JPC SYSTEMIC PATHOLOGY

RESPIRATORY SYSTEM

August 2026 P-V26

HISTOPATHOLOGIC DESCRIPTION:

Lung: Multifocally, approximately 50% of alveolar septa are thickened up to 10X normal by fibrin, edema, increased numbers of macrophages and lymphocytes, and small aggregates of brown globular pigment within macrophages (hemosiderin). Multifocally, lumina of smaller bronchi, bronchioles, and alveoli contain variable amounts of edema admixed with hemorrhage and previously described inflammatory cells. Endothelial cells lining small to medium caliber blood vessels and alveolar capillaries occasionally form viral syncytial cells with up to six nuclei. Diffusely, perivascular, peribronchiolar, and subpleural interstitium and interlobular septa are markedly expanded by homogenous pale eosinophilic fluid or increased clear space (edema), previously described inflammatory cells, and ectatic lymphatics. Multifocally alveolar septa are discontinuous, forming large confluent airways (emphysema).

Kidney: Endothelial cells lining glomerular capillaries and renal vessels are often swollen (degeneration) or necrotic with occasional syncytial cell formation. Multifocally, the tunica media of arteries and arterioles is characterized by myocyte degeneration and fibrin deposition, and the tunica adventitia is mildly expanded by fibrosis and multifocally infiltrated or surrounded by lymphocytes and fewer macrophages and plasma cells. Rarely, glomerular tuft architecture is lost and replaced by thick, homogenous, eosinophilic collagenous material (glomerular sclerosis) or glomerular tufts are markedly shrunken (glomerular obsolescence). Medullary tubules occasionally contain granular basophilic material (mineral).

GENERAL DISCUSSION:

• Hendra virus (HeV) is an enveloped, single stranded RNA virus; Paramyxoviridae family; with tropism for endothelial cells that causes an acute interstitial pneumonia with severely damaged endothelium and widespread alveolar edema
• Fruit bats (flying foxes) in the genus Pteropus are the reservoir host
• Closely related to Nipah virus of swine which is also a member of this genus

PATHOGENESIS:

• The pathogenesis of HeV is not completely understood; fruit bats are the source of infection but transmission is poorly understood
• Virus localizes in blood vessel walls in the lungs and other organs leading to endothelial cell damage and endothelial syncytial cell formation

TYPICAL CLINICAL FINDINGS:

• Nonspecific; fever, anorexia, respiratory distress, and nasal discharge

TYPICAL GROSS FINDINGS:

• Marked pulmonary edema and congestion with gelatinous distention of sub-pleural lymphatics; ventral consolidation of the lungs; excess thoracic and pericardial fluid
• Petechial hemorrhages on pleural surfaces; patchy hemorrhages of varying sizes within the parenchyma
• Fluid may ooze from cut surfaces; airways are often blocked by white to blood-tinged foam; trachea may contain abundant froth
• Other gross findings include visceral edema and petechial and ecchymotic hemorrhages in the perirenal tissue, stomach, and intestines

TYPICAL LIGHT MICROSCOPIC FINDINGS:

• Diffuse serofibrinous alveolar edema with vasculitis, capillary thrombosis, ectatic lymphatics, hemorrhage, and increased numbers of alveolar macrophages
• Endothelial syncytial cells are inconsistently present in small blood vessels and alveolar capillaries
• Subtle vascular lesions may be present in a wide range of organs including lung, heart, and kidney
• Inclusion bodies are not visible by light microscopy (except in guinea pig neurons)

DIFFERENTIAL DIAGNOSIS:

• Pulmonary form of African horse sickness
  • Pulmonary edema, distended subpleural lymphatics, froth within the airways and failure of lungs to collapse
  • Occurs in Africa, Middle East, India, Pakistan and Spain
• Equine viral arteritis (Arterivirus)
  • Interstitial pneumonia with vasculitis, distended lymphatics and bronchioalveolar edema
  • Occurs throughout the world
• Anaphylaxis
  • Alveolar and interlobular edema, bronchoconstriction, congestion, emphysema and blood-tinged froth within airways
• Heart failure
  • Edema with low protein content within interstitium and alveoli, erythrocytes and hemosiderin-laden macrophages (“heart failure cells”) within alveoli; white froth within airways

COMPARATIVE PATHOLOGY:

• Golden hamster: Highly susceptible to experimental infection
• Cat: Develop a fatal pneumonia similar to the equine disease
• Guinea pig: Develop generalized vascular disease and encephalitis, but little or no pulmonary disease

References:

1. Caswell JL, Williams KJ. Respiratory system. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 2. 7th ed. Elsevier; 2025:569.
2. Lopez, A. and Martinson, SA. Respiratory System, Thoracic Cavities, Mediastinum, and Pleurae. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 7th ed. Elsevier; 2022:604.