Gross Findings: This subadult female Bald Eagle was in excellent body condition, with abundant subcutaneous, visceral, and epicardial fat stores. The bird was in fair postmortem condition. The pancreas was reddened and had 1-2 dozen multifocal pinpoint to 5 x 5 mm diameter tan foci throughout (Fig. 1A). The spleen was dark purple and had 1-2 dozen pinpoint white foci throughout (Fig. 1B). The liver contained 1-2 dozen linear black foci throughout ranging from 1 x <1 mm to 2 x 1 mm. The gall bladder was distended with dark green, slightly viscous bile. The external surface of the cerebellum was dark red. Remaining tissues not described were within normal limits.

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Histopathological Findings: In one region of the cerebrum there are small perivascular cuffs of mononuclear cells and rare heterophils with scattered foci of gliosis, and rare acutely necrotic neurons (Fig. 2A). There are foci of acute necrosis in multiple organs, including the pancreas (Fig. 2B), liver, spleen, kidney (Fig. 2C), and adrenal glands. The heart has small foci with loss of myofibers and replacement by degenerate mononuclear cells (Fig 2D).  In addition, there is fibrinoid vascular necrosis of a few arteries in the portal triads of the liver, moderate lymphoid depletion in the spleen, marked heterophilic inflammation segmentally in the ureters, and a small amount of acute myofiber degeneration in the pectoral and thigh skeletal muscles. Incidentally, there is mild to moderate parasitism present, with trematode eggs and black pigment within the liver with associated fibrosis and inflammation, moderate numbers of Capillarid nematodes and eggs with bipolar plugs in the mucosa of the esophagus, 3-4 degenerate parasites in the mucosa of the small intestine, and rare protozoal cysts within small numbers of myofibers in the heart.

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Ancillary Test(s): In-house testing detected the avian influenza matrix gene, the H5 gene, and the Eurasian lineage clade H5 2.3.4.4 strain in tracheal and cloacal swabs by rRT-PCR; the H7 gene was not detected. Additional testing performed at the USDA National Veterinary Services Laboratories (NVSL) confirmed the Eurasian lineage H5 clade 2.3.4.4 and the N1 gene by rRT-PCR on the tracheal swab. The final report from NVSL with avian influenza hemagglutinin cleavage site analysis was pending at the time of this publication. In the current HPAI outbreak in wild birds in the USA beginning in January 2022, EA/AM 2.3.4.4 H5N1 is the common reassortant virus that is causing disease.

Morphologic Diagnoses:

1. Severe multifocal to coalescing acute pancreatic necrosis
2. Mild mononuclear perivascular encephalitis with moderate multifocal gliosis and mild focally extensive acute neuronal necrosis
3. Mild multifocal acute hepatic, spleen, renal, and adrenal cortical necrosis
4. Mild multifocal subacute cardiac myofiber necrosis
5. Marked segmental heterophilic ureteritis
6. Liver, portal arteries: Moderate multifocal segmental to circumferential fibrinoid vascular necrosis

Disease: Highly pathogenic avian influenza (HPAI)

Etiology: Highly pathogenic avian influenza is caused by an influenza A virus. Avian influenza virus is deemed highly pathogenic based on its ability to cause morbidity and mortality in domestic poultry.  The cause of the current HPAI outbreak in wild birds in the United States that began in January 2022 is H5 clade 2.3.4.4b.

Distribution: Worldwide

Host range: In general, all wild birds are susceptible to HPAI, though waterfowl (i.e., geese, swans, and ducks) are most commonly infected. Apparently healthy waterfowl have been shown to carry HPAI, and some groups of wild birds, such as raptors, appear to be more susceptible to disease than others. Songbirds appear to be low risk for HPAI infection compared to other wild birds. Mammals, including humans, may also be infected.

