Influenza for USMLE Step 2
Influenza for the USMLE Step 2 Exam
Etiology
- Influenza Virus:
- Caused by influenza viruses from the Orthomyxoviridae family. There are three main types:
- Influenza A: Responsible for most seasonal epidemics and occasional pandemics. Subtypes are based on surface proteins:
- Hemagglutinin (H): Binds to sialic acid receptors, allowing viral entry.
- Neuraminidase (N): Facilitates viral release.
- Influenza B: Causes seasonal outbreaks but is generally less severe than influenza A.
- Influenza C: Causes mild illness and is less common.
- Antigenic Drift vs. Antigenic Shift:
- Antigenic drift: Minor mutations in surface proteins (H or N), leading to annual seasonal influenza outbreaks.
- Antigenic shift: Major genetic reassortment, particularly in influenza A, resulting in new strains and potential pandemics.
Transmission
- Respiratory Droplets:
- Spread via coughing, sneezing, or talking, with transmission occurring through inhalation of droplets or touching contaminated surfaces followed by contact with the eyes, nose, or mouth.
- Seasonality:
- Influenza outbreaks peak in winter months in temperate regions, with increased transmission in colder weather.
Pathophysiology
- Viral Replication:
- The virus binds to sialic acid receptors on respiratory epithelial cells via hemagglutinin, leading to viral replication and destruction of epithelial cells. This impairs mucociliary clearance, promoting secondary infections.
- Immune Response:
- Cytokines (e.g., interferons, IL-6, TNF-α) are released, leading to fever, malaise, and myalgias. The systemic response is responsible for many flu symptoms.
Clinical Features
- Incubation Period:
- Typically 1-4 days.
- Systemic Symptoms:
- Abrupt onset of high fever, severe myalgias, headache, and profound fatigue. These symptoms distinguish influenza from the common cold.
- Respiratory Symptoms:
- Dry cough, sore throat, and rhinorrhea. These are common and often worsen as the illness progresses.
- Gastrointestinal Symptoms:
- Common in children and include nausea, vomiting, and diarrhea.
Complications
- Primary Viral Pneumonia:
- Worsening respiratory symptoms with hypoxia and high fever. Chest X-ray shows bilateral infiltrates.
- Secondary Bacterial Pneumonia:
- A more common complication, typically caused by Streptococcus pneumoniae, Staphylococcus aureus, or Haemophilus influenzae. It presents with recurrent fever, productive cough, and lobar consolidation on chest X-ray.
- Exacerbation of Chronic Conditions:
- Influenza can worsen asthma, COPD, or cardiovascular conditions.
- Other Complications:
- Rare complications include myocarditis, rhabdomyolysis, and Guillain-Barré syndrome.
Diagnosis
- Clinical Diagnosis:
- During flu season, diagnosis is often based on the sudden onset of fever, myalgias, and respiratory symptoms.
- Laboratory Testing:
- Rapid influenza diagnostic tests (RIDTs): Quick results but low sensitivity.
- RT-PCR: The gold standard, with high sensitivity and specificity, differentiating between influenza A and B.
Management
Antiviral Therapy
- Neuraminidase Inhibitors:
- Oseltamivir (oral) and zanamivir (inhaled) inhibit viral release and are effective against both influenza A and B. Treatment should begin within 48 hours of symptom onset to reduce illness duration and complications.
- Baloxavir:
- A newer antiviral, effective in a single oral dose, that inhibits viral replication by targeting the endonuclease enzyme.
Supportive Care
- Symptomatic Treatment:
- Antipyretics (e.g., acetaminophen) for fever and myalgias, hydration, and rest. Cough suppressants may be used if necessary.
Prevention
- Annual Influenza Vaccine:
- The most effective preventive measure. Available as inactivated vaccines (IIV) and live attenuated vaccines (LAIV), the vaccine is updated yearly to cover circulating strains.
- Vaccination is recommended for all individuals ≥6 months, especially high-risk groups (e.g., elderly, pregnant women, healthcare workers).
- Hand Hygiene:
- Regular handwashing and alcohol-based hand sanitizers help prevent transmission.
Key Points
- Influenza is caused by influenza A and B viruses and presents with abrupt onset of fever, myalgias, and respiratory symptoms.
- Diagnosis is often clinical during flu season but can be confirmed by RT-PCR or RIDT.
- Complications include primary viral pneumonia, secondary bacterial pneumonia, and exacerbation of chronic diseases.
- Neuraminidase inhibitors (e.g., oseltamivir) or baloxavir should be started within 48 hours of symptom onset to be most effective.
- Annual influenza vaccination is recommended for all individuals ≥6 months and is the best method of prevention.