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Viral Pneumonia

Definition and Overview

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Common Causative Agents

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Pathogenesis

1. Primary viral pneumonia - infection initiated in and confined to the respiratory tract (influenza, RSV)
2. Respiratory onset with systemic spread - infection begins in the airway but causes systemic disease (measles, varicella)
3. Pulmonary involvement secondary to systemic infection - respiratory tract is a secondary target (CMV, disseminated HSV)

Influenza - the Prototype

• Virus reaches the lung by contiguous spread from the upper airway or inhalation of small-particle aerosols
• Initial infection of ciliated respiratory epithelium of the trachea, bronchi, and lower respiratory tract causes widespread epithelial cell destruction
• Viral replication inhibits sodium channels → electrolyte/water shifts → fluid accumulation in alveolar lumen
• Infected cells undergo apoptosis (caspase activation) and inhibition of host mRNA translation
• "Danger signals" released from dying cells activate resident macrophages
• Inflammatory mediators (chemokines, cytokines) activate pulmonary endothelium and recruit neutrophils within the first 1-2 days
• Innate immune responses include type I interferon production, which upregulates MX1 gene (encodes GTPase that interferes with viral replication)
• NK cells and cytotoxic T cells kill infected pneumocytes; eventually, antibody responses to hemagglutinin and neuraminidase augment clearance

RSV and Parainfluenza (Children)

Influenza Hemagglutinin and Neuraminidase

• Hemagglutinin (H1, H2, H3): attaches virus to sialic acid residues on surface polysaccharides; acidic endosome triggers conformational change allowing viral envelope-cell membrane fusion and genome release
• Neuraminidase (N1, N2): cleaves sialic acid to release newly formed virions from infected cells
• Antigenic drift: spontaneous mutations alter H and N epitopes → seasonal epidemics
• Antigenic shift: replacement of H and/or N genes via recombination with animal flu viruses → pandemics (essentially all individuals susceptible)

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Clinical Features

Adults (Fishman's Pulmonary Diseases, p. 2219)

• Nonproductive cough (frothy, pink-tinged sputum in severe cases)
• Cyanosis and hypoxemia (severe disease)
• Nonspecific findings: conjunctivitis, rhinitis, tracheal tenderness, increased respiratory rate
• Diffuse rales and wheezes
• Sputum - scant, few PMNs, minimal bacteria on Gram stain
• Influenza: abrupt onset with prominent systemic symptoms - fever, chills, headache, severe myalgia, malaise - followed by cough and dyspnea

Children (Rosen's Emergency Medicine, p. 3192)

• More common in winter season with gradual onset
• Cough, congestion, low-grade fever
• Tachypnea may be the only finding; also retractions, rales, wheezing
• Grunting, cyanosis, lethargy, dehydration, and apnea in severe cases
• Young infants: apneic episodes with minimal fever

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Radiographic Findings

• Hyperinflation and peribronchial thickening
• Diffuse increase in interstitial markings
• Patchy consolidation (lobular atelectasis or alveolar pneumonia)

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Risk Factors for Severe Disease

• Extremes of age (infants, elderly)
• Malnutrition and alcoholism
• Underlying cardiopulmonary disease (valvular heart disease, COPD)
• Pregnancy (especially 2nd and 3rd trimester; also postpartum) - 10% of 1957 influenza fatalities were pregnant women; 35% ICU admission and 7% mortality in H1N1 2009 pandemic
• Immunosuppression
• Comorbidities: diabetes, obesity, heart disease, immunosuppression (especially for influenza pandemics)

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Complications

Bacterial Superinfection (Major Complication)

• Typical viral illness → partial or complete recovery → recurrence of fever + cough + dyspnea 2-14 days later
• CXR shows new lobar infiltrates
• Clinical course typical of bacterial pneumonia

1. Disruption of epithelial barrier
2. Loss of mucociliary clearance (markedly decreased, demonstrated with labeled particles in influenza)
3. Virally-induced type I interferon response downregulates host antibacterial defenses
4. Impaired innate immunity

Other Complications

• ARDS (severe primary viral pneumonia)
• Dehydration (especially children)
• Bronchiolitis obliterans
• Apnea (infants < 3 months)
• Thromboembolic disease (especially COVID-19 in pregnancy)

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Diagnosis

• Clinical - seasonal presentation, viral symptoms prodrome, typical age/epidemiology
• Children: do not require CXR or viral testing routinely in classic winter presentations with fever, cough, congestion, and wheezing (Rosen's EM, p. 3192)
• Rapid antigen tests (influenza, RSV)
• PCR-based respiratory panels (most sensitive; detect multiple viruses simultaneously)
• Sputum - scant; Gram stain shows minimal bacteria (helps distinguish from bacterial)
• Serology - retrospective, limited clinical utility

