JPC SYSTEMIC PATHOLOGY

DIGESTIVE SYSTEM

March 2025

C-V04

 

SIGNLAMENT (JPC #2741020): Chilean flamingo

 

HISTORY: This animal was weak and ataxic. Sections of the heart were positive via immunohistochemistry and in situ hybridization for West Nile virus.

 

HISTOPATHOLOGIC DESCRIPTION: Heart: Multifocally affecting 50% of the myocardium, separating and surrounding cardiomyocytes and Purkinje fibers, multifocally infiltrating the endocardium and the epicardium, and extending into the adjacent epicardial adipose tissue, are numerous lymphocytes, plasma cells, and macrophages. Adjacent cardiomyocytes are often shrunken, hypereosinophilic, or fragmented with loss of cross striations, have pyknotic or karyolytic nuclei, and are surrounded by eosinophilic cellular and karyorrhectic debris (necrosis). Rare cardiomyocytes are pale, swollen, and vacuolated (degeneration). The epicardial adipose tissue is diffusely composed of shrunken and variably sized adipocytes (fat atrophy). 

 

MORPHOLOGIC DIAGNOSIS: 1. Heart: Pancarditis, lymphoplasmacytic and histiocytic, multifocal to coalescing, moderate, subacute, with cardiomyocyte degeneration and necrosis, Chilean flamingo (Phoenicopterus chilensis), avian.

2. Heart, epicardium: Fat atrophy, diffuse, mild. 

 

ETIOLOGIC DIAGNOSIS: Flaviviral myocarditis

 

CAUSE: West Nile virus

 

GENERAL DISCUSSION:

• West Nile virus (WNV), an arbovirus of the family Flaviviridae, genus Flavivirus (enveloped ssRNA virus), was first identified in Uganda in 1937 and introduced into the northeastern US in 1999
• Zoonotic disease that can cause fatal disease in humans, horses, birds, and other mammalian and reptilian species
• Horses are the most susceptible domestic animals 
• Sylvatic transmission with a bird-mosquito-bird cycle maintains WNV in the environment, with humans and horses becoming infected during the urban transmission cycle and considered incidental dead-end hosts
• Brain and heart are defined features of disease in birds
• Corvids (crows, magpies, jays) are major amplifying hosts- high titer viremia and high mortality
• Finches/sparrows (low titer viremia and variable mortality) may be primary maintenance hosts in North America
• Divided into two genetic lineages; pathogenicity depends on virus strain:
• Lineage 1 WNV in North America (made up of different clades, some of which are virulent)
• Lineage 2 WNV (Africa) strains are nonpathogenic or cause mild disease in humans/horses
• Mosquitos (especially Culex and Aedes) are considered the primary maintenance vectors in the US; ticks (Argas and Amblyomma) are capable of harboring the virus; frequency of disease related to seasonal mosquito activity (i.e. summer and fall)

 

PATHOGENESIS:

• Transmitted via mosquito bites and (rarely) by direct contact, fomites, or ingestion
  • Signs of WNV in naïve areas: increased wild bird mortality and increased number of cases of equine encephalitis
• Susceptibility (in avians) varies by species; high viremia and mortality in corvids (crows, magpies, jays), Passeriformes, geese, Psittaciformes; low viremia/mortality in chickens and pigeons; turkeys resistant
• In susceptible species: Injection of virus-containing saliva from mosquito> replication in local tissues and lymph nodes> high-titer viremia> viral invasion of multiple organ systems with widespread necrosis, hemorrhage, and inflammation 
  • Virus can be detected in blood 30-45 minutes after mosquito feeding in birds: local replication is not necessary for primary viremia
  • Dissemination occurs before development of a full adaptive immune response, so the innate immune system (especially macrophages) are important in resistance to infection
  • Cellular targets include CNS (cerebellum) and PNS; cardiomyocytes; macrophages and monocytes; multiple epithelial cell types; oocytes; fibrous connective tissues
  • Mechanism for crossing blood-brain-barrier in CNS infection is unclear
• The disassociation of virion density from lesion severity suggests an immunopathologic mechanism in the pathogenesis of WNV disease in horses

 

TYPICAL CLINICAL FINDINGS:

• Weakness and recumbency; ataxia, tremors, abnormal head posture, circling
• Sudden death without premonitory signs, especially in crows

 

TYPICAL GROSS FINDINGS:

• Nonspecific and inconsistent; often none
• Hemorrhage of the brain and meninges, especially cerebellum
• Splenomegaly; splenitis
• Heart: Enlargement, flaccidity, pale streaks within the myocardium
  • Myocardial hemorrhage; mottled, pale foci
• Intraosseous calvarial hemorrhage
• Intestinal mucosal hemorrhage
• Renal and pulmonary congestion or hemorrhage

 

TYPICAL LIGHT MICROSCOPIC FINDINGS:

