JPC SYSTEMIC PATHOLOGY

DIGESTIVE SYSTEM

November 2024

D-V25

 

Signalment (JPC #1941253): A 90-day-old chicken. 

 

HISTORY: This bird was from a flock of 21,000 layers that had experienced 8% mortality and a sudden drop in egg production.

 

HISTOPATHOLOGIC DESCRIPTION: Pancreas: Affecting 75% of the pancreas are multifocal to coalescing regions of pancreatic acinar architecture loss and stromal collapse with replacement by eosinophilic cellular and karyorrhectic debris, fibrin, edema, and hemorrhage (lytic necrosis). Multifocally, scattered acinar cells are shrunken and individualized, with hypereosinophilic cytoplasm and pyknotic or karyorrhectic nuclei (single cell death). In less affected areas, acinar cells are often shrunken with loss of zymogen granules (atrophy). Low to moderate numbers of lymphocytes, fewer heterophils, and macrophages are scattered throughout the parenchyma and interlobular connective tissue extending into the adjacent mesentery. Areas of necrosis and inflammation also extend into the peripancreatic adipose tissue, where adipocytes at the periphery exhibit loss of cellular detail (fat necrosis) and replacement by fibrin, necrotic debris, basophilic finely granular mineral, and acicular cholesterol clefts (fat saponification). Lymphatics are mildly ectatic with intraluminal protein and fibrin and increased adjacent clear space (edema). 

 

MORPHOLOGIC DIAGNOSIS: Pancreas: Pancreatitis, necrotizing, subacute, multifocal to coalescing, severe, with serositis and peripancreatic fat necrosis and saponification, chicken, avian.

 

ETIOLOGIC DIAGNOSIS: Orthomyxoviral pancreatitis and serositis

 

ETIOLOGY: Avian orthomyxovirus

 

CONDITION: Highly pathogenic avian influenza (HPAI)

 

SYNONYMS: Fowl plague

 

GENERAL DISCUSSION:

• Influenza viruses are relatively host-specific Orthomyxoviruses (enveloped, negative-sense, ssRNA viruses) that are spread worldwide in their hosts (wild waterfowl and shorebirds)
• Influenza viruses readily undergo genetic variation via a propensity for genetic drift caused by point mutations and genetic shift caused by genomic recombination
• Influenza virus is divided into three genera:
  • Influenza A affects birds, swine, horses, humans, and others
  • Influenza B has been isolated from humans only, causes seasonal flu
  • Influenza C is rare, causes mild to subclinical infections in humans and swine
• Influenza A viral strain identification is based on antigenic variation in the virus’ two major surface antigen envelope glycoproteins, hemagglutinin (H) and neuraminidase (N), of which 16 (15 or 18 per other references) and 9 are currently recognized, respectively
  • There is no cross-protection between strain subtypes, so viral subtyping is important
• Avian influenza (AI) is caused by a type A influenza virus that affects chickens, turkeys, ducks, pheasants, quail, many wild birds, and other poultry; infection results in variable disease from asymptomatic to acute, fatal infections 
  • Most outbreaks in the US are in turkeys and ducks
• Within the US, enzootic forms of AI typically cause mild to moderate respiratory disease
• Reservoirs: 
  • Wild waterfowl and shorebirds are the major natural reservoir (asymptomatic, although H5N1 Hong Kong (2002) subtype is lethal in waterfowl), with transmission along wild bird migratory pathways; 
  • Manmade reservoirs include live bird markets and commercial swine facilities (transmission has been reported from swine to turkeys)
• Divided into categories:
  • HPAI: high pathogenicity avian influenza, also known as high pathogenic notifiable AI - HPNAI
    • Strains that cause high pathogenicity; outbreaks of these strains are increasing in frequency
    • Subtypes H5 or H7 with hemagglutinin cleavage sites similar to those of virulent viruses are considered HPAI regardless of their pathogenicity in vivo
  • LPNAI: low pathogenicity notifiable AI
    • Subtypes H5 and H7 that are not highly pathogenic in vivo and lack hemagglutinin cleavage sites similar to those of virulent viruses
    • These are notifiable because history has proven that prevention of HPNAI is based on successful control of H5 and H7 LPAI
  • LPAI: low pathogenicity AI
    • Subtypes other than H5 and H7 that are not highly pathogenic in vivo and lack hemagglutinin cleavage sites similar to those of virulent viruses
• AI zoonotic transmission is rare but possible