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Treatment and Management

Outpatient Management (Goldman-Cecil Medicine, p. 3050)

Inpatient Management (Goldman-Cecil Medicine, p. 3085-3098)

Specific Antiviral Therapies

• Influenza: Oseltamivir (oral), Zanamivir (inhaled), Baloxavir
• RSV: Supportive care; Ribavirin (severe/immunocompromised); Palivizumab (prophylaxis in high-risk infants)
• Varicella pneumonia: Acyclovir (reduces mortality, especially in pregnancy)
• CMV pneumonia (immunocompromised): Ganciclovir or Valganciclovir
• HSV pneumonia: Acyclovir

Children

• Most viral pneumonias resolve without therapy
• Consider antibiotics if severely ill or when bacterial co-infection cannot be excluded
• Supportive care: hydration, oxygen, respiratory support as needed

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Special Populations

Pregnancy (Murray & Nadel's Respiratory Medicine, p. 3027)

• Viral pneumonia carries significantly higher maternal mortality
  • 1918 pandemic: maternal mortality up to 50%
  • 1957 epidemic: pregnant women = 10% of all fatalities
  • H1N1 2009: 35% ICU admission, 7% mortality in infected pregnant women
• Early treatment with oseltamivir or zanamivir is beneficial
• Varicella pneumonia in pregnancy: 35% mortality (vs. 10% in other adults); treat with acyclovir
• Inactivated influenza vaccine is recommended in all trimesters; live attenuated vaccine is contraindicated

Immunocompromised

• Broader spectrum of pathogens (CMV, HSV, VZV, adenovirus)
• More severe disease; higher mortality
• Lower threshold for antiviral therapy and invasive diagnostics

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Prevention

• Annual influenza vaccination (recommended for all ages > 6 months, all trimesters in pregnancy)
• COVID-19 vaccination
• RSV prophylaxis with palivizumab (high-risk infants; premature, BPD, congenital heart disease)
• Respiratory isolation, hand hygiene
• Varicella-zoster immune globulin within 96 hours of varicella exposure in susceptible pregnant women

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• Robbins, Cotran & Kumar - Pathologic Basis of Disease, pp. 663-664
• Fishman's Pulmonary Diseases and Disorders, pp. 2219-2222
• Murray & Nadel's Textbook of Respiratory Medicine, pp. 3027-3029
• Rosen's Emergency Medicine - Concepts and Clinical Practice, pp. 3192-3193
• Goldman-Cecil Medicine, pp. 3050-3098

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🫁 Clinical Reference Guide: Rapid Identification & Treatment of Viral vs. Bacterial Pneumonia

For Medical Students | Based on Harrison's, Murray & Nadel, Robbins, Fishman's, Goldman-Cecil, Rosen's EM

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SECTION 1 - First Glance: How to Start Thinking

1. How did it start? - Abrupt onset favors bacterial or influenza; gradual/insidious favors atypical or viral
2. What does the sputum look like? - Productive, purulent = bacterial; scant, non-purulent = viral/atypical
3. What season is it, and what is the patient's age? - Winter + child = think RSV/parainfluenza; any season + elderly/comorbid = think bacterial until proven otherwise

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SECTION 2 - Side-by-Side Comparison

2A. Clinical Features

Teaching pearl: Relative bradycardia means the pulse does NOT rise by the expected 10 bpm per 1°C of fever - this is a clue to atypical organisms (Legionella, Mycoplasma). - Murray & Nadel, p. 985

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2B. Chest X-Ray (CXR) Patterns

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2C. Laboratory Findings

Key teaching point: CRP, ESR, and PCT help trend direction, but no single lab test reliably differentiates viral from bacterial pneumonia. The Harriet Lane Handbook states directly: "CBC or inflammatory markers (CRP, ESR, procalcitonin) are not reliable to differentiate bacterial vs viral pneumonia."

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2D. Common Pathogens by Category

Bacterial CAP - Typical

Bacterial CAP - Atypical ("walking pneumonia")

Viral CAP

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SECTION 3 - Severity Assessment: Where Does This Patient Go?

