has been proposed.

Distribution: WMC occurs mainly in the United States and Canada, with a few cases reported in Europe. Avian reoviruses in general have a worldwide distribution.

Seasonality: This disease occurs primarily in the winter months when crows form large roosts and have close contact with one another.

Host range: WMC primarily affects wild American Crows in North America with no age predilection. There have been sporadic reports in other corvids in Europe, including a Magpie (Pica pica) in Great Britain, a Hooded Crow (Corvus corone cornix) in Finland, and a Carrion Crow (Corvus corone) in Belgium. Other avian reoviruses have been isolated from wild birds, including storks, herons, buzzards, and pigeons, with no signs of disease.

Transmission: Fecal-oral transmission is the primary mode of transmission.

Clinical signs: Birds are often found dead. Reported clinical signs have included obtundation, increased respiratory effort, weakness, head tilt, and difficulty standing.

Pathology: On gross examination, the body condition varies from good to poor. The intestines, ceca, and/or colon are often purple to black, dilated, thin-walled, or turgid. The lumina can contain fibrinonecrotic or mucohemorrhagic cores. Splenomegaly and pinpoint tan foci in the spleen may be present. Occasionally there are pinpoint to 1-2 mm in diameter tan foci in the liver. Microscopically there is necrohemorrhagic enteritis and typhlocolitis, and fibrinonecrotizing splenitis. Occasionally, necrotizing hepatitis occurs. Some birds may have minimal to no lesions aside from splenic and/or bursal lymphoid depletion.

Diagnosis: Confirmation of Orthoreovirus sp. infection can be made by PCR or virus isolation. Other available methods include immunohistochemistry, in-situ hybridization, and transmission electron microscopy.

Public health concerns: None. Avian reoviruses do not infect humans or other mammals.

Wildlife population impacts: Epizootics of WMC can occur in consecutive or nonconsecutive years. Where there are large roosting colonies, hundreds of crows may be found dead due to this disease.

Management: There is no treatment for avian reovirus infections. These viruses are relatively resistant outside the host and may survive on feathers, wood shavings, metal, and glass for up to ten days, and even longer in water. There are vaccines available to control avian reovirus infections in domestic poultry.

References:

• Cornell Wildlife Health Lab. 2018. Avian Reovirus. Cornell University College of Veterinary Medicine Animal Health Diagnostic Center. https://cwhl.vet.cornell.edu/disease/avian-reovirus. Accessed online June 2022.
• Dandár E, Huhtamo E, Farkas SL, Oldal M, Jakab F, Vapalahti O, Banyai K. 2014. Complete genome analysis identifies Tvärminne avian virus as a candidate new species within the genus Orthoreovirus. J Gen Virol 95:898–904. https://doi.org/10.1099/vir.0.060699-0
• Forzán MJ, Renshaw RW, Bunting EM, Buckles E, Okoniewski J, Hynes K, Melendez R, Ableman A, Laverack M, Fadden M, Dastjerdi A, Schuler K, Dubovi EJ. 2019. A novel orthoreovirus associated with epizootic necrotizing enteritis and splenic necrosis in American crows (Corvus brachyrhynchos). J Wildl Dis 55(4): 812-822. https://doi.org/10.7589/2019-01-015
• Huhtamo E, Uzcátegui NY, Manni T, Munsterhjelm R, Brummer-Korvenkontio M, Vaheri A, Vapalahti O. 2007. Novel Orthoreovirus from Diseased Crow, Finland. Emerg Infect Dis 13(12): 1967-1969. https://doi.org/10.3201/eid1312.070394
• Lawson B, Dastjerdi A, Shah S, Everest D, Núñez A, Pocknell A, Hicks D, Horton DL, Cunningham AA, Irvine RM. 2015. Mortality associated with avian reovirus infection in a free-living magpie (Pica pica) in Great Britain. BMC Vet Res. 11:20. https://doi.org/10.1186/s12917-015-0329-5
• Meteyer C, Docherty D, Ip H, Ramsay N, Saito E, Oaks L. 2009. Reovirus-associated necrotizing enteritis in American crows. In: Program and abstracts of the 58th annual international conference, Wildlife Disease Association, Blaine, Washington, 2–7 August, p. 79.