Viral Pneumonia

Definition & Overview

Common Causative Agents

Pathogenesis

1. Viral attachment and entry: Viruses attach to and enter respiratory epithelial cells (e.g., influenza hemagglutinin binds sialic acid residues on cell-surface polysaccharides, followed by endosomal fusion).
2. Cytopathic injury: Viral replication inhibits host cell mRNA translation and activates caspases, causing apoptosis of pneumocytes. Sodium channel inhibition causes fluid shifts leading to alveolar flooding.
3. Inflammatory cascade: Dying cells release danger signals activating resident macrophages; inflammatory mediators (chemokines, cytokines) recruit neutrophils into the interstitium within 1-2 days.
4. Impaired host defenses: Mucociliary clearance is disrupted, predisposing to bacterial superinfection (most commonly S. aureus, which can be life-threatening).
5. Innate immune response: Type I interferon production upregulates MX1 gene (encodes a GTPase that interferes with viral replication); NK cells and cytotoxic T cells kill infected cells.

• Antigenic drift - spontaneous point mutations in hemagglutinin/neuraminidase genes; causes annual epidemics
• Antigenic shift - reassortment of entire gene segments from animal influenza strains (swine/avian); causes pandemics (e.g., 1918, 2009 H1N1)

Clinical Features

Adults

• Gradual onset with cough, fever, myalgia, headache, malaise
• May have rhinorrhea and pharyngitis
• Tachypnea, dyspnea, pleuritic chest pain in more severe disease
• Crackles on auscultation; wheezing possible

Children (Rosen's Emergency Medicine)

• More common in winter season
• Gradual onset with cough, congestion, and low-grade fever
• Tachypnea may be the only physical finding in mild cases
• Retractions, rales, and wheezing are common
• Severe: grunting, cyanosis, lethargy, dehydration, apnea (especially in infants < 3 months)

Radiology

• Hyperinflation
• Peribronchial thickening (cuffing)
• Diffuse bilateral interstitial infiltrates
• Patchy areas of consolidation (lobular atelectasis)

Portable chest X-ray showing bilateral interstitial pattern characteristic of viral pneumonia - Rosen's Emergency Medicine

Special Populations

Pregnancy (Murray & Nadel)

Diagnosis

• Clinical: Winter presentation + fever + cough + interstitial infiltrates strongly suggests viral etiology
• Children generally do not require CXR or viral testing if clinical picture is clear
• Rapid antigen tests / PCR: for influenza, RSV, SARS-CoV-2
• Serology: Useful for retrospective diagnosis
• Lab findings: leukopenia or normal WBC (in contrast to leukocytosis in bacterial pneumonia); lymphopenia common with SARS-CoV-2

Management

Outpatient (Goldman-Cecil Medicine)

Inpatient (Goldman-Cecil Medicine)

• Influenza: Oseltamivir 75 mg twice daily × 5 days PLUS cover for bacterial co-infection (beta-lactam ± macrolide or respiratory fluoroquinolone)
• Other viral pneumonias: Treat as bacterial co-infection empirically (beta-lactam + macrolide or respiratory quinolone)
• SARS-CoV-2 severe: Dexamethasone, remdesivir; consider baricitinib or tocilizumab for cytokine storm

Supportive care

• Supplemental oxygen, escalating to high-flow nasal cannula or mechanical ventilation if needed
• Hydration
• Antipyretics/analgesics
• Bronchodilators if wheezing

Complications

• Bacterial superinfection (most common and serious - especially S. aureus, S. pneumoniae)
• ARDS (in severe influenza, COVID-19)
• Dehydration (especially in children)
• Bronchiolitis obliterans (rare, post-viral)
• Apnea (in infants)
• Secondary bacterial pneumonia (particularly S. aureus co-infection in influenza)
• Myocarditis (influenza, adenovirus)

Key Distinguishing Features: Viral vs. Bacterial Pneumonia

• Murray & Nadel's Textbook of Respiratory Medicine (Viral Pneumonia in Pregnancy chapter)
• Robbins, Cotran & Kumar Pathologic Basis of Disease, pp. 663-664
• Rosen's Emergency Medicine, p. 3192 (Pediatric Viral Pneumonia)
• Goldman-Cecil Medicine International Edition, Chapter 85 (Outpatient & Inpatient Management Tables)

Mode of Transmission - Viral Pneumonia

Routes and sites of viral entry. Respiratory droplet/aerosol is the primary route for pneumonia-causing viruses. - Sherris & Ryan's Medical Microbiology, 8th Ed.

