JPC SYSTEMIC PATHOLOGY

RESPIRATORY SYSTEM

September 2026

P-B04

HISTOPATHOLOGIC DESCRIPTION

Lung: Diffusely, the alveoli and, to a lesser extent, the bronchi and bronchioles are filled with an exudate composed of many foamy macrophages, intact and necrotic neutrophils, fewer lymphocytes, occasional large colonies of 2 µm coccobacilli, eosinophilic beaded to fibrillar material (fibrin), and pale eosinophilic homogenous fluid (edema). Often surrounding bacterial colonies are necrotic leukocytes with streaming nuclei (oat cells). Alveolar septa are either expanded by fibrin and edema and lined by hyperplastic type II pneumocytes, or exhibit loss of architecture and replacement with eosinophilic cellular and karyorrhectic debris (lytic necrosis). Bronchial epithelial cells are often either cuboidal to attenuated, shrunken, and hypereosinophilic with a pyknotic nucleus (necrotic), or are lost, occasionally sloughed into the lumen, and replaced by a mat of necrotic debris and fibrin (fibrinonecrotic membrane). Subepithelial connective tissue of bronchi and bronchioles is expanded by moderate numbers of lymphocytes and plasma cells, necrotic cellular debris, fibrin, hemorrhage, and edema. Multifocally, the tunica intima and tunica media of small caliber blood vessels are blurred and expanded by fibrin, edema, karyorrhectic debris, and hemorrhage (necrotizing vasculitis). The luminae of affected vessels often contain variably organized layers of fibrin that either partially or completely occlude the vessel and are adhered to disrupted endothelium (fibrin thrombi). Less affected vessels are lined by hypertrophied, reactive endothelium. Interlobular septa and pleura are diffusely expanded by fibrin, necrotic debris, few inflammatory cells, occasional large colonies of coccobacilli, ectatic lymphatics, and marked edema. Multifocally, the pleural surface is covered with mats of fibrin.

GENERAL DISCUSSION

• Encapsulated, nonmotile, non-spore forming, facultative anaerobic, gramnegative, and pleomorphic coccobacillus
• Causes severe, often fatal, fibrinonecrotizing lobar pneumonia with accompanying fibrinous pleuritis
• Causes one of the most common, highly contagious, and important worldwide respiratory diseases of growing pigs; can cause disease without primary viral infection
• Can also cause septicemia (young pigs) and otitis media/interna with vestibular syndrome (weaned pigs)
• Two biovars and 19 recognized serovars; molecular techniques and serotyping necessary for biovar/serovar classification in outbreaks (To, 2026)
• Molecular typing with both genotype and serotype analysis is recommended as gold standard for accurate ID of APP isolates (To, 2026; To, 2023).

PATHOGENESIS

• May be carried in the nasopharynx of apparently healthy animals (carriers are the principal method of introduction onto a naive farm)
• Transmission via contaminated fomites or aerosol droplets > colonization of the tonsil, adherence to cilia of epithelium lining terminal bronchioles and alveolar type I pneumocytes (binds poorly to tracheal and bronchial epithelium and cilia) > phagocytosis by macrophages (interstitial, alveolar, intravascular), with prolonged survival within phagosome > release of pore-forming Apx exotoxin > macrophage lysis and release of bacterium; activated macrophages release interleukins and TNF > recruits neutrophils > phagocytosis by neutrophils > immediate killing by neutrophils
• Lesions due to injury and lysis of all cell populations within the respiratory system, including the vascular system (→ vasculitis and leukocyte necrosis)
• Activation and killing of intravascular macrophages > release of cytokines, proteolytic enzymes, oxygen free radicals (hydrogen peroxide, hydroxyl radical, superoxide anion) > damage to capillary and postcapillary venule endothelium > activation of coagulation cascade, hemorrhage, edema, effusion, thrombi, ischemia > pulmonary coagulative necrosis
• Requires iron to grow and replicate; utilizes porcine transferrin and iron from lysed red blood cells as iron source
• Main virulence factors:
  • Three (3) cytotoxins of the RTX (repeats in toxin) family (Apx I, II, III)) cause cytolysis of neutrophils, alveolar macrophages, erythrocytes, endothelium, and epithelium
  • Apx I and III are highly toxic to macrophages, neutrophils, and surrounding tissue in high concentrations
  • Apx toxin likely mediates leukocyte necrosis
  • A fourth Apx toxin is expressed in vivo, but not in vitro – significance of this toxin is unclear.
  • Lipopolysaccharide (LPS) induces macrophage activation and secretion of neutrophil chemoattractants, procoagulant activity, and complement activation; can bind hemoglobin to aid in transfer of iron molecule
  • Capsule impairs phagocytosis and may inhibit complement activation
• Some serovars are nonencapsulated, which can hamper accurate serodetection without molecular diagnostics (To, 2023)
• Additional virulence factors include:
  • Fimbrial adhesins (type 4 fimbriae facilitate adherence), outer membrane proteins, iron-binding proteins, metalloproteinase, and urease
  • A variety of antioxidant enzymes that facilitate survival amongst released cytotoxins and reactive oxygen species (ROS), including superoxide dismutase (remove oxygen free radicals), catalase, and hydrogen peroxide reductase
  • Urease-positive activity is utilized to differentiate A. pleuropneumoniae from other V-factor-dependent Pasteurellaceae species isolated from pigs

TYPICAL CLINICAL FINDINGS

• Disease is most common in pigs 6-weeks to 6-months of age, with peak mortality between 10-16 weeks of age, though all ages are susceptible

