USMLE Guide: Influenza

Introduction

Influenza, commonly known as the flu, is a highly contagious viral respiratory infection that affects millions of people worldwide each year. This guide aims to provide a comprehensive overview of influenza, including its etiology, clinical presentation, diagnosis, treatment, and prevention.

I. Etiology

Influenza is caused by RNA viruses belonging to the Orthomyxoviridae family, with three main types: Influenza A, B, and C. Influenza A viruses are further classified into subtypes based on their surface glycoproteins, hemagglutinin (H) and neuraminidase (N). These subtypes include H1N1, H3N2, and other less common strains.

II. Clinical Presentation

The clinical presentation of influenza can vary, but commonly includes the following symptoms:

• Fever: Sudden onset of high-grade fever (>101°F or 38.3°C).
• Respiratory symptoms: Cough, sore throat, nasal congestion, and rhinorrhea.
• Systemic symptoms: Fatigue, myalgia, headache, and malaise.
• Gastrointestinal symptoms: Nausea, vomiting, and diarrhea (more common in children).

III. Diagnosis

The diagnosis of influenza is primarily clinical, based on the characteristic symptoms mentioned above during flu season. However, laboratory confirmation may be required in certain situations, including:

• Rapid antigen tests: Quick and widely available, but less sensitive and specific compared to other methods.
• Reverse transcription-polymerase chain reaction (RT-PCR): Highly sensitive and specific, detecting viral RNA in respiratory samples.
• Viral culture: Time-consuming but provides definitive identification of the viral strain.

IV. Treatment

The management of influenza primarily focuses on supportive care, symptomatic relief, and antiviral therapy in certain cases:

• Supportive care: Adequate hydration, rest, and antipyretics (e.g. acetaminophen) to manage fever.
• Symptomatic relief: Over-the-counter medications for cough, sore throat, and nasal congestion (e.g. antitussives, analgesics, decongestants).
• Antiviral therapy: Recommended in high-risk individuals or severe cases within 48 hours of symptom onset. Neuraminidase inhibitors (e.g. oseltamivir, zanamivir) are commonly used.

V. Prevention

Preventing influenza is crucial to reduce its spread and complications. Key preventive measures include:

• Vaccination: Annual influenza vaccination is recommended for all individuals aged 6 months and older, especially high-risk groups (e.g. elderly, healthcare workers).
• Hand hygiene: Frequent handwashing with soap and water or use of alcohol-based hand sanitizers.
• Respiratory hygiene: Covering mouth and nose when coughing or sneezing, and using tissues or elbows instead of hands.
• Social distancing: Avoiding close contact with sick individuals, crowded areas, and large gatherings during flu season.

Conclusion

Influenza is a common respiratory infection caused by influenza viruses, primarily affecting the respiratory system with a range of symptoms. Diagnosis is mainly clinical, but laboratory confirmation may be necessary in certain cases. Treatment involves supportive care and, in specific situations, antiviral therapy. Prevention through vaccination and adopting preventive measures is crucial to reducing influenza transmission and associated complications.