Influenza for USMLE Step 3
Influenza for the USMLE Step 3 Exam
Etiology
- Influenza Virus:
- Influenza is caused by influenza A and influenza B viruses from the Orthomyxoviridae family.
- Influenza A is responsible for most seasonal epidemics and pandemics due to its ability to undergo:
- Antigenic drift: Small mutations in hemagglutinin (H) or neuraminidase (N) proteins, causing yearly epidemics.
- Antigenic shift: Major genetic reassortment between different viral strains, leading to pandemics.
- Influenza B causes seasonal outbreaks, usually less severe than influenza A, and does not undergo antigenic shift.
Transmission
- Respiratory Droplets:
- Influenza is primarily spread by respiratory droplets from coughing, sneezing, or talking. It can also be spread by contact with contaminated surfaces.
- Seasonality:
- Influenza peaks during the winter months in temperate climates due to increased transmission in colder weather.
Pathophysiology
- Viral Infection:
- The virus binds to sialic acid receptors on respiratory epithelial cells via hemagglutinin, leading to viral entry, replication, and destruction of the respiratory epithelium. This results in impaired mucociliary clearance, making the lungs vulnerable to secondary infections.
- Cytokine Release:
- The immune response leads to the release of cytokines such as interferons, TNF-α, and IL-6, causing fever, malaise, and myalgias.
Clinical Features
- Incubation Period:
- Typically 1-4 days.
- Systemic Symptoms:
- Sudden onset of fever, severe myalgias (especially in the back and legs), headache, and profound fatigue.
- Respiratory Symptoms:
- Dry cough, sore throat, rhinorrhea. Respiratory symptoms often worsen with time.
- Gastrointestinal Symptoms:
- More common in children, with nausea, vomiting, and diarrhea.
Complications
- Primary Viral Pneumonia:
- Characterized by worsening dyspnea, hypoxia, and high fever. Chest X-ray shows bilateral diffuse infiltrates.
- Secondary Bacterial Pneumonia:
- Occurs after initial improvement, with recurrence of fever and productive cough. Common pathogens include Streptococcus pneumoniae, Staphylococcus aureus, and Haemophilus influenzae. Chest X-ray typically shows lobar consolidation.
- Other Complications:
- Exacerbation of chronic conditions: Worsening of asthma, COPD, or heart failure.
- Rare complications: Myocarditis, pericarditis, rhabdomyolysis, and Guillain-Barré syndrome.
Diagnosis
- Clinical Diagnosis:
- During flu season, influenza is often diagnosed clinically based on the sudden onset of fever, myalgias, and respiratory symptoms.
- Laboratory Testing:
- Rapid influenza diagnostic tests (RIDTs): Provide results quickly but have lower sensitivity.
- RT-PCR: The gold standard, with high sensitivity and specificity, used to confirm the diagnosis and differentiate between influenza A and B.
Management
Antiviral Therapy
- Neuraminidase Inhibitors:
- Oseltamivir (oral) and zanamivir (inhaled) block viral release from infected cells. Most effective when started within 48 hours of symptom onset, reducing illness duration and severity. Effective against both influenza A and B.
- Baloxavir:
- A newer antiviral that inhibits viral replication. It is effective with a single oral dose and works against both influenza A and B.
Supportive Care
- Symptomatic Treatment:
- Antipyretics (e.g., acetaminophen) for fever, rest, hydration, and cough suppressants for relief of symptoms.
Prevention
- Annual Vaccination:
- The influenza vaccine is updated annually to match circulating strains. Both inactivated influenza vaccines (IIV) and live attenuated influenza vaccines (LAIV) are available.
- Vaccination is recommended for all individuals ≥6 months, especially high-risk groups such as the elderly, pregnant women, healthcare workers, and those with chronic illnesses.
- Hygiene Practices:
- Handwashing and alcohol-based hand sanitizers reduce transmission. Respiratory hygiene, such as covering coughs and sneezes, is also important.
Key Points
- Influenza is caused by influenza A and B viruses, transmitted via respiratory droplets, and characterized by sudden onset of fever, myalgias, and respiratory symptoms.
- Diagnosis is often clinical during flu season, but RT-PCR is the most sensitive and specific test for confirmation.
- Complications include primary viral pneumonia, secondary bacterial pneumonia, and exacerbation of chronic diseases.
- Antiviral treatment with neuraminidase inhibitors (e.g., oseltamivir) or baloxavir is most effective when started within 48 hours of symptom onset.
- Annual influenza vaccination is recommended for all individuals ≥6 months and is the most effective prevention method.