 

PATHOGENESIS: 

• Viral gene transcription occurs in the nucleus, viral protein production occurs in the cytoplasm, and virions bud from the plasma membrane of the host cell
• Transmission via inhalation, ingestion, or contact with infected secretions/tissues (respiratory secretions, feces, feathers; spread on fomites between farms)
• Ligand-receptor binding to host cell: Viral envelope glycoproteins (hemagglutinin, neuraminidase) bind to target cell membrane receptors (sialyloligosaccharides on respiratory epithelium) à virus enters host cell à viral replication in upper airway epithelium à cytolytic effect on respiratory epithelium à airway epithelial necrosis with lesser necrosis of alveolar epithelium; infection is typically restricted to respiratory tract, but may become viremic and/or induce cytokine storm, with marked endothelial tropism (in chickens, rarely in ducks) with variable spread to other cell types à +/- myocarditis, myositis, encephalitis
• Viral infection of airway epithelium à impaired ciliary beating & epithelial necrosis, à impaired mucociliary clearance +/- impaired alveolar macrophage function à secondary bacterial infection
• Virulence factors:
  • Hemagglutinin: Involved in ligand-receptor interaction enabling viral attachment and entry into specific target host cells
  • Neuraminidase: Involved in shedding of virus from infected host cells
  • PB2 protein influences virulence and host preference
  • PB1-F2 is a nonstructural protein that associates mostly with the mitochondria; contributes to virulence through apoptosis induction of cells of the innate immune system, suppression of early interferon response, increased viral replication or delayed viral clearance and increased inflammation 
  • NS1 is a non-structural protein present in the cytosol of affected cells and contributes to virulence by interfering with host antiviral response
• Host specificity and cross-species infection is determined by receptors on host epithelial cells lining the respiratory tract and virus-encoded polymerases, especially PB2
• AI virus causes damage by direct replication in cells, tissues and organs, through indirect effects from production of cytokines and other cellular mediators, and from ischemia due to vascular thrombosis

 

TYPICAL CLINICAL FINDINGS:

• Variable morbidity and mortality; depend on species, virulence, concurrent infections; often asymptomatic in reservoir hosts (wild waterfowl and shorebirds) 
• LPAI outbreaks are most common:
  • Respiratory signs (coughing, sneezing, rales, lacrimation), depression, and reduced feed and water intake, occasionally diarrhea
  • Turkeys: egg production drop and egg shell abnormalities are common
  • May cause severe disease if birds are secondarily infected with live Pasteurella vaccine, E. coli, or Bordetella avium (causative agent of turkey coryza)
• HPAI outbreaks are less common, and usually seen in chickens:
  • Sudden death or severe acute disease without preceding signs
  • Mortality may reach 100%
  • Precipitous drops in egg production
  • Listlessness, respiratory signs (similar to but less prominent than LPAI), enteric signs (diarrhea), or nervous system signs (tremors, opisthotonus, paresis, paralysis, vestibular degradation/torticollis)

 

TYPICAL GROSS FINDINGS:

• Highly variable
• LPAI: 
  • Respiratory tract: Swollen sinuses, oculonasal discharge, mild to moderate tracheitis/congestion/hemorrhage, sinusitis, air sacculitis, conjunctivitis; fibrinopurulent bronchopneumonia if secondary bacterial infection occurs
  • Ovarian atresia and oviduct involution, +/- egg yolk peritonitis 
  • Firm pancreas with pale mottling/hemorrhage 
• HPAI: In poultry, variety of edematous, hemorrhagic and necrotic lesions in visceral organs and skin. If peracute death, may have no gross lesions
  • Chickens: Edema of the head, neck, and feet +/- petechia/ecchymoses; cyanosis of the head; vesicles and ulceration of the comb, wattles
    • Blotchy red discoloration of the legs
    • Petechiae of serosal and mucosal surfaces, esp epicardium, ventriculus, proventriculus; fibrinous exudates on air sacs, oviduct, pericardial sac, and/or peritoneum
    • Hemorrhage/necrosis of the ventriculus, proventriculus, pectoral muscles, pancreas, spleen, heart, kidney (+/- urate deposition)
    • Interstitial pneumonia with edema, may be congested or hemorrhagic
    • Atrophy of cloacal bursa and thymus
  • Turkeys: Congestion/hemorrhage in trachea, pancreas, breast muscle, coronary fat, intestine, bursa of Fabricius, kidneys; encephalitis and pancreatitis have been reported 
  • HPAI (H5N1) in domestic ducks causes corneal opacity due to loss of corneal endothelium 