CURB-65 Score (for hospitalization triage in CAP)

CURB-65 is easy to use at the bedside. PSI is more accurate for predicting short-term mortality but requires more variables. Neither reliably identifies need for ICU - use clinical judgment + ATS/IDSA severe CAP criteria for ICU decisions. - Murray & Nadel, p. 907-913

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SECTION 4 - Treatment

4A. Viral Pneumonia Treatment

Key rule: All inpatients with viral pneumonia should be treated for possible bacterial coinfection with standard CAP antibiotic regimens. - Goldman-Cecil Medicine, p. 3085-3098

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4B. Bacterial Pneumonia Treatment

Outpatient CAP (no comorbidities)

• Amoxicillin 1 g TID × 5 days OR
• Doxycycline 100 mg BID × 5 days OR
• Azithromycin 500 mg day 1, then 250 mg days 2-5 (where macrolide resistance is low)

Outpatient CAP (comorbidities: COPD, DM, renal/hepatic disease, or recent antibiotics)

• Respiratory fluoroquinolone: Levofloxacin 750 mg OD × 5 days or Moxifloxacin 400 mg OD × 5 days OR
• Beta-lactam + macrolide combination

Inpatient CAP - Non-severe (no risk factors for MRSA or Pseudomonas)

• Beta-lactam + macrolide: ampicillin 1.5-3 g q6h OR ceftriaxone 1-2 g OD PLUS azithromycin 500 mg OD or clarithromycin 500 mg BID
• Monotherapy: Levofloxacin 750 mg OD or Moxifloxacin 400 mg OD
• If macrolide + fluoroquinolone contraindicated: beta-lactam + doxycycline 100 mg BID
• Newer agents: Omadacycline or Lefamulin (step-down capable)

Inpatient CAP - Severe

• Beta-lactam + macrolide OR beta-lactam + respiratory fluoroquinolone
• MRSA suspected: add vancomycin 15 mg/kg q12h (adjust by levels) or linezolid 600 mg q12h
• Pseudomonas suspected: piperacillin-tazobactam 4.5 g q6h, cefepime 2 g q8h, meropenem 1 g q8h

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4C. Atypical Bacterial Pneumonia Treatment

Note: Beta-lactams are NOT effective against atypical organisms (no cell wall).

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SECTION 5 - Bacterial Superinfection After Viral Pneumonia

Day 1-5: Viral illness (flu symptoms, low-grade fever, myalgia)
    ↓
Day 5-7: Apparent improvement
    ↓
Day 7-14: Sudden RECURRENCE of fever + productive cough + pleuritic pain
          New lobar infiltrates on CXR = BACTERIAL SUPERINFECTION

• Staphylococcus aureus (including MRSA) - most common and most lethal post-influenza
• Streptococcus pneumoniae
• Haemophilus influenzae

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SECTION 6 - Special Populations Quick Reference

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SECTION 7 - Diagnostic Algorithm (Step-by-Step)

Patient with fever + cough + respiratory symptoms
               │
    ┌──────────▼──────────────┐
    │   Clinical assessment   │
    │   Onset? Sputum? Season?│
    │   Comorbidities? Travel?│
    └──────────┬──────────────┘
               │
    ┌──────────▼──────────────────────────────────────────┐
    │  Chest X-Ray (always obtain if admitting or severe) │
    │  Lobar consolidation → likely bacterial             │
    │  Bilateral interstitial → likely viral              │
    │  Cavitation → S. aureus, anaerobes, Klebsiella, TB  │
    └──────────┬──────────────────────────────────────────┘
               │
    ┌──────────▼──────────────────────────────────────────┐
    │  Labs: CBC, CRP, PCT, blood cultures (if admitting) │
    │  Sputum Gram stain + culture                        │
    │  Rapid viral tests (flu, RSV, COVID)                │
    │  Urinary Legionella Ag + pneumococcal Ag            │
    │  Multiplex PCR if available                         │
    └──────────┬──────────────────────────────────────────┘
               │
    ┌──────────▼──────────────────────────────────────────┐
    │  CURB-65 → decide site of care                      │
    │  0-1: Outpatient   2: Supervised   ≥3: Admit        │
    └──────────┬──────────────────────────────────────────┘
               │
    ┌──────────▼────────────────────────────────────┐
    │  TREAT empirically - don't wait for cultures  │
    │  Viral + inpatient → ALWAYS add antibiotics   │
    │  for bacterial coinfection                    │
    └───────────────────────────────────────────────┘

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SECTION 8 - High-Yield Memory Anchors

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Sources

• Robbins, Cotran & Kumar - Pathologic Basis of Disease, pp. 663-664
• Fishman's Pulmonary Diseases and Disorders, pp. 2219-2222
• Murray & Nadel's Textbook of Respiratory Medicine, pp. 984-985, 907-913
• Rosen's Emergency Medicine - Concepts and Clinical Practice, pp. 3192-3193
• Goldman-Cecil Medicine, pp. 3050-3103
• The Harriet Lane Handbook (Johns Hopkins), Pulmonary section