Influenza

Definition | Aetiology | Pathophysiology | Risk Factors | Signs and Symptoms | Investigations | Management | Referral

Definition

Influenza (flu) is an acute viral respiratory infection caused by influenza viruses. It can range from a mild self-limiting illness to severe complications, particularly in high-risk groups.

Aetiology

Influenza is caused by the influenza virus, an RNA virus belonging to the Orthomyxoviridae family. There are three main types:

  • Influenza A: most severe, responsible for seasonal epidemics and pandemics (e.g., H1N1, H3N2).
  • Influenza B: less severe but can still cause outbreaks.
  • Influenza C: causes mild respiratory illness, not associated with epidemics.

Pathophysiology

Influenza infects the respiratory epithelium, leading to:

  • Destruction of airway epithelial cells.
  • Increased mucus production and inflammation.
  • Impaired mucociliary clearance, increasing risk of secondary bacterial infections.
  • Systemic symptoms due to cytokine release (fever, myalgia).

Risk factors

  • Age <5 years or >65 years.
  • Chronic lung disease (e.g., asthma, COPD).
  • Immunosuppression (e.g., chemotherapy, HIV, transplant patients).
  • Pregnancy.
  • Obesity (BMI >40).
  • Healthcare workers (increased exposure risk).

Signs and symptoms

Symptoms:

  • Sudden onset fever (≥38°C).
  • Dry cough.
  • Myalgia (muscle aches).
  • Fatigue and malaise.
  • Headache.
  • Sore throat and nasal congestion.

Signs:

  • Fever.
  • Tachypnoea (fast breathing).
  • Pharyngeal erythema (red throat).
  • Wheeze or crackles (if complicated by pneumonia).

Investigations

Influenza is usually diagnosed clinically during flu season. Investigations may be needed in severe or high-risk cases:

  • Viral PCR (nasopharyngeal swab): gold standard for confirming influenza.
  • Chest X-ray: if pneumonia is suspected, may show patchy infiltrates.
  • Full blood count (FBC): may show normal or slightly raised white cell count.
  • C-reactive protein (CRP): raised in secondary bacterial infection.
  • Oxygen saturation: assess need for supplemental oxygen.

Management

1. Supportive Care:

  • Rest and hydration.
  • Paracetamol: for fever and myalgia.
  • Oxygen therapy: if SpO₂ <92%.

2. Antiviral Therapy (if indicated):

  • Oseltamivir (Tamiflu) 75 mg BD for 5 days: recommended in high-risk patients or severe cases.
  • Most effective if started within 48 hours of symptom onset.

3. Antibiotics (only if secondary bacterial infection suspected):

  • If bacterial pneumonia develops, consider co-amoxiclav or doxycycline.

Referral

Refer to secondary care if any of the following are present:

  • Severe respiratory distress.
  • SpO₂ persistently <92% despite oxygen.
  • Haemodynamic instability (hypotension, tachycardia).
  • Altered mental status (suggesting encephalopathy).
  • High-risk patients: pregnant women, immunocompromised individuals, or those with severe comorbidities.
 
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