Influenza

Influenza causes a relatively debilitating illness and should not be confused with the common cold. The incubation period is usually 1–3 d and the illness usually starts abruptly with a fever, headache, shivering and generalised muscle aching.

• Viral organisms—influenza A, influenza B, coronavirus (specific strains)

• Avian (bird) influenza—H5N1 influenza A strain

• Swine flu—swine variety of H1N1 influenza A

Clinical criteria

During an influenza epidemic:

• fever >38 °C plus

• one respiratory symptom:

  • – dry cough

  • – sore throat plus

• one systemic symptom

  • – myalgia

  • – headache

  • – prostration or weakness

  • – chills or rigors

Diagnosis Nasal/throat swabs, PCR assay, viral culture, specific AB assay

Complications

• Tracheitis, bronchitis, bronchiolitis

• Secondary bacterial infection

• Pneumonia due to S. aureus (mortality up to 20%)

• Encephalomyelitis (rare)

• Toxic cardiomyopathy with sudden death (rare)

• Depression (a common sequela)

Management

Advice to the patient includes:

• rest in bed until the fever subsides and patient feels better

• analgesics—aspirin is effective or ibuprofen or paracetamol

• fluids—maintain high fluid intake (water and fruit juice); freshly squeezed lemon juice and honey preparations help

• consider antiviral drugs (within 36 h), e.g.:

  • – zanamivir (Relenza) (adult and child >5 yrs) 10 mg by inhalation bd for 5 d or

  • – oseltamivir (Tamiflu) 75 mg (o) bd for 5 d (child: 2 mg/kg ↑ 75 mg)

These agents have questionable benefit in a low-risk population but treatment for vulnerable patients is appropriate

Prophylaxis Influenza vaccination offers some protection for up to 70% of the population for about 12 months.