Back
ditki Logo
medical & biological sciences
All Access Pass - 1 FREE Month!
Institutional email required, no credit card necessary.

Influenza for ABIM

Free One-Month! Institutional (.edu or .org) email required
Log in or start your One-Week Free Trial!
Influenza for the American Board of Internal Medicine Exam
Etiology
  • Influenza Virus:
    • Influenza is caused by influenza viruses, which are enveloped, single-stranded RNA viruses classified into three types: Influenza A, B, and C. Influenza A and B are the primary causes of seasonal outbreaks, while influenza C causes mild respiratory illness.
    • Influenza A:
    • The most pathogenic, responsible for annual epidemics and occasional pandemics due to antigenic shift. Influenza A viruses are further classified based on surface glycoproteins:
    • Hemagglutinin (H): Responsible for viral entry into host cells.
    • Neuraminidase (N): Facilitates viral release from infected cells.
    • The two most common subtypes in humans are H1N1 and H3N2.
    • Influenza B:
    • Causes seasonal outbreaks, typically less severe than influenza A, and is not prone to antigenic shift but can undergo antigenic drift.
    • Antigenic Shift vs. Drift:
    • Antigenic drift: Gradual accumulation of point mutations in hemagglutinin or neuraminidase leading to seasonal epidemics.
    • Antigenic shift: Abrupt genetic reassortment between different influenza strains (typically in influenza A), resulting in novel subtypes and potential pandemics.
Transmission
  • Respiratory Droplets:
    • Influenza spreads primarily through respiratory droplets when an infected individual coughs or sneezes. Transmission can also occur via contact with contaminated surfaces, followed by self-inoculation of mucosal surfaces.
  • Seasonality:
    • In temperate climates, influenza peaks during the winter months, whereas in tropical regions, it may occur year-round.
Pathophysiology
  • Viral Replication:
    • After inhalation, the virus infects epithelial cells in the upper and lower respiratory tracts by binding to sialic acid receptors via hemagglutinin. Viral replication in epithelial cells results in cellular damage, desquamation, and impaired mucociliary clearance.
Influenza viral entry
  • Immune Response:
    • Viral replication induces an innate immune response, including the release of cytokines (e.g., interferons, TNF-α, IL-6), which cause systemic symptoms such as fever, malaise, and myalgias.
  • Complications:
    • The virus can cause direct viral pneumonia or predispose to secondary bacterial infections by damaging the respiratory epithelium and impairing local immune defenses.
Clinical Features
  • Incubation Period:
    • The incubation period for influenza is typically 1-4 days (average 2 days).
  • Systemic Symptoms:
    • Influenza presents with sudden onset of:
    • High fever: Typically