• Heart: Multifocal, lymphoplasmacytic myocarditis with necrosis
• Necrotizing splenitis; pancreatitis; adrenalitis; nephritis; hepatitis
• Spleen: Lymphoid depletion
• Central nervous system
  • Hemorrhage in cerebellar folia, cerebrum, brain stem, cervical spinal cord
  • Degeneration and necrosis of cerebellar molecular layer and Purkinje cells; cerebral neurons only rarely affected
  • Lymphoplasmacytic meningoencephalitis; perivascular cuffing; gliosis
• Enterocolitis, lymphoplasmacytic, chronic

 

ULTRASTRUCTURAL FINDINGS:

• Virions 35-45 nm in diameter, dense core surrounded by a thin, diffuse outer layer (typical flaviviral morphology)
• Virions in cytoplasmic vacuoles, less frequently in rough endoplasmic reticulum
• 100 nm smooth membrane vesicles (SMV) in dilated rough endoplasmic reticulum and vacuoles (characteristic of flaviviruses)
• Marked vesiculation and vacuolation of the cytoplasm, with disorganization of the rough endoplasmic reticulum and Golgi apparatus

 

ADDITIONAL DIAGNOSTIC TESTS:

• Immunohistochemistry (lacks specificity) 
• In situ hybridization 
• Reverse transcriptase-polymerase chain reaction (RT-PCR) assays
• Virus isolation (requires Biosafety level 3 lab)
• Hemagglutination inhibition; serology (rising titer)

 

DIFFERENTIAL DIAGNOSIS:

• Exotic Newcastle’s disease (Paramyxoviridae; rubulavirus, N-V10) or highly pathogenic avian influenza (Orthomyxoviridae; influenzavirus A, D-V25): Both cause mortality in chickens and turkeys; however, WNV is seldom a fatal infection in these species
  • CNS lesions, congestion, edema, and hemorrhage can be present in both and in various tissues
• Eastern equine encephalitis (Togaviridae; alphavirus): Arbovirus known to be pathogenic for birds

 

COMPARATIVE PATHOLOGY:

• Birds of prey: Classic triad of lesions is myocarditis, endophthalmitis, and meningoencephalomyelitis
  • Raptors can also become infected through consumption of infected prey or scavenged carcasses
  • Owls and other free-living birds myodegeneration to severe lymphohistiocytic necrotizing myositis
  • Penguins: Clinical signs when observed, include anorexia, weakness, abnormal behavior, vomiting, and dyspnea from excessive tracheal or pulmonary mucus secretion; multifocal lymphoplasmacytic meningoencephalitis 
• Pelicans and cormorants: Head tilt, ataxia, and disorientation; lymphohistiocytic myocarditis, encephalitis or meningoencephalitis
• Flamingos: Most significant viral infection in flamingos due to resultant widespread mortality
• Psittacine: Myocarditis (myocardial pallor and epicardial petechiae, lymphoplasmacytic and histiocytic myocarditis with myocardial necrosis and hemorrhage, reduced body condition, severe chronic systemic inflammation, lymphoplasmacytic enteritis 
• Vertebrates (other than birds) generally have low-titer viremia
• Horses: Pronounced, if not exclusive, tropism for the brain and spinal cord, leading to neuronal necrosis, predominantly lymphocytic polioencephalomyelitis extending from basal nuclei, brain stem to sacral spinal cord (worst lesions in ventral horns of thoracolumbar segments), gliosis/glial nodules
• Antigen scarce in lesions; no antigen in the PNS
• IgM-capture enzyme-linked immunosorbent assay is more sensitive than RT-PCR in horses
• Sheep: Reported cases of WNV induced lymphoplasmacytic meningoencephalitis in California
• Camelids: Fever, seizures, ataxia, lethargy, recumbency, and a high mortality rate are characteristic; histologic lesions of polioencephalitis
• Dogs: Usually no clinical disease (rare reports of CNS and/or cardiac disease)
• Alligators: Multiorgan involvement (similar to wild birds) described in juveniles
  • Lymphohistiocytic proliferative cutaneous lesions of American alligators 
• Humans with WNV: Non-specific flu-type symptoms; fatalities in older people and children; predominately brainstem lymphohistiocytic glial nodules
• NHP: Has been found in outdoor-housed NHPs, natural and experimental infections are generally subclinical, +/- encephalitis in Bonnet macaques; ataxia, tremors, cranial nerve deficits and nystagmus; nonsuppurative meningoencephalitis

 

Arboviruses:

• Alphavirus (Togaviridae): EEE (N-V09), WEE, VEE
• Flavivirus (Flaviviridae): WNV (C-V04, N-V19), Japanese encephalitis virus, St. Louis encephalitis virus, Dengue, yellow fever (D-V23), tick-borne encephalitis virus
• Bunyavirus (Bunyaviridae): California encephalitis virus, LaCrosse virus
• Phlebovirus (Bunyaviridae): Rift Valley fever virus (D-V18), sandfly fever virus

 

 

REFERENCES:

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