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Influenza

Specialties > Child health

Background knowledge 🧠

Definition

  • Influenza is a highly contagious viral respiratory illness caused by influenza viruses (types A, B, and C).
  • Characterised by sudden onset of fever, cough, and muscle aches.
  • Commonly known as the flu, it can lead to serious complications, especially in vulnerable populations.
  • Seasonal flu epidemics typically occur during the winter months.

Epidemiology

  • Worldwide distribution with seasonal epidemics in temperate regions.
  • In the UK, influenza activity typically peaks between December and March.
  • Annual attack rates: 5-10% in adults and 20-30% in children.
  • Higher morbidity and mortality rates in elderly, pregnant women, and individuals with comorbidities.
  • Annual flu vaccination is recommended for at-risk groups.

Aetiology and Pathophysiology

  • Caused by influenza viruses, primarily Influenza A and B.
  • Influenza A is subtyped based on hemagglutinin (H) and neuraminidase (N) proteins (e.g., H1N1).
  • Influenza B is categorised into lineages (e.g., Victoria and Yamagata).
  • Virus enters the respiratory tract, leading to inflammation and necrosis of respiratory epithelium.
  • Cytokine release contributes to systemic symptoms.
  • Transmission primarily via respiratory droplets.

Types

  • Influenza A: Most virulent; causes pandemics.
  • Influenza B: Causes seasonal epidemics; generally less severe.
  • Influenza C: Rare; causes mild respiratory illness.
  • Zoonotic Influenza: Includes avian and swine flu, caused by transmission of animal influenza viruses to humans.

Clinical Features 🌡️

Symptoms

  • Sudden onset of high fever (38°C or higher).
  • Dry cough and sore throat.
  • Severe myalgia and arthralgia.
  • Headache and photophobia.
  • Fatigue and malaise.
  • Nasal congestion and rhinorrhoea (less common).

Signs

  • Fever (often high, above 38°C).
  • Tachycardia relative to fever (tachycardia may be less pronounced in elderly).
  • Pharyngeal erythema.
  • Clear chest on auscultation (unless secondary infection).
  • Lymphadenopathy (occasionally).

Investigations 🧪

Tests

  • Clinical diagnosis is often sufficient in peak flu season.
  • PCR testing of respiratory samples (nasopharyngeal swab) is the gold standard.
  • Rapid antigen tests available but less sensitive.
  • Chest X-ray if pneumonia is suspected (may show patchy infiltrates).
  • Full blood count: may show leukopenia or lymphopenia.
  • CRP and procalcitonin may help distinguish bacterial co-infection.

Management 🥼

Management

  • Supportive care: Rest, hydration, and analgesia (paracetamol or ibuprofen).
  • Antiviral therapy (e.g., oseltamivir) if started within 48 hours of symptom onset in at-risk patients.
  • Consider hospital admission for severe cases or at-risk groups.
  • Monitor for complications such as pneumonia or exacerbation of chronic conditions.
  • Annual vaccination recommended for all at-risk populations.
  • Infection control measures to prevent spread (e.g., isolation, hand hygiene).

Complications

  • Primary viral pneumonia (rare but severe).
  • Secondary bacterial pneumonia (more common in elderly).
  • Exacerbation of underlying chronic conditions (e.g., COPD, heart failure).
  • Acute otitis media, especially in children.
  • Myocarditis or pericarditis (rare).
  • Encephalitis or Guillain-Barré syndrome (rare).

Prognosis

  • Generally self-limiting in healthy individuals.
  • Higher risk of severe disease and mortality in at-risk populations (elderly, immunocompromised, pregnant women).
  • Most recover within 7-10 days, although fatigue may persist longer.
  • Mortality rates higher during pandemic years (e.g., H1N1 in 2009).

Key Points

  • Influenza is a significant cause of morbidity and mortality, especially in at-risk groups.
  • Vaccination is the most effective preventive measure.
  • Antiviral treatment is most effective when given early.
  • Complications can be severe, particularly in those with comorbidities.
  • Public health measures are critical in managing outbreaks and pandemics.

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Child health