Influenza Viral Pneumonia 

Introduction

Influenza viral pneumonia is a severe lower respiratory tract infection caused by influenza viruses, characterized by diffuse alveolar injury, severe hypoxemia, and potential progression to ARDS, multiorgan failure, and death. It represents one of the most important viral causes of ICU admission during seasonal epidemics and pandemics.

From an exam and critical care perspective, influenza pneumonia is crucial because:

  • It causes primary viral pneumonia
  • It predisposes to secondary bacterial pneumonia
  • It may produce cytokine storm–mediated lung injury
  • It often requires advanced ventilatory and ECMO support


Etiology and Virology

Type

Clinical Importance

Influenza A

Most severe disease, pandemics

Influenza B

Seasonal epidemics, less severe

Influenza C

Mild disease


Influenza A Subtypes

Defined by surface glycoproteins:

  • Hemagglutinin (HA) → Viral entry
  • Neuraminidase (NA) → Viral release

Examples:

  • H1N1
  • H3N2
  • Avian influenza (H5N1, H7N9)


Mechanisms of Viral Evolution

Antigenic Drift

  • Minor mutations
  • Causes seasonal epidemics

Antigenic Shift

  • Major genetic reassortment
  • Leads to pandemics
  • Seen only in Influenza A


Epidemiology

  • Peak incidence: Winter months
  • Spread: Respiratory droplets and aerosols
  • Incubation period: 1–4 days
  • Infectivity: Begins 1 day before symptoms


High-Risk Groups

  • Elderly
  • Pregnant women
  • Chronic lung disease
  • Chronic cardiac disease
  • Diabetes mellitus
  • Immunocompromised patients
  • Morbid obesity
  • Children <5 years
  • Healthcare workers


Pathogenesis

Influenza pneumonia occurs via direct viral cytopathic injury plus host inflammatory response.


Stepwise Pathophysiology

1. Viral Entry

  • Virus attaches via hemagglutinin to respiratory epithelial sialic acid receptors
  • Viral replication occurs in:
    • Trachea
    • Bronchi
    • Alveoli


2. Epithelial Damage

Results in:

  • Loss of mucociliary clearance
  • Exposure of basement membrane
  • Necrosis of respiratory epithelium


3. Immune Response and Cytokine Storm

  • Release of:
    • IL-6
    • TNF-α
    • Interferons

Leads to:

  • Diffuse alveolar damage
  • Capillary leak
  • ARDS


4. Secondary Bacterial Superinfection

Common organisms:

  • Staphylococcus aureus (including MRSA)
  • Streptococcus pneumoniae
  • Haemophilus influenzae


Types of Influenza-Associated Pneumonia

1. Primary Viral Pneumonia

Most severe form.

Features:

  • Rapid progression
  • Severe hypoxemia
  • ARDS
  • High mortality


2. Secondary Bacterial Pneumonia

Occurs after initial improvement.

Clues:

  • Biphasic illness
  • Recurrent fever
  • Purulent sputum
  • Focal consolidation


3. Mixed Viral-Bacterial Pneumonia

Common in ICU patients.


Histopathology

Typical findings:

  • Diffuse alveolar damage
  • Hyaline membrane formation
  • Interstitial inflammation
  • Necrotizing bronchiolitis


Clinical Features

Typical Influenza Symptoms

  • Fever
  • Myalgia
  • Headache
  • Malaise
  • Dry cough
  • Sore throat


Features Suggesting Pneumonia

  • Progressive dyspnea
  • Hypoxemia
  • Tachypnea
  • Cyanosis
  • Hemoptysis (rare)


Red Flag ICU Features

  • Respiratory failure
  • Shock
  • Altered sensorium
  • Multiorgan dysfunction


Investigations


Laboratory Findings

CBC

  • Leukopenia or leukocytosis
  • Lymphopenia common

Inflammatory Markers

  • Elevated CRP
  • Elevated Procalcitonin (suggests bacterial coinfection)

