Influenza

Influenza is an acute viral respiratory infection caused by Orthomyxoviridae viruses. It remains a major cause of global morbidity and mortality, with significant health, social, and economic impacts. Epidemics occur seasonally, while pandemics arise from new influenza A subtypes.

• Types of Influenza:
  • 🅰️ Influenza A: Multiple animal reservoirs (birds, pigs). Causes pandemics via antigenic shift (e.g. 1918, 2009 H1N1).
  • 🅱️ Influenza B: Only humans; less severe, seasonal outbreaks. Still significant in children.
  • 🇨 Influenza C: Mild, self-limiting, rarely causes outbreaks.
  • 🇩 Influenza D: Emerging, mainly cattle; not yet a proven human pathogen.

🧬 Virology: Antigenic Drift and Shift

• Antigenic Drift: Gradual point mutations in Haemagglutinin (H) and Neuraminidase (N). Explains why seasonal flu vaccines need yearly updates.
• Antigenic Shift: Abrupt reassortment between strains (e.g. bird + pig + human). Creates novel subtypes with little human immunity → pandemics.

📡 Transmission and Spread

• Spread via droplets, aerosols, and fomites.
• Contagious from 1–2 days before symptoms until ~7 days after (longer in children or immunocompromised).
• Closed settings (care homes, schools, prisons) accelerate outbreaks.

🔬 Viral Structure

• Haemagglutinin (H) → binds host epithelial cells (entry).
• Neuraminidase (N) → cleaves sialic acid to release new virions (spread).
• Other proteins (M2, NS1) target host immunity — basis for some antiviral drugs.

🤒 Clinical Presentation

• Rapid onset: high fever, chills, myalgia, headache, cough, sore throat, fatigue.
• Children: often vomiting, abdominal pain, febrile seizures.
• Adults: high absenteeism, prolonged fatigue (“post-viral syndrome”).

⚠️ Complications

• Pneumonia: Viral or secondary bacterial (esp. S. pneumoniae, S. aureus).
• Cardiac: Myocarditis, arrhythmias, heart failure exacerbation.
• Neurological: Guillain-Barré, encephalitis, acute necrotising encephalopathy (rare, severe in children).
• Exacerbations: COPD/asthma flare-ups, decompensated diabetes.

🧪 Investigations

• RT-PCR: 🥇 Gold standard for diagnosis (nasopharyngeal swab).
• Rapid antigen tests: Point-of-care, less sensitive.
• Chest X-ray: Indicated if pneumonia suspected.

💊 Management

• Supportive care: Rest, hydration, paracetamol/NSAIDs, oxygen if hypoxic.
• Antivirals (best if started ≤48h from onset):
  • 💊 Oseltamivir (Tamiflu): Oral, 75 mg BD x 5d.
  • 💨 Zanamivir: Inhaled, useful in Oseltamivir resistance.
  • 💉 Peramivir: IV option (severe cases, limited use in UK).
  • 🧬 Baloxavir: Newer single-dose cap-dependent endonuclease inhibitor (licensed in Japan/US, not yet routine in UK).
• Prophylaxis: Antivirals for vulnerable contacts during outbreaks (care homes, oncology wards).
• Hospitalisation: For hypoxia, severe pneumonia, or vulnerable high-risk patients.

💉 Prevention and Vaccination

• Annual influenza vaccination: Updated seasonally based on WHO global surveillance.
• Priority groups: Elderly, pregnant women, children (UK nasal spray programme), healthcare workers, immunocompromised, chronic illness patients.
• 🧼 Hygiene: handwashing, masks in outbreaks, respiratory etiquette.
• COVID-19 lessons: Flu + COVID co-infection worsens outcomes → dual vaccination campaigns now common in UK.

📊 Public Health & Modern Considerations

• 🛰️ WHO Global Influenza Surveillance and Response System (GISRS) monitors strains for vaccine design.
• 📉 Economic burden: lost productivity + NHS winter pressures.
• 🌍 Emerging zoonotic threats: Avian flu (H5N1, H7N9) & swine flu (H1N1) highlight pandemic risk.
• 📦 Stockpiles of antivirals and PPE remain part of UK’s national preparedness strategy.

🧑‍⚕️ Teaching Point

Think of influenza not just as a seasonal illness, but as a global health threat with pandemic potential. Always consider secondary bacterial pneumonia if a patient initially improves then deteriorates. Vaccination + early antiviral use in high-risk groups saves lives.