JPC SYSTEMIC PATHOLOGY

RESPIRATORY SYSTEM

September 2026 P-V18

HISTOPATHOLOGIC DESCRIPTION:

Lung: Affecting approximately 40% of this section, there are multifocal areas of atelectasis and consolidation centered around or adjacent to bronchi and bronchioles. Diffusely affecting the bronchioles, and to a lesser extent the bronchi, the epithelium is circumferentially either attenuated, degenerate, and/or necrotic, sloughed, and replaced by karyorrhectic and cellular debris with infiltrating macrophages, neutrophils, lymphocytes, and plasma cells. This necrotic and inflammatory material forms an exudate that variably fills bronchial and bronchiolar lumina which are mildly to moderately ectatic. The epithelium of less affected bronchi is often hyperplastic, piling up to three cell layers thick, forming folds into the lumina, or is eroded and replaced by similar necrosis and inflammation which infiltrates through the lamina propria. Alveolar septa surrounding bronchi and bronchioles are diffusely thickened up to three times normal by lymphocytes, plasma cells, macrophages, and type II pneumocyte hyperplasia. Alveolar septa are often fragmented and necrotic with replacement by karyorrhectic and cellular debris that extends into the alveolar lumina which contain scattered neutrophils, macrophages, fibrin, and necrotic debris. There is mild perivascular and interlobular edema, and vessels are often lined by hypertrophied (reactive) endothelium.

GENERAL DISCUSSION:

• Single-stranded RNA orthomyxovirus: viral gene transcription in nucleus, viral protein production in cytoplasm, and virions bud from plasma membrane
• Causes outbreaks of severe nonfatal respiratory disease or as an endemic part of porcine respiratory disease complex
• Three genera of orthomyxoviruses
  • Influenza A: Most significant; causes disease in swine, horses, humans, and other species
  • Influenza B: Human isolates only
  • Influenza C: Rare and cause mild to subclinical infections in humans and swine
• Genetic drift leads to point mutations and several genomic variants
  • Antigenic variation of viral strains is dependent on 15 hemagglutinin (H) and 9 neuraminidase (N) envelope glycoproteins
  • Used to track outbreaks and develop vaccine strains
• Swine act as hosts for reassortment among avian, porcine, and human influenza viruses because swine possess cell-surface receptors for avian and human viral strains
• Usually restricted to respiratory tract but may cause myocarditis, myositis, and encephalitis in carnivores and horses

PATHOGENESIS:

• Spread by aerosol or contact with secretions and infect epithelial cells lining respiratory tract inducing necrotizing bronchitis/bronchiolitis
• Virulence proteins:
  • Hemagglutinin binds to sialic acid (mammals: SAα2,6; birds: SAα2,3) and mediates fusion with host cell membrane
  • Neuraminidase destroys receptors in mucus that mimic viral glycoprotein receptors, cleaving sialic acid and facilitating entry into host cells by degrading mucins
  • Polymerases: direct cell processes towards viral replication
    • PB1: catalyst for viral mRNA transcription
    • PB2: most important in pathogenicity; removes 5’ caps from host pre-mRNA molecules (cap-snatching) for use as primers for viral mRNA production
    • PA- assists w/ “cap snatching”
  • NS1: Present only in host cell cytoplasm; interferes with host antiviral response by inhibiting apoptosis of viral infected cells and promoting it of cell that attack viral infected cells
• Subtypes H1N1, H3N2, and H1N2 are common in North America
• IgA and IgG are maximally elevated 1-4 weeks after exposure
• Virus shed for 5-7 days but cleared from lung 72 hours after infection
• Infection maintained in swine population by continuous exposure of naïve animals, no evidence of long-term carriers

TYPICAL CLINICAL FINDINGS:

• Outbreaks:
  • Acute, rapid spread to all ages of pigs
  • Coughing, fever, stiffness
  • Pigs recover in 5-14 days with low mortality
• Endemic form:
  • Pigs develop secondary bacterial pneumonia from subclinical influenza infection
  • Signs not specific for influenza and require further diagnostics for confirmation

TYPICAL GROSS FINDINGS:

• Cranioventral lobular distribution of atelectasis, sharply demarcated from normal lung
• Affected lobules are red, firm, and collapsed
• Adjacent lobules may be emphysematous
• Mediastinal and bronchial lymph nodes are enlarged and edematous
• Secondary bacterial pneumonia may obscure lesions

TYPICAL LIGHT MICROSCOPIC FINDINGS:

• Hallmark lesion is necrotizing bronchitis/bronchiolitis
• Alveolar epithelium may also be necrotic
• Attenuation of bronchiolar epithelial cells; hyperplasia of airway epithelium in later lesions, +/- bronchiolitis obliterans
• Exudate in bronchiolar lumina composed of necrotic debris and neutrophils
• Atelectasis
• Peribronchiolar, perivascular, and interstitial lymphocytes and plasma cells

ADDITIONAL DIAGNOSTIC TESTS:

• Selecting tissue from cranioventral areas early in course of disease is critical for histologic and virologic diagnosis
• PCR and sequencing
• ELISA on nasal or bronchial swabs
• IHC on fixed tissue (to type A nucleoproteins)
• Virus isolation
• ISH within formalin-fixed, paraffin-embedded tissue

DIFFERENTIAL DIAGNOSIS:

• PCV-2 (PMWS, H-V11): Granulomatous interstitial to bronchointerstitial pneumonia and lymphadenopathy
• Proliferative and necrotizing pneumonia: Severe acute disease; entire lung may be affected, alveolar luminal exudate and septal thickening with type II pneumocyte hyperplasia; generalized lymphadenopathy; etiology unclear, may be related to multiple viruses including PRRSV, PCV-2, H1N1 or H3N2 influenza
• Pseudorabies (Suid herpesvirus-1, E-V01, N-V07): Lung lesions inconsistent; cranioventral distribution of scattered throughout the lung; bronchointerstitial pneumonia with necrosis and sloughing bronchiolar epithelium; intranuclear inclusions
• Nipah virus (paramyxovirus): Lymphocytic and/or fibrinoid vasculitis; lymphoid necrosis; syncytial cells and intracytoplasmic viral inclusion bodies
• Porcine reproductive and respiratory syndrome virus (Arterivirus, P-V25): Interstitial pneumonia with necrotic alveolar macrophages and aggregates of free chromatin; lymphadenopathy; lymphocytic infiltrates in multiple organs (often perivascular)
• Inhaled toxic gases: rare in animals; water-soluble gases (eg sulfur dioxide and ammonia)→ upper respiratory injury; less soluble gases (eg nitrogen dioxide)→ bronchiolar and alveolar injury, massive mortality in cattle, pigs, and chickens with pulmonary edema, congestion, necrosis of bronchial and bronchiolar epithelium, alveolar hemorrhage, and fibrin exudation

COMPARATIVE PATHOLOGY:

• Also affects feral swine and wild boar
• Mink are naturally susceptible to Influenza A from both swine and human origin (Clayton MJ et al, J Vet Diagn Invest. 2022)
• Syrian hamsters are experimentally susceptible to human influenza A and B viruses, and may transmit influenza A but not influenza B; subclinical signs, natural infection not reported
• Guinea pigs are also experimentally susceptible to influenza A & B but they readily transmit both to other guinea pigs. Natural infection not reported in guinea pigs.
• Waterfowl serve as reservoir of nearly all influenza A viruses
• Poultry and occasionally wild birds (Wünschmann A, Vet Pathol. 2024): High and low path avian influenza (see D-V25)
  • LPAI: Amino acid adjacent to hemagglutinin cleavage site is only susceptible to proteases found in the GI and respiratory tracts
  • HPAI: Has a sequence of multiple basic amino acids that allows cleavage by furin-like proteases that are present in many tissues→ systemic infection
• Ostrich: Bacterial flora of trachea may affect viability of influenza A for further confirmation testing
• Equine influenza: H3N8, high morbidity/low mortality, mainly in foals; foci of consolidation separated by unaffected to hyperinflated lung; similar histo to swine influenza, tracheitis is common
• Canine: H3N8: self-limiting upper respiratory tract disease; tracheobronchial epithelial cell necrosis with a unique lesion that similarly affects the bronchial glands; H3N2: much more severe subtype, death common
• Feline: No unique virus, but susceptible to human H1N1 and avian H5N1 and H7N7; ingestion of H5N1 infected chicken liver induces systemic infection with targeting of endothelial cells
• Reptiles: Recent discovery of sialic acid receptors within the GI and respiratory systems of snakes suggest possible susceptibility to influenza A. (E Silva YC et al, J Comp Pathol. 2024)

References:

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