 

TYPICAL LIGHT MICROSCOPIC FINDINGS:

• LPAI: Pneumonia, heterophilic to lymphocytic tracheitis and bronchitis 
  • Turkeys: Pancreatic acinar necrosis 
• HPAI: Multi-organ necrosis and/or inflammation (lymphohistiocytic in non-fatal cases), varies by strain/species
  • Most severely affected: Brain, heart, lung, pancreas, 1º/2º lymphoid organs
  • Edema, congestion, hemorrhage, perivascular lymphoid cuffing (vascular damage), and necrosis
  • Classic lesions in chickens: Cyanosis and edema of the head, vesicles and ulceration on the combs, edema of the feet, blotchy red discoloration of the shanks, petechiae in the abdominal fat and various mucosal/serosal surfaces; necrosis or hemorrhage in the mucosa of the gizzard and proventriculus 
  • Turkeys: Lesions highly variable; hemorrhagic pneumonia, hemorrhage in liver/kidneys, hemorrhagic necrosis in spleen/pancreas, encephalitis and encephalomalacia (Mumu, J Vet Diagn Invest. 2021)
  • In reservoir waterfowl species there are no gross pathognomonic lesions, but  multifocal to coalescing hemorrhagic necrosis in the pancreas, subepicardium and myocardium, and severely congested lungs with edema can be seen. 

 

ADDITIONAL DIAGNOSTIC TESTS:

• Rapid real time RT-PCR, virus isolation (acute infection often required), IHC, serology (ELISA, agar gel immunodiffisuion, hemagglutination-inhibition)
• HPAI may be detected in formalin-fixed paraffin-embedded tissues with RNAscope In-Situ Hybridization. (Gaide, J Vet Diagn Invest, 2023) 
• Confirmation of HPNAI requires molecular characterization and/or inoculation into susceptible chickens

 

DIFFERENTIAL DIAGNOSIS:

• Newcastle disease (rubulavirus, Avian paramyxovirus-1, N-V10): Edema of head; congestion, edema, lymphohistiocytic inflammation, and possibly hemorrhage of the respiratory tract; hemorrhage and necrosis of Peyer’s patches and cecal tonsils; neurological signs; drop in egg production
  • Disease caused by HPNAI is similar to velogenic viscerotropic Newcastle disease
• Other paramyxoviral infections (rubulavirus, Avian paramyxoviruses 2-9): Pathogenicity varies with infected species and virus strain
• Mycoplasma gallisepticum and E. coli: Fibrinous airsacculitis, pericarditis and perihepatitis, with hyperplastic lymphoid follicles
• Chlamydia psittaci (D-B12): Airsacculitis, pericarditis, fibrinous perihepatitis, chlamydial organisms, vasculitis
• Pasteurella multocida (fowl cholera): Heterophilic pneumonia with bacilli, hemorrhage, and thrombosis
• Infectious bronchitis (gallid coronavirus): Lymphocytic and heterophilic inflammation of the upper respiratory tract, with sloughing of respiratory epithelium; eggshell abnormalities
• Infectious laryngotracheitis (alphaherpesvirus, gallid herpesvirus-1, P-V11): Hemorrhagic laryngotracheitis, with epithelial syncytia and intranuclear inclusion bodies
• West Nile Virus (C-V04, N-V19): May cause pancreatitis in addition to other lesions (e.g. myocarditis, meningoencephalitis, etc.) in susceptible species, but poultry apparently do not typically develop clinical disease; most susceptible species include passerines such as crows, ravens, etc. and some raptors

 

COMPARATIVE PATHOLOGY:

• In mammalian species, influenza A typically is a respiratory disease with high morbidity but low mortality unless complicated by secondary bacterial infection, and may be a component of respiratory disease complex; humans may naturally transmit disease to ferrets (H1N1, H5N1), pigs (H1N1), cats, and dogs
• Swine (see P-V18): 
  • Swine serve as hosts for reassortment of avian and human influenza strains because they possess cell surface receptors for both, which could result in novel pathogenic strains à potential for human pandemic influenza; this reassortment is now recognized to potentially occur in other species
  • Important respiratory disease of pigs, manifests as epidemics of rapidly-spreading nonfatal respiratory disease or endemically as part of the porcine respiratory disease complex 
  • Hallmark lesion is necrotizing bronchitis/bronchiolitis
  • In North America, subtypes H1N1, H3N2, and H1N2 are common 
  • Samples should be collected early in the disease course from the cranioventral areas of the lung for histologic and virologic diagnosis
• Horses: Colloquially termed “equine influenza virus” (EIV)
  • Infection with influenza A is widespread in most intensively managed horse populations, with outbreaks in naïve populations often in fall and winter
  • All equine influenza viruses currently belong to the H3N8 subtype, H7N7 is historical and is considered extinct 
  • Disease is characterized by high morbidity and low mortality unless secondary bacterial pneumonia develops
• Dogs: 
  • Equine-origin H3N8 influenza A virus can occur in dogs and typically causes self-limiting upper respiratory tract disease, with unique lesion of bronchial gland epithelial necrosis, hyperplasia, and neutrophilic infiltrates. 
  • Avian-origin H3N2 influenza A has also been reported in outbreaks.
• Cats: 
  • May be susceptible to influenza A from human (H1N1) and avian (H5N1, H7N7) subtypes via inhalation with lesions centered on alveolar epithelium (interstitial pneumonia, pulmonary edema)
  • Ingestion of H5N1-infected chicken liver caused systemic infection targeting endothelium à widespread hemorrhage, lymphoid necrosis
• Cattle: Bovine influenza has similar pathogenesis and mechanism of injury as equine influenza
• Ferrets: susceptible to several human influenza virus strains, used in influenza research due to similarities with humans in clinical disease 
• Nonhuman primates: Influenza A and B have been reported uncommonly; most information is based on experimental infection
• Snakes may play a role in Influenza A epidemiology due to possessing receptors for binding the virus in their respiratory and digestive tracts. (Silva, J Comp Pathol. 2024)

 

REFERENCES:

1. Abdul-Aziz T, Fletcher OJ. Chapter 5: Cardiovascular System. In: Abdul-Aziz T, Fletcher OJ, Barns HJ, eds. Avian Histopathology. 4th ed. Madison, WI: Omnipress; 2016: 145, 159-160.
2. Ajithdoss DK, Torchetti MK, Badcoe L, Bradway DS, Baszler TV. Pathologic findings and viral antigen distribution during natural infection of ring-necked pheasants with H5N2 highly pathogenic avian influenza virus A. Vet Pathol. 2017;54(2):312-315.
3. Barthold SW, Griffey SM, Percy DH. Pathology of Laboratory Rodents and Rabbits. 4th ed. Ames, IA: Blackwell Publishing; 2016: 220.
4. Caswell JL, Williams KJ. Respiratory system. In: Maxie MG. ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 2. 6th ed. St. Louis, MO: Elsevier; 2017:526-527, 567, 577, 587.
5. Crespo R, Franca MS, Fenton H, Shivaprasad HL. Galliformes and Colubriformes. In: Terio KA, McAloose D, St. Leger J, eds. Pathology of Wildlife and Zoo Animals. London, UK: Academic Press; 2018:755.
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7. Gaide N, Crispo M, Jbenyeni A, Bleuart C, et al. Validation of an RNAscope assay for the detection of avian influenza A virus. J Vet Diagn Invest. 2023;35(5):500-506.
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20. Swayne DE, Suarez DL, Sims LD. Influenza. In: Swayne DE et al, eds. Diseases of Poultry. 14^th ed., Hoboken, NJ: John Wiley & Sons, Inc.; 2020: 210-256.
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