JPC SYSTEMIC PATHOLOGY
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 approximately 75% of the pancreas are multifocal to coalescing areas 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 vacuolated cytoplasm and pyknotic or karyorrhectic nuclei (single cell necrosis). In less affected areas, acinar cells are 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 (edema) with intraluminal protein and fibrin.
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
- 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 15 (16 per one reference, 18 per another) and 9 (11 per one reference) 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)
- AI is divided into two categories: low pathogenic AI (LPAI) and high pathogenic AI (HPAI, also known as high pathogenic notifiable AI or HPNAI)
- Strains that cause high pathogenicity are considered HPAI (or HPNAI); outbreaks of these strains are increasing in frequency
- Strains of subtypes H5 or H7 with hemagglutinin cleavage sites similar to those of virulent viruses are considered HPAI regardless of their pathogenicity in vivo
- Strains of subtypes H5 and H7 that are not highly pathogenic in vivo and lack hemagglutinin cleavage sites similar to those of virulent viruses are considered low pathogenicity notifiable AI (LPNAI); these are notifiable because history has proven that prevention of HPNAI is based on successful control of H5 and H7 LPAI
- Subtypes other than H5 and H7 what are not highly pathogenic in vivo and lack hemagglutinin cleavage sites similar to those of virulent viruses are considered low pathogenicity AI (LPAI)
- AI zoonotic transmission is rare but possible
- Recent outbreak of AI in the literature: H5N2 HPAI arising from reassortment was first reported in Washington State (December 2014), then Idaho and Oregon (Jan/Feb 2015), then in a commercial turkey flock in Minnesota (March 2015), then 15 other states; selected reports:
- 2015, Washington state: ring-necked pheasants, congestion of cerebellar meningeal blood vessels was the only consistent gross finding (Ajithdoss 2017)
- 2015, Minnesota (as well as Arkansas, Iowa, Missouri, North Dakota, South Dakota, and Wisconsin): H5N2 HPAI in turkeys and chickens, findings included frank blood in trachea, mouth, beak, body cavities filled with serosanguinous fluid, pulmonary hemorrhage, cecal tonsil necrosis, focal pancreatic acinar necrosis (Fitzpatrick 2017)
- 2014-2015, Iowa: H5N2 HPAI in chickens (report in commercial layer facilities), lesions included diffuse edema, multifocal hemorrhage, catarrhal exudate of the oropharynx, multifocal tracheal hemorrhage, less commonly epicardial petechiae, splenic hemorrhage, pancreatic necrosis
- 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 (respiratory secretions, feces; spread on fomites between farms) à 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 mucociliary clearance via impaired ciliary beating and epithelial necrosis, +/- impaired alveolar macrophage function à predisposed to 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:
- Typically causes respiratory signs (coughing, sneezing, rales, lacrimation), depression, and reduced feed and water intake; egg production drop and egg shell abnormalities are common in affected turkeys
- May cause severe disease if birds are secondarily infected with live Pasteurella vaccine, coli, or Bordetella
- HPAI outbreaks are less common, and usually seen in chickens:
- Sudden death or severe acute disease without preceding signs, mortality may reach 100%, clinical signs may relate to respiratory, enteric (may have diarrhea), or nervous systems
TYPICAL GROSS FINDINGS:
- Highly variable
- LPAI: swollen sinuses, oculonasal discharge, mild to moderate tracheitis, sinusitis, air sacculitis, conjunctivitis; ovarian atresia and oviduct involution, +/- egg yolk peritonitis; fibrinopurulent bronchopneumonia if secondary bacterial infection occurs
- classical lesions of chickens: edema of the head and feet, cyanosis of the head, vesicles and ulceration of the combs, blotchy red discoloration of the legs, petechiae of multiple serosal and mucosal surfaces, and hemorrhage/necrosis of the ventriculus and proventriculus
- Other lesions include fibrinous exudates on air sacs, oviduct, pericardial sac, and/or peritoneum; small necrotic foci in multiple organs (skin, comb, wattles, liver, kidney, spleen, lungs); congestion, edema, hemorrhages of multiple organs
- Lesions of HPAI in turkeys are not well described; encephalitis and pancreatitis have been reported
- HPAI (H5N1) in domestic ducks frequently and characteristically causes corneal opacity due to loss of corneal endothelium (Yamamoto 2016)
TYPICAL LIGHT MICROSCOPIC FINDINGS:
- Edema, congestion, hemorrhage, perivascular lymphoid cuffing (vascular damage), and necrosis in the myocardium, spleen, lungs, brain, wattles, liver, kidney, pancreas, skeletal muscle, and lymphoid necrosis
ADDITIONAL DIAGNOSTIC TESTS:
- Rapid real time- RT-PCR, ELISA, virus isolation (acute infection often required), IHC, serology
- Recent study (Nuradji 2017) shows that feathers are a good tissue source for HPAI H5N1 viral antigen detection in chickens and ducks
- Recent report (Wang 2018) of rapid detection technique using RT-PCR and pyrosequencing to detect a mutation of hemagglutinin cleavage site associated with increased pathogenicity in H5N1
- Confirmation of HPNAI requires molecular characterization and/or inoculation into susceptible chickens
- Newcastle disease (rubulavirus, Avian paramyxovirus-1): 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: 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, see P-V11): Hemorrhagic laryngotracheitis, with epithelial syncytia and intranuclear inclusion bodies
- West Nile Virus: May cause pancreatitis in addition to other lesions (i.e. 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
- 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, Janke 2014):
- 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
- 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
- Recent report (Bernardino 2016) of pyrosequencing for rapid detection of clade association in EIV
- H3N8 influenza A virus was first recognized in dogs in 2004, is thought to have arisen from equine influenza virus (canine and equine respiratory epithelium express similar sialyloligosaccharides), and typically causes self-limiting upper respiratory tract disease, with unique lesion of bronchial gland epithelial necrosis, hyperplasia, and neutrophilic infiltrates
- Dogs are rarely infected with other influenza A subtypes, with several recent outbreaks around the US from H3N2 (Watson 2017)
- May be susceptible to influenza A from human (H1N1) and avian (H5N1, H7N7) subtypes via inhalation with lesions centered on alveolar epithelium
- 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
- Guinea pigs: experimentally susceptible to unadapted human influenza viruses, high seroprevalence, but natural disease is not reported
- Ferrets: susceptible to several human influenza virus strains, used in influenza research due to similarities with humans in clinical to disease
- Nonhuman primates: Influenza A and B have been reported uncommonly; most information is based on experimental infection
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- Bernardino PN, Mapes SM, Corbin R, Pusterla N. Pyrosequencing as a fast and reliable tool to determine clade affiliation for equine influenza A virus. Jour Vet Diag Invest. 2016;28(3):323-326.
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- Janke BH. Influenza A virus infections in swine: pathogenesis and diagnosis. Vet Pathol. 2014;51:410-426.
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