ABG

  • Hypoxemia
  • Respiratory alkalosis early
  • Metabolic acidosis late


Microbiological Diagnosis

Gold Standard

✔ RT-PCR from respiratory samples

Samples:

  • Nasopharyngeal swab
  • Endotracheal aspirate
  • Bronchoalveolar lavage


Other Tests

  • Rapid antigen tests (less sensitive)
  • Viral culture (rarely used)


Radiological Findings

Chest X-ray Findings

  • Bilateral interstitial infiltrates
  • Patchy alveolar opacities
  • Diffuse ground glass opacities
  • ARDS pattern in severe disease


CT Chest Findings

Typical CT Patterns:

  • Ground glass opacities
  • Consolidation
  • Crazy paving pattern
  • Air bronchograms
  • Diffuse alveolar damage


Severity Assessment

Predictors of Severe Disease

  • Age >65
  • Pregnancy
  • Immunosuppression
  • Rapidly worsening hypoxemia
  • High lactate
  • Multilobar infiltrates


Complications

Pulmonary

  • ARDS
  • Necrotizing pneumonia
  • Pneumothorax
  • Pulmonary hemorrhage


Extrapulmonary Complications

Neurological

  • Encephalitis
  • Guillain–Barré syndrome
  • Reye syndrome (aspirin in children)

Cardiac

  • Myocarditis
  • Pericarditis

Musculoskeletal

  • Myositis
  • Rhabdomyolysis

Others

  • Sepsis
  • Multiorgan failure


Management

General Principles

  • Early antiviral therapy
  • Supportive care
  • Prevention of secondary infection
  • Lung protective ventilation


Antiviral Therapy

Neuraminidase Inhibitors

Oseltamivir (First Line)

✔ Drug of choice

Dose:

  • Standard: 75 mg twice daily
  • Severe/ICU: Often 150 mg BD (expert practice; guideline evidence mixed)

Duration:

  • Minimum 5 days
  • Severe disease: 7–10 days or longer


Dose Adjustment:

  • Required in renal failure


Alternative Antivirals

Drug

Indication

Zanamivir

Inhaled therapy

Peramivir

IV option

Baloxavir

Single dose therapy (mild disease mainly)


Timing of Antivirals

  • Most effective within 48 hours
  • ICU patients benefit even if started late


Antibiotic Therapy

Indicated if:

  • Suspected bacterial coinfection
  • Severe pneumonia
  • Septic shock

Empiric Coverage

  • MRSA coverage often recommended
  • CAP guidelines followed


Respiratory Support

Oxygen Therapy

  • Target SpO₂: 92–96%

HFNC / NIV

  • Useful in moderate hypoxemia
  • Requires close monitoring

Mechanical Ventilation

Lung Protective Strategy

  • Tidal volume: 4–6 mL/kg IBW
  • Plateau pressure <30 cmH₂O
  • Driving pressure <15


Adjunct ARDS Therapies

  • Prone ventilation
  • Neuromuscular blockade (early severe ARDS)
  • Conservative fluid strategy
  • ECMO in refractory hypoxemia


ECMO Indications

  • PaO₂/FiO₂ <80 despite optimal care
  • Refractory hypercapnia
  • Severe lung compliance reduction


Corticosteroids

Controversial

  • Routine use NOT recommended
  • May increase viral replication
  • Consider only if:
    • Septic shock
    • ARDS with other indication


Infection Control

  • Droplet precautions
  • Airborne precautions during aerosol-generating procedures
  • Isolation protocols


Prevention


Vaccination

✔ Most effective preventive strategy

Recommended For:

  • Elderly
  • Healthcare workers
  • Chronic disease patients
  • Pregnant women


Chemoprophylaxis

  • Oseltamivir for high-risk exposure


Prognosis

Mortality Predictors

  • ARDS
  • Shock
  • Bacterial coinfection
  • Delayed antiviral therapy
  • Immunosuppression