Background Information

Aetiology

Influenza is an acute respiratory illness caused by RNA viruses of the family Orthomyxoviridae (influenza viruses).

There are 3 types of influenza virus:

• Influenza A: most common and more virulent (responsible for outbreaks, epidemics and pandemics)
• Influenza B (often co-circulates with influenza A)
• Influenza C (often causes a mild / asymptomatic infection, similar to the common cold)

Complications

Respiratory complication (most common):

• Pneumonia (this can be caused by the virus itself, or secondary bacterial infection – esp. Staphylococcus aureus)
• Acute bronchitis
• Otitis media
• Exacerbation of asthma and COPD

Non-respiratory complications:

• Cardiac complications (myocarditis, pericarditis) (exacerbation of underlying chronic heart failure is common)
• Febrile convulsions
• Myalgia, myositis and rhabdomyolysis
• Neurological complications (e.g. Reyes syndrome, encephalomyelitis, transverse myelitis, Guillain-Barré syndrome, aseptic meningitis, and encephalitis)
• Toxic shock syndrome

Diagnosis

Clinical Features

Typical features of uncomplicated influenza:

• Fever
• Coryzal symptoms (nasal congestion, runny nose, cough)
• Systemic symptoms (headache, malaise, myalgia, arthralgia)
• Less common – GI symptoms, ocular symptoms (e.g. photophobia, conjunctivitis, lacrimation, pain upon eye movement)

Features of complicated influenza:

• Presence of LRTI (e.g. hypoxaemia, dyspnoea, presence of lung infiltrate)
• CNS involvement
• Significant exacerbation of an underlying medical condition

Investigation and Diagnosis

Influenza is mostly diagnosed clinically​​​​​​, based on typical clinical features and when influenza is known to be circulating in the community.

Testing is not necessary in all patients:

• Rapid influenza diagnostic tests should be done in those with complicated influenza (but do not delay antiviral treatment while awaiting laboratory confirmation)
• PCR-based laboratory testing is important in the following to identify potential antiviral resistance:
  • Patients who develop symptoms despite antiviral prophylaxis, or
  • Persistent infection despite antiviral treatment

Management

Patients with complicated influenza will typically require admission to hospital.

Approach for other patients:

• All patients should be offered self-care management
• Do not routinely give anti-virals, see below for specific indications

Self-Care Management

Advise the patient to:

• Drink adequate fluids
• Consider paracetamol / ibuprofen for pain or fever
• Rest in bed if patient feels fatigue

Advice on expected symptoms resolution timing

• Fever and systemic symptoms usually resolve after ~1 week
• Cough and fatigue may persist up to 2 weeks after resolution of fever

Antiviral Therapy

Indications for Antiviral

Anti-viral is only indicated if ALL of the following are present:

• Patient presented <48 hours of symptom onset (<36 hours for zanamivir in children)
• The national surveillance scheme indicates that influenza is circulating
• Patient is considered at risk of influenza complications – ANY of the following
  • <6 m/o and >65 y/o
  • Pregnant women
  • Asplenia / hyposplenism
  • Chronic respiratory disease (e.g. severe asthma, COPD, bronchiectasis, cystic fibrosis)
  • Chronic heart disease
  • Chronic kidney disease
  • Chronic liver disease
  • Chronic neurological conditions (including stroke and TIA)
  • Diabetes mellitus
  • Immunosuppression (due to disease or treatment)
  • Morbid obesity (BMI >40)

Choice of Antiviral

Antiviral of choice (if indicated): oral oseltamivir / inhaled zanamivir (neuraminidase inhibitor)

• Oseltamivir is generally 1st line, zanamivir is only used in patients who are severely immunosuppressed and the dominant strain has a high risk of oseltamivir resistance (e.g. H1N1)
• Zanamivir is not licensed in <5 y/o; oseltamivir can be used from 2 weeks old onwards

References