Pulmonary infections (except tuberculosis)

Overview: Pneumonia Classification

By histological pattern:

• Fibrinopurulent alveolar exudates → acute bacterial pneumonia
• Mononuclear interstitial infiltrates → viral / atypical pneumonia
• Granulomas + cavitation → chronic pneumonia

By pattern of spread:

1. Bronchopneumonia: patchy distribution throughout both lungs; starts in bronchi/bronchioles → extends to adjacent alveoli
2. Lobar pneumonia: entire lobe involved; homogeneous exudate; ↑ virulence or ↑ host vulnerability; S. pneumoniae responsible for >90%

Other classifications: Acute/chronic; community-acquired/nosocomial/opportunistic; bacterial/viral/fungal; normal/immunocompromised host

---

A) Community-Acquired Pneumonia

S. pneumoniae (most common cause)

• At-risk groups: chronic disease (CHF, COPD, DM), Ig defects, ↓/absent splenic function
• Lower and right middle lobes most frequently involved (aspiration of laryngeal flora)

4 stages of lobar pneumonia:

1. Congestion: heavy, red, boggy lung; protein-rich fluid + neutrophils + bacteria in alveoli
2. Red hepatization: firm; alveoli packed with neutrophils, RBCs, fibrin
3. Grey hepatization: dry, gray, firm; RBCs lysed; fibrinopurulent exudate persists
4. Resolution: exudate enzymatically digested → resorbed / ingested by macrophages / organized by fibroblasts; pleural fibrinous pleuritis may resolve or leave fibrous adhesions

Complications: abscess formation, empyema (pus in pleural cavity), scarring, dissemination (meningitis, arthritis, endocarditis)

Legionella pneumonia (Legionnaire’s disease)

• Causative agent: Legionella pneumophila
• Also causes Pontiac fever (self-limited URTI without pneumonia)
• Thrives in water-cooling towers, domestic water tubing
• Transmission: inhalation of aerosolized organism or aspiration of contaminated water
• At risk: smokers >55 yrs, immunocompromised, organ-transplant recipients

Other community-acquired bacteria

• Haemophilus influenzae, Moraxella catarrhalis, S. aureus, Klebsiella pneumoniae, Pseudomonas aeruginosa

Atypical pneumonia

• Clinical features inconsistent with typical pneumonia: moderate sputum, no consolidation, moderate ↑ WBC, no alveolar exudate
• Most common cause: Mycoplasma pneumoniae (children and young adults; closed communities)
• Others: Chlamydia pneumoniae, Coxiella burnetii, RSV, influenza/parainfluenza, adenovirus
• Pathogenesis: organism → respiratory epithelial necrosis → ↓ mucociliary clearance → secondary bacterial infection; extends to alveoli → interstitial inflammation
• Morphology: red-blue congestion (macro); bronchiolitis + widened alveolar septa with lymphoplasmacytic infiltrate (micro)
• Clinical: fever, headache, malaise → cough with minimal sputum

---

B) Nosocomial Pneumonia

• Hospital-acquired; common in ICU patients, ventilated, immunosuppressed, prolonged antibiotic therapy
• Most common agents: Gram-negative rods (Enterobacteriaceae, Pseudomonas) and S. aureus

---

C) Aspiration Pneumonia

• Weakened patients aspirating gastric contents (unconscious or vomiting)
• Chemical irritation from gastric acid + bacteria → lung damage
• Abscess formation is a common complication
• Agents: aerobic (S. pneumoniae, S. aureus, H. influenzae) + anaerobic oral flora (Bacteroides, Prevotella, Fusobacterium)

Lung Abscess

• Localized suppurative necrosis forming one or more large cavities
• Causes: aspiration (dental/sinus infection, oral surgery, alcoholism, coma), gastric aspiration, complication of necrotizing pneumonia, neoplastic obstruction
• Rupture into airways → air-fluid level on X-ray
• Rupture into pleural cavity → bronchopleural fistula → pneumothorax or empyema
• Clinical: foul-smelling, purulent/bloody sputum; malaise; spiking fevers

---

D) Opportunistic Infections

Candida albicans

• Most common disease-causing fungus; normal flora of oral cavity, GI, vagina
• Invasive candidiasis: bloodborne dissemination → renal/myocardial abscesses, endocarditis, brain, candida pneumonia

Aspergillus

• Opportunistic pathogen in immunocompromised
• Invasive aspergillosis: necrotizing pneumonia → vascular invasion → systemic dissemination
• Aspergilloma: fungus ball in preformed lung cavity
• ABPA (Allergic bronchopulmonary aspergillosis): type I hypersensitivity in asthma patients; worsened symptoms

💡 High-yield: Lobar pneumonia = S. pneumoniae >90%; 4 stages: congestion → red hepatization → grey hepatization → resolution. Atypical = Mycoplasma; no consolidation; interstitial infiltrate. Legionella = cooling towers; smokers. Aspiration → abscess. Aspergillus = immunocompromised; aspergilloma = fungus ball.