Raptor species from which HPAI virus has been detected in the current HPAI outbreak in the USA include American Kestrel (Falco sparverius), Bald Eagle, Barred Owl (Strix varia), Broad-winged hawk (Buteo platypterus), Cooper’s Hawk (Accipiter cooperii), Crested Caracara (Caracara plancus), Eastern Screech Owl (Megascops asio), Golden Eagle (Aquila chrysaetos), Great Horned Owl (Bubo virginianus), Harris Hawk (Parabuteo unicinctus), Long-eared Owl (Asio otus), Merlin (F. columbarius), captive Northern Harrier (Circus hudsonius), Osprey (Pandion haliaetus), Peregrine Falcon (F. peregrinus), Prairie Falcon (Falco mexicanus), Red-shouldered Hawk (B. lineatus), Red-tailed Hawk (B. jamaicensis), Rough-legged Hawk (B. lagopus), Sharp-shinned Hawk (A. striatus), Short-eared Owl (A. flammeus), Snowy Owl (B. scandiacus), Swainson’s Hawk (Buteo swainsoni), and Western Screech Owl (Megascops kennicottii).

Transmission: HPAI virus is found in oral and nasal secretions and in feces. It is maintained and spread by a fecal-oral transmission and inhalation. Migratory waterfowl may spread avian influenza viruses over long distances. HPAI virus is also found within tissues, thus making raptors susceptible to infection because they will feed on infected waterfowl.

Clinical signs: There are often no clinical signs associated with HPAI in wild birds. When present, neurologic signs are most common and include swimming in circles, twisted necks, and tremors. Clinical signs in poultry are highly variable, including neurologic, respiratory, circulatory, and enteric abnormalities.

Pathology: Gross findings vary depending on the species affected. Common findings include white to red foci within the pancreas indicative of necrosis and hemorrhage, white foci in the spleen and liver, and reddening of the cerebellum.  However, some cases may have no obvious gross changes. Microscopic findings are variable and include inflammation in the meninges and brain, gliosis, neuronal necrosis, necrosis in the pancreas, liver, and spleen, necrosis and/or inflammation in the heart, and occasionally necrosis in the kidneys, adrenal glands, gonad, and intestine. In a small number of Bald Eagles examined at NWHC, encephalitis, gliosis, renal necrosis, myocardial necrosis and inflammation, and splenic necrosis were the most common histologic lesions.

Diagnosis: Tracheal and cloacal swabs can be used for identifying infected wild birds that are not showing clinical signs. As gross and microscopic lesions are not necessarily specific to HPAI, confirmation using PCR on tracheal or cloacal swabs or tissues with microscopic changes is required.

Public health concerns: The HPAI virus has the potential to infect humans, though reports are rare. HPAI should be considered a human health concern and appropriate precautions such as personal protective equipment and hand washing should be used if handling sick or dead birds.

Wildlife population impacts: The susceptibility of wild birds in North America to recently discovered strains of HPAI continues to be investigated. As of March 14, 2023, HPAI virus has been detected in 6,444 live, hunter harvested, or dead wild and captive wild birds in all states except Hawaii, of which 1,215 have been raptors. As some populations of wild birds are more susceptible to HPAI, the emergence of this disease may be especially detrimental in certain threatened or endangered species.

Management: Removal and disposal of potentially infected carcasses by properly trained individuals may be warranted to reduce scavenger, domestic animal, or human exposure. Visit unaffected sites first (if necessary) and then go to affected sites to avoid cross contamination of sites. Work areas/surfaces, equipment, and vehicles should be washed to remove organic material and then disinfected after use and between sites. Limiting contact with pet birds and poultry for 5 days after potential exposure to an HPAI infected bird may be warranted to prevent spread of potentially infectious agents. 

References:

• Cornell Wildlife Health Lab. 2018. Avian Influenza. Cornell University College of Veterinary Medicine Animal Health Diagnostic Center. https://cwhl.vet.cornell.edu/system/files/public/cwhl-fact-sheets-ai.pdf. Accessed March 2022.
• USDA Animal and Plant Health Inspection Service. Last modified March 15, 2023. 2022-2023 Detections of Highly Pathogenic Avian Influenza in Wild Birds. USDA Animal and Plant Health Inspection Service.  https://www.aphis.usda.gov/aphis/ourfocus/animalhealth/animal-disease-information/avian/avian-influenza/hpai-2022/2022-hpai-wild-birds. Accessed March 2023.
• USDA Animal and Plant Health Inspection Service. June 2016. Highly Pathogenic Avian Influenza and North American Wild Birds: Frequently Asked Questions. USDA Animal and Plant Health Inspection Service.  https://www.aphis.usda.gov/aphis/ourfocus/animalhealth/animal-disease-information/avian/avian-influenza/ai-wild-birds. Accessed March 2023.