Influenza

Aetiology

Influenza A and B virus. Predominantly a winter-season illness in Australia — take a detailed travel history where seasonality doesn’t fit.

Diagnosis

Clinical criteria (adequate to diagnose during a recognised epidemic): fever >38°C, plus at least one respiratory symptom, plus at least one systemic symptom — typically dry cough, sore throat, coryza, myalgia, headache, rigors/ chills, prostration/weakness.

Confirmatory testing: PCR of a nose or throat swab.

Complications

Tracheitis, bronchitis, bronchiolitis; secondary bacterial infection (staphylococcal pneumonia — mortality up to 20%); rare toxic cardiomyopathy with sudden death; rare encephalomyelitis; depression is a recognised sequela.

Management

• Supportive care (rest, analgesia, fluids) and isolation at home for most patients.
• Neuraminidase inhibitor therapy (oseltamivir or zanamivir):
  • Oseltamivir — adult 75 mg BD for 5 days; renal impairment: 30 mg BD (CrCl 30–60), 30 mg daily (CrCl 10–30); child <1 year 3 mg/kg BD; child <15 kg 30 mg BD, 15–23 kg 45 mg BD, 23–40 kg 60 mg BD.
  • Zanamivir (age ≥5) — 10 mg inhaled BD for 5 days.
  • If started within 48 hours of symptom onset in an otherwise healthy adult, may reduce symptom duration by around 1 day — a modest benefit that must be weighed against side effects (nausea, vomiting, headache, neuropsychiatric events).
• Offer treatment regardless of symptom duration in patients who: need hospitalisation for influenza, have established complications, or have moderate/high-severity pneumonia during flu season.
• Consider treatment in patients at high risk of complications: age ≥65, pregnant women, Down syndrome, heart disease, obesity (BMI>30), chronic respiratory conditions, severe neurological conditions, immunocompromise, other chronic illness, Aboriginal and Torres Strait Islander people of any age, children ≤5, aged-care/long-term residential facility residents, people experiencing homelessness.
• Consider treatment to reduce transmission risk in patients who are hospitalised or in aged care, or who have household contacts at high risk of complications.

Prevention

Annual influenza vaccination recommended for everyone ≥6 months of age. Funded under the National Immunisation Program for: children under 5, adults ≥65, all Aboriginal and Torres Strait Islander people, pregnant women, and people with an eligible chronic condition (check the current Australian Immunisation Handbook for the specific eligible-condition list).