Case of the Month - February 2026

Signalment and History

This juvenile Canada goose (Branta canadensis) was admitted to a rehabilitator with neurologic clinical signs. The goose was euthanized at admission. The necropsy was performed in a biosafety cabinet of the Minnesota Veterinary Diagnostic Laboratory.

Gross Findings

The animal was in a fair post-mortem preservation. It was underweight based on the small amount of internal fat.

Histopathology

Brain: Numerous perivascular infiltrates (PVIs), with up to 5 layers, were present throughout the forebrain. The PVIs mainly consisted of lymphocytes, plasma cells, and fewer histiocytes. The parenchyma had multifocally an increased cellularity due to glial proliferation and infiltration by inflammatpry cells including lymphocytes, plasma cells, histiocytes and occasional degenerate heterophils. Scattered neurons are hypereosinophilic with pyknotic nuclei and angular cell borders. 

Pancreas: Few foci of exocrine pancreas were necrotic as evidenced by the loss of cellular detail, hypereosinophilia of cells, the presence of vacuoles in the parenchyma, karyorrhexis, and minimal inflammatory infiltration.

Kidney: The epithelial cells of the collecting ducts in the medullary cones are distended and contain eosinophilic granular material (interpreted as developmental stages of a coccidian parasite (presumptive Eimeria truncata).

Figure 1: (A) Forebrain with increased cellularity due to the presence of glial cells, infiltrating inflammatory cells, and perivascular infiltrates. (B) Forebrain with perivascular cuff (PVC). The PVC is composed of numerous lymphocytes and plasma cells. Adjacent to the PVC are acidophilic neurons (black arrow) and glial cells (white arrow). (C) Immunohistochemistry of forebrain, showing strong intracytoplasmic and nuclear immunolabeling of neurons and glial cells for influenza A virus antigen. (D) Pancreas, focus of acinar necrosis, characterized by vacuolated cells with pyknotic and karyorrhectic nucleus. (E) Immunohistochemistry of pancreas with strong intracytoplasmic immunolabeling of acinar cells for influenza A virus antigen. (F) Kidney, epithelial cells of collecting ducts in medullary cones are distended by eosinophilic granular debris interpreted as developmental stages of a coccidian parasites (presumptive Eimeria truncata; white arrow). Some epithelial cells are degenerated or necrotic (black arrow).

Comments

The current highly pathogenic avian influenza (HPAI) H5N1 clade 2.3.4.4b strain was introduced into North America during the winter of 2021–2022. In Minnesota, the virus was first detected in 2022 and has since continued to circulate among wild bird populations. Unlike previous incursions, this strain has been associated with significant morbidity and mortality in a wide range of wild bird species, particularly geese, raptors, cranes, and vultures, reflecting a notable shift from the traditionally asymptomatic reservoir status of many wild birds. Besides birds, the virus has infected and caused mortality in a variety of mammals in Minnesota, including red foxes, striped skunks, domestic cats, an American black bear, and goat kids. While the virus has also been detected in dairy cows since 2024, human infection have so far not been reported in Minnesota.

In Canada geese, the infection is frequently fatal and is characterized by a multisystemic, necrotizing disease process. Prominent lesions include encephalitis with neuronal necrosis, pancreatic necrosis, and myocarditis with associated cardiomyocyte necrosis. These findings are consistent with the marked endotheliotropism of HPAI viruses, resulting in systemic viral dissemination and widespread tissue injury.

The increased susceptibility and mortality observed in wild bird species with this clade are likely related to viral adaptation, an expanded host range, and transmission dynamics such as predation and scavenging, particularly in raptors. This changing epidemiologic pattern highlights the importance of continued surveillance and reinforces the role of HPAI as a significant cause of mortality in free-ranging avian populations.

As a secondary and incidental finding, renal coccidiosis was identified, characterized by intraepithelial and intratubular gamonts within collecting ducts. The parasite is presumed to be Eimeria truncata. The exact identification and interpretation was precluded by the extent of postmortem autolysis. In adult birds, this condition is typically incidental, whereas severe infection may cause disease in juvenile waterfowl. E. truncata has a direct life cycle initiated by ingestion of sporulated oocysts containing infective sporozoites; however, the mechanism by which the parasite reaches the kidneys following intestinal infection is not fully understood. Uniquely, portions of the life cycle—particularly merogony and gametogony—occur within renal tubular epithelial cells, resulting in renal coccidiosis, a key distinguishing feature of this parasite in geese.

References

Neufeld JL, Embury-Hyatt C, Berhane Y, Manning L, Ganske S, Pasick J. Pathology of Highly Pathogenic Avian Influenza Virus (H5N1) Infection in Canada Geese (Branta canadensis); Preliminary Studies. Veterinary Pathology. 2009;46(5):966-970. doi:10.1354/vp.08-VP-0168-E-FL

Couty, M., Guinat, C., Fornasiero, D. et al. The role of wild birds in the global highly pathogenic avian influenza H5 panzootic, 2020–2023. npj biodivers 5, 1 (2026). https://doi.org/10.1038/s44185-025-00114-5

Smith DA. Palaeonathae: Apterygiformes, Casuariiformes, Rheiformes, Struthioniformes; Tinamiformes. In: Terio KA, McAloose D, St. Leger J, ed. Pathology of Wildlife and Zoo Animals, Cambridge, MA Academic Press; 2018: 644