Primary Routes for Pneumonia-Causing Viruses

1. Large-Particle Respiratory Droplets (Most Common)

• Generated by coughing, sneezing, and talking
• Droplets are heavy (>5 µm) and travel only short distances (generally within 2 meters) before settling
• Require close proximity between source and recipient
• Principal route for: Influenza A & B, RSV, parainfluenza, coronavirus, SARS-CoV-2

"Influenza is spread person to person, primarily through large-particle respiratory droplet transmission (e.g., coughing or sneezing), which requires close proximity between the person who is the source and the person who is the recipient because droplets generally only travel short distances." - Red Book 2021, p. 724

2. Small-Particle Aerosols (Airborne Transmission)

• Particles <5 µm that remain suspended in air for extended periods
• Can travel beyond 2 meters and linger in enclosed spaces
• Longer-range airborne transmission may occur under unique conditions (e.g., aerosol-generating medical procedures like intubation, bronchoscopy)
• Particularly relevant for: Measles (highly airborne), Varicella-Zoster, SARS-CoV-2 (in poorly ventilated spaces)

"Large-droplet and small-particle aerosols traveling over a short range (generally within 2 meters) appear to be the predominant route; longer-range airborne transmission may also occur rarely under unique conditions." - Goldman-Cecil Medicine, p. 3520

3. Contact Transmission (Direct & Indirect)

Direct Contact

• Hand-to-mucosa self-inoculation: Virus picked up from contaminated surfaces or secretions transferred by touching the nose, eyes, or mouth
• Virus can survive on hands and surfaces for up to 24 hours (influenza)
• Important for: Influenza, RSV, rhinovirus, adenovirus

Indirect Contact (Fomites)

• Virus deposited on inanimate objects (doorknobs, toys, bedside rails) and transferred to mucous membranes
• RSV is classically transmitted this way - survives up to 6 hours on surfaces
• Route: Nose/mouth → hand or object → nose (rhinovirus, coronavirus, adenovirus)

"Another mode of transmission comes from contact with influenza virus from droplet-contaminated hands or surfaces, where it can remain for up to 24 hours, with transfer from hands to mucosal surfaces of the face." - Red Book 2021

4. Skin-to-Air-to-Respiratory Tract

• Skin vesicle discharge → air → respiratory tract
• Primary route for Varicella-Zoster virus (chickenpox/shingles)
• Also relevant for Smallpox virus

5. Zoonotic Transmission (Animal-to-Human)

"Infection by avian viruses can occur after direct contact with infected birds or their excreta, exposure to contaminated environments and infectious aerosols, ingestion of inadequately cooked food, and sometimes by inoculation into the conjunctiva." - Goldman-Cecil Medicine

6. Vertical (Mother-to-Child) Transmission

• Less commonly causes pneumonia directly but relevant for congenital/perinatal disease
• In utero / transplacental: CMV, rubella, parvovirus B19
• Intrapartum (birth canal): HSV, hepatitis B/C, HIV
• Postpartum via breastfeeding: CMV, HTLV, HIV

Transmission Summary by Virus

Key Epidemiological Points

• Incubation period of influenza: 1-4 days (mean 2 days); patients are infectious 24 hours before symptom onset - Red Book 2021
• Peak viral shedding in nasal secretions: first 3 days of illness; ceases within 7 days in most adults but can persist 10+ days in young children and immunocompromised patients
• The receptor binding site determines transmissibility: human influenza strains bind α-2,6-linked sialosaccharides (upper airway); avian strains prefer α-2,3-linked sialosaccharides (distal bronchioles/alveoli) - which is why avian flu is less efficiently transmitted human-to-human but causes more severe lower lung disease - Goldman-Cecil Medicine

Infection Control Implications

• Jawetz, Melnick & Adelberg's Medical Microbiology, 28th Ed. - Modes of Transmission
• Sherris & Ryan's Medical Microbiology, 8th Ed. - Table 7-1 Common Routes of Transmission
• Red Book 2021 (AAP Committee on Infectious Diseases) - Influenza Epidemiology
• Goldman-Cecil Medicine International Edition - Influenza Pathobiology, p. 3520