Case of the Month - September 2025

Signalment and History

This nestling Cooper’s hawk (Accipiter cooperi) was presented with neurological signs that did not improve despite supportive care. The animal was euthanized and submitted for necropsy.

Gross Findings

The bird was underweight, with an enlarged liver and spleen. 

Figure 1: The liver is moderately enlarged as evidenced by its rounded margins and its significant extension caudally into the coelomic cavity.

Histopathology

Heart (Figure 2): Approximately 50% of the myocardium is affected by a multifocal to coalescing, moderate to severe inflammatory infiltrate composed predominantly of lymphocytes and macrophages, which infiltrates and partially replaces myocardial fibers. Affected cardiomyocytes are variably separated, compressed, and occasionally lost, resulting in disruption of normal myocardial architecture. 

Brain (Figure 3. A&B): The neuroparenchyma is diffusely and markedly hypercellular, characterized by pronounced gliosis and widespread infiltration by lymphocytes, plasma cells, and macrophages. Multifocal perivascular cuffs composed predominantly of lymphocytes and histiocytes are consistently present throughout the gray and white matter and typically measure one to two cell layers in thickness.

Eyes (pecten, Figure 4): The pecten of both eyes is moderately expanded by multifocal accumulations of mononuclear inflammatory cells, primarily lymphocytes and macrophages, which surround and expand the capillaries. The vascular architecture of the pecten is preserved, but intervascular spaces are widened by inflammatory infiltrates.

Liver: Scattered randomly throughout the hepatic parenchyma are small, multifocal clusters of degenerate and occasionally necrotic hepatocytes. These foci are surrounded by small numbers of lymphocytes and plasma cells, with minimal disruption of the surrounding hepatic cords.

Figure 2: Heart, marked mononuclear inflammation, H&E. Inset: Strong cytoplasmic immunolabeling for WNV antigen, IHC-DAB. 

Figure 3: A. Brain, perivascular cuffing and gliosis, H&E. B. Brain, neuronal necrosis (arrow) H&E. C. Strong cytoplasmic immunolabeling for WNV antigen, IHC-DAB. 

Figure 4: Eye, pecten with marked mononuclear inflammation characterized by the perivascular infiltration. H&E.  

Ancillary Testing

• Molecular diagnostics: A brain sample was positive for West Nile virus (targeting the nucleoprotein gene) by PCR (performed at the Minnesota Department of Health).
• Immunohistochemistry: Heart (Figure 2. Inset) - Strong and abundant immunolabeling for WNV envelope protein antigen was present in the cytoplasm of cardiomyocytes.
  Brain (Figure 3. C)- Strong and abundant immunolabeling for WNV antigen was present in cytoplasm of glial cells and neurons and their processes.
• Bacteriology: Liver: no growth by aerobic culture

Morphologic Diagnosis

1. Brain, meningoencephalitis, lymphohistiocytic, widespread, marked
2. Heart, myocarditis, lymphocytic and histiocytic, widespread, marked
3. Eyes, pectenitis, non-suppurative, multifocal, moderate
4. Liver, hepatitis, lymphoplasmacytic and histiocytic, widespread, moderate, with mild hepatocellular necrosis

Etiology

West Nile virus (Flavivirus).

Discussion

West Nile virus (WNV) is a mosquito-borne flavivirus capable of infecting a broad range of avian species. The first recognized cases in North America were identified in American crows during the 1999 outbreak in the greater New York City area. Transmission occurs primarily through the bite of infected Culex mosquitoes; however, direct contact with infected animals and oral ingestion of WNV-infected tissues have also been documented as alternative routes.

The epidemic spread westwards throughout the entire North American continent in subsequent years and reached the spring of Midwest in 2002. Ever since the Midwest (including Minnesota) is experiencing recurrent waves of WNV infection in birds, humans, and the occasional horse, fox squirrel, white tailed deer, reindeer, seal, new world camelid, and sheep. Among the birds, birds of prey, including Cooper’s, bald eagles, red-tailed hawks, and great horned owls (among others) are commonly infected and succumb to the disease. In the Midwest region, cases typically occur from summer (as early as mid-June) through fall, peaking in late August to early September, with risk persisting into October.

Gross lesions in affected raptors are uncommon but if they occur have been described in brain, heart, and eyes. Microscopic changes are common and may follow a characteristic pattern dominated by severe mononuclear encephalitis, myocarditis, and pectenitis, as exemplified in this case. Diagnosis is confirmed by PCR, virus isolation, or immunohistochemistry (IHC). The cause of the hepatic necrosis was not apparent in the presented specific case. Cellular inflammatory infiltrates in the liver can be a nonspecific finding in raptors with systemic infections and may be interpreted as a reactive state.

WNV infection is a zoonotic disease, and human infections can occur during the transmission season usually after mosquito bites but rarely also due to direct inoculation of virus e.g. into cut wounds while performing postmortem examinations of infected birds. The 2025 summer and fall season was particularly active with respect to WNV infections with 110 symptomatic human cases reported in Minnesota alone, including two deaths. What may initially present as typical itchy mosquito bites can progress to more serious illness, including fever, severe headache, vomiting, muscle fatigue, or weakness. Individuals experiencing these symptoms should seek evaluation from a healthcare professional.

References                                                                        

1. Wünschmann A, Shivers J, Bender J, Carroll L, Fuller S, Saggese M, van Wettere A, Redig P. Pathologic Findings in Red-Tailed Hawks (Buteo jamaicensis) and Cooper's Hawks (Accipiter cooperi) Naturally Infected with West Nile Virus," Avian Diseases. 2004;48(3), 570-580.
2. Wünschmann A, Shivers J, Carroll L, Bender J. Pathological and Immunohistochemical Findings in American Crows (Corvus brachyrhynchos) Naturally Infected with West Nile Virus. Journal of Veterinary Diagnostic Investigation. 2004;16(4):329-333. doi:10.1177/104063870401600413
3. West Nile Virus Cases Continue to Rise in Minnesota – The Public Health Laboratory Respond. https://www.health.mn.gov/about/org/phl/annualreports/2025/westnile.html?utm_source=chatgpt.com

Funding provided by the Minnesota Environment and Natural Resources Trust Fund.