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Case History: In January, an adult female 4360-g Bald Eagle (Haliaeetus leucocephalus) was observed on the ground within a few miles of the Lower Wisconsin River in Wisconsin, USA. The eagle was unable to fly, ataxic, shivering, convulsing, and vomiting.

The bird was taken to a rehabilitation facility where seizures, tachypnea, intermittent open mouth breathing, and rapid constriction and dilation of the pupils were observed. The eagle died overnight.

Gross Findings: On external examination, a small amount of fecal material is present in the feathers around the vent. On internal examination, there is a moderate amount of subcutaneous, visceral and epicardial fat. Kidneys are dark red (congestion). The cerebellum is diffusely red. The gall bladder is severely distended with dark green bile. A 2 x 0.6 x 0.2 cm green and tan, firm plaque is present on the right abdominal air sac. The mid-esophagus contains a 3.8 x 1 x 1 cm twig. The proventriculus is empty. The ventriculus contains a single tan small nematode and a small amount of green-brown material. Intestines contain scant tan to green digesta.

Histopathological Findings: Diffusely, hepatocytes contain one or more variably-sized intracytoplasmic round, clear vacuoles (lipid; Fig. 1A). There is bile stasis (Fig. 1B). Multifocally in the brainstem, there are small perivascular hemorrhages (Fig. 1C). Renal tubular epithelial cells occasionally contain intracytoplasmic clear, round vacuoles (suspect lipid) or brown-orange pigment (Fig 1D). Occasional tubules in medullary cones are dilated with attenuated epithelium and contain eosinophilic intraluminal material. There is scattered renal mineralization. There is severe splenic lymphoid depletion. The grossly observed air sac lesion consists of a granuloma with a core of eosinophilic necrotic debris surrounded by a thick layer of macrophages and multinucleated giant cells and rimmed by fibrous connective tissue. A single sarcocyst is present within the cytoplasm of a cardiomyofiber. There is moderate freeze artifact.

Photomicrographs with H&E stain from a Bald Eagle found dead in Wisconsin, USA
Figure 1. Photomicrographs with H&E stain from a Bald Eagle (Haliaeetus leucocephalus) found dead in Wisconsin, USA. (A) Hepatocytes contain one or more variably-sized intracytoplasmic round, clear vacuoles (arrowheads). (B) Bile ducts are plugged with bile (asterisk). (C) Multifocally in the brainstem, there are small perivascular hemorrhages (arrows). (D) Small round vacuoles (arrowheads) are occasionally present in the cytoplasm of renal tubular epithelial cells.  

Laboratory findings: No lead was detected in the liver. Brain cholinesterase levels in duplicate samples were 11 umoles/min/goT and 10.26 umoles/min/goT after incubation indicating a 31% and 36% decrease below the control average, respectively. This is suggestive of a non-lethal exposure to an organophosphate pesticide. An avian influenza virus matrix RT-PCR screen using tracheal and cloacal swabs was negative. A West Nile virus RT-PCR screen using pooled liver, kidney, and spleen was negative. Virus isolation using pooled liver, kidney, and spleen was negative. Routine and fungal culture of the air sac lesion yielded no growth. Aerobic and anaerobic culture of the intestine yielded light mixed growth of no significance.

Morphologic Diagnoses:

  1. Hepatic lipidosis, diffuse, severe with bile stasis
  2. Brainstem perivascular hemorrhage, multifocal, mild, acute
  3. Renal tubular epithelial cell vacuolization, multifocal, mild
  4. Air sac granuloma, focal, chronic
  5. Cardiac sarcocyst, focal

Disease: Wisconsin River Bald Eagle Syndrome (WRBES)

Etiology: Undetermined

Host range: Bald eagles typically in good body condition.

Distribution: Occurs within several miles of the Lower Wisconsin River, Wisconsin, USA.

Seasonality: Primarily winter, but cases have occurred from November to April.

Clinical signs: Weakness, seizures, tremors, vestibular signs, difficulty or inability to fly or walk, falling from the roost, vomiting, and green feces.

Pathology: The most consistent histopathological finding is diffuse hepatic lipidosis. Changes in the brain may include perivascular hemorrhage and cuffing (both mononuclear cells and granulocytes) and astrocytes with swollen nuclei (resembling Alzheimer Type II cells). There may be vacuolization of renal tubular epithelial cells.

Diagnosis: Diagnostically compatible field signs and histopathological lesions of WRBES with no detected etiology in a Bald Eagle found near the Lower Wisconsin River.

Public health concerns: None

Wildlife population impacts: None


McLaughlin GS, Beheler KA, Sileo L, Strom S, Thomas NJ, Meteyer CU, Green DE, Miller KJ. 2004. Winter mortality of bald eagles along the Lower Wisconsin River. In: Proceedings of the Joint Conference of the American Association of Zoo Veterinarians, American Association of Wildlife Veterinarians, and Wildlife Disease Association: health and conservation of captive and free-ranging wildlife, San Diego, CA, August 28–September 3, pp. 312–313.

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