Overview
Definition
An acute, highly contagious viral respiratory infection.
Epidemiology
Causes seasonal epidemics during winter months.
Etiology & Risk Factors
- Influenza A and B viruses (Orthomyxoviridae family).
- Undergoes antigenic drift (minor mutations causing seasonal epidemics) and antigenic shift (major reassortment causing pandemics, only Influenza A).
Clinical Symptoms
- Abrupt, sudden onset of high fever, severe myalgias (muscle aches), and extreme fatigue
- Non-productive cough, sore throat, headache
- Symptoms are typically much more severe and sudden than the common cold
Clinical Approach
Diagnosis
- Rapid Influenza Diagnostic Tests (RIDTs) - antigen detection
- RT-PCR (most sensitive and specific, often bundled in a respiratory viral panel)
Management
- Supportive care is the mainstay for most healthy individuals
- Antivirals (Oseltamivir/Tamiflu, a neuraminidase inhibitor): Most effective if started within 48 hours of symptom onset. Indicated for hospitalized patients, severe disease, or high-risk individuals.
- Prevention: Annual Influenza vaccination
Complications
- Secondary bacterial pneumonia (Classic board question: Staphylococcus aureus pneumonia following influenza infection)
- Exacerbation of underlying COPD or heart failure