Influenza

Influenza, commonly known as flu, is a contagious respiratory illness caused by influenza viruses. It affects the nose, throat, and sometimes the lungs. Although many cases are mild, influenza can cause severe illness and death, especially in vulnerable groups. Annual vaccination and good public health measures are key to prevention.

Causes

Influenza is caused by infection with influenza viruses. There are three main types affecting humans:

1. Influenza A
2. Influenza B
3. Influenza C (causes milder symptoms)

Influenza A and B are responsible for seasonal epidemics. Influenza A viruses are further classified into subtypes based on surface proteins (e.g., H1N1, H3N2).

Pathophysiology

Once inhaled, the virus binds to epithelial cells in the respiratory tract using the haemagglutinin protein. It then enters these cells and replicates, leading to cell death and inflammation. This damage and the body’s immune response cause the typical symptoms. In severe cases, the infection can spread to the lower respiratory tract, causing pneumonia and systemic inflammatory responses.

Signs and Symptoms

• Sudden onset of high fever
• Chills and shivering
• Muscle aches and body pain
• Headache
• Sore throat
• Dry, persistent cough
• Fatigue and weakness
• Nasal congestion or runny nose
• Sometimes gastrointestinal symptoms like nausea, vomiting (more common in children)

Risk Factors

• Young children (especially under 5 years)
• Elderly people (over 65 years)
• Pregnant women
• People with chronic conditions (e.g., asthma, diabetes, heart disease)
• Immunosuppressed individuals (e.g., chemotherapy patients, transplant recipients)
• Healthcare workers
• Residents of nursing homes or long-term care facilities

Investigations

Influenza is often diagnosed clinically, but investigations may include:

• Nasopharyngeal swabs for PCR testing: Gold standard for detecting influenza virus
• Rapid antigen tests: Available but less sensitive than PCR
• Chest X-ray: If pneumonia is suspected
• Full blood count and CRP: If concerned about bacterial superinfection

Management

1. Non-Pharmacological Management

• Rest: Essential to allow the immune system to fight the infection
• Hydration: Drink plenty of fluids to prevent dehydration
• Isolation: Stay at home to prevent spreading the virus
• Respiratory hygiene: Cover coughs/sneezes and dispose of tissues properly
• Hand hygiene: Regular handwashing to minimise transmission

2. Pharmacological Management

• Antiviral medications: Oseltamivir (Tamiflu) or Zanamivir (Relenza) may be considered if given within 48 hours of symptom onset, particularly in high-risk groups.
• Antipyretics and analgesics: Paracetamol or ibuprofen for fever and body aches.
• Antibiotics: Not indicated unless there is a secondary bacterial infection.

3. Surgical Management

There is no surgical management for influenza. However, in rare cases where complications like empyema or lung abscess develop, surgical drainage might be necessary.

Complications

• Secondary bacterial pneumonia (commonest serious complication)
• Otitis media (especially in children)
• Exacerbation of chronic illnesses (e.g., COPD, asthma, heart failure)
• Viral pneumonia
• Encephalitis (rare)
• Myocarditis and pericarditis (heart involvement)
• Death (especially in vulnerable populations)

References

• NICE Clinical Knowledge Summary (CKS): Influenza (Updated 2023)
• Public Health England (PHE) Guidance: Influenza Prevention and Management
• Royal College of General Practitioners (RCGP) Seasonal Influenza Guidance
• Royal College of Physicians (RCP) Acute Care Toolkit: Managing Flu in Hospitals
• GMC Good Medical Practice: Safe Care and Infection Control
• NHS Inform: Seasonal Flu Vaccination and Treatment Advice