Influenza

 

• Background

  Pathophysiology

  • Influenza A and B are RNA viruses of the Orthomyxoviridae family. A and B vary in matrix proteins and nucleoproteins. Their surface glycoproteins include hemagglutinin and neuraminidase, the types of which are used to classify influenza A strains.
  • Influenza A is more common, severe, and causes epi/pandemics like H5N1 (aVIan) and H1N1 (swINe). Influenza B is milder and, unlike A, occurs almost exclusively in humans.
  • Spread through droplets, so masks help prevent transmission.
  • 1-3 days incubation.

  Epidemiology

  • Incidence peaks in colder months, 'flu season'. Possible reasons for this are increased indoor human contact due to the weather, cold and humidity aiding viral survival, and low vitamin D levels leading to impaired immune function.
  • 20% of the population are infected each flu season. Most remain asymptomatic carriers, with only 1 in 4 (5% of the population) becoming symptomatic, usually with mild upper respiratory tract symptoms. Only 1 in 5 symptomatic individuals (1% of the population) present to a doctor, and even then it is often not recognized as flu. As with many infectious outbreaks, we only see the tip of the iceberg.

• Presentation

  • Quick onset.
  • Respiratory symptoms: cough, coryza.
  • Systemic symptoms: fever, rigors, fatigue, anorexia.
  • Pain: headache, myalgia.
  • Other symptoms: nausea and vomiting, conjunctivitis, photophobia.

• Diagnosis

  Usually a clinical diagnosis, but nasal/throat swabs for viral culture or PCR are useful for confirmation in severe disease and for epidemiological surveillance.

• Management

  Prevention:

  • Vaccination reduces infection, symptomatic disease, and complications by around 50%.
  • Route: whole trivalent vaccine IM, or fragmented nasal vaccine if <13 years old.
  • Indications: toddlers, age >65, diabetes, respiratory disease (COPD, asthma), organ failure (heart, liver, kidney), immunosuppressed.

  Treatment:

  • Rest, paracetamol, and hydration.
  • Neuraminidase inhibitors can be given if over 1 years old and it is within the first 48 hours of symptoms. May reduce symptom duration by 1 day, but evidence on reducing complications is unclear. Oseltamivir (oral) can cause nausea and vomiting, and zanamivir (nasal) can cause bronchospasm so is avoided in asthma and COPD.

• Complications

  • Pneumonia can result directly from influenza itself or, more commonly, secondary bacterial infection (Strep. pneumo, Staph. aureus, H. influenzae). Implicated in many influenza deaths.
  • Other local infections: bronchitis, sinusitis, otitis media.
  • Neurological complications: encephalitis, post-influenza encephalopathy in children, Guillain-Barré syndrome.