• Peracute/acute: High morbidity and mortality; sometimes death with no preceding clinical signs; blood-stained froth at nose and mouth
• Subacute/chronic: Coughing, dyspnea, lethargy, fever, anorexia, hypoxia
• Survivors of acute disease may become carriers
• +/- extrapulmonary manifestations (uncommon) such as arthritis, endocarditis, hepatitis, and lymph node abscesses

TYPICAL GROSS FINDINGS

• Fibrinosuppurative, hemorrhagic, and necrotizing lobar pneumonia or pleuropneumonia
• Lesions are most commonly caudodorsal with concentration around major bronchi near the hilus, though all lobes can be affected; uni- or bilateral
• Dark red, firm to hard and/or pale, friable foci of coagulative necrosis 1-10 cm in diameter
• Fibrinous pleuritis, adhesions, and abscesslike nodules surrounded by a thick connective tissue capsule

• Hemorrhagic and fibrinous pleural, pneumonic, and pericardial fluid
• Large, edematous, congested bronchial and mediastinal lymph nodes

TYPICAL LIGHT MICROSCOPIC FINDINGS

• Fibrinous pleuritis, fibrinosuppurative bronchopneumonia, and necrotizing pneumonia including thrombosis, vasculitis, edema, fibrin deposition, and neutrophilic and histiocytic inflammation
• Leukocyte necrosis is a prominent histologic feature
• Degenerate “streaming” alveolar leukocytes (similar to oat cells in bovine shipping fever pneumonia) bordering areas of coagulative necrosis
• Extrapulmonary vascular lesions (thrombosis and fibrinoid necrosis) possible, especially in the kidney
• Chronic cases may have sequestrum formation with multiple pulmonary abscesses and large (2 to 10 cm) pieces of necrotic lung encapsulated by connective tissue

ADDITIONAL DIAGNOSTIC TESTS

• Isolation of organism, in conjunction with suppurative bronchopneumonia with necrosis of neutrophils, is required for definitive diagnosis
• Culture or lung smear (identified via antigen detection such as ELISA or latex agglutination)
• Real-time PCR

DIFFERENTIAL DIAGNOSIS

• Gross:
• Salmonella enterica serovar Choleraesuis (formerly Salmonella choleraesuis): Fibrinonecrotic or hemorrhagic pneumonia; multifocal necrotizing hepatitis, splenomegaly, disseminated lymphadenopathy, and colitis
• Glaesserella parasuis (formerly Haemophilus parasuis, Glasser's Disease; P-B18): Occasionally suppurative bronchopneumonia; usually fibrinous meningitis, polyserositis, and/or polyarthritis
• Actinobacillus suis: Difficult to differentiate; causes pneumonia and septicemia in swine
• Microscopic:
• Pasteurella multocida: Suppurative bronchopneumonia with or without pleuritis; usually no vasculitis
• Actinobacillus suis: Difficult to differentiate; causes pneumonia and septicemia in swine
• Streptococcus suis: Occasionally associated with bronchopneumonia
• Trueperella pyogenes (formerly Arcanobacterium pyogenes): Chronic suppurative bronchitis, bronchiolitis, and pulmonary abscesses in sheep, cattle, and swine
• Swine influenza virus (P-V18): Causes extensive bronchiolar necrosis

COMPARATIVE PATHOLOGY

• Actinobacillus spp. in other species (A. pleuropneumoniae only affects swine):
• Foals: Actinobacillus equuli (U-B02)
  • Most common cause of embolic suppurative nephritis in foals
  • May also cause hepatitis, interstitial pneumonia, and fibrinous mesenteric lymphadenitis and peritonitis
• Cattle: Actinobacillus lignieresii in cattle (see H-B07); causes deep glossitis (wooden tongue) and/or stomatitis
• Fibrinous bronchopneumonia in domestic animals:
• Mannheimia haemolytica (P-B12), Histophilus somni (N-B03), Mycoplasma bovis, Mycoplasma mycoides ssp. mycoides small colony type (P-B02)

REFERENCES

1. Balestrin E, Wolf JM, et al. Molecular detection of respiratory coinfections in pig herds with enzootic pneumonia: a survey in Brazil. J Vet Diagn Invest. 2022;34(2):310-313.
2. Caswell JL, Williams KJ. Respiratory System. In: Maxie MG, ed. Jubb, Kennedy & Palmer's Pathology of Domestic Animals. Vol 2. 7th ed. St. Louis, MO: Elsevier; 2026:501-503,530-531.
3. Lopez A, Martinson SA. Respiratory System, Thoracic Cavities, Mediastinum, and Pleurae. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 7th ed. Elsevier; 2022:628-29.
4. To H, Kon M, Kawashima N, et al. Characterization of atypical Actinobacillus pleuropneumoniae serovar 1 isolates. J Vet Diagn Invest. 2026;38(2):259-264.
5. To H, Tsutsumi N, Ito S, Gottschalk M, Nagai S. Characterization of nonencapsulated Actinobacillus pleuropneumoniae serovar K12:O3 isolates. J Vet Diagn Invest. 2023;35(6):766-771.
6. Soutter F, Priestnall SL, et al. An Experimental Dermal Oedema Model for Apx Toxins of Actinobacillus pleuropneumoniae. J Comp Pathol. 2022;195:12-18.
7. Stanton JB, Zachary JF. Mechanisms of Microbial Infections. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 7th ed. Elsevier; 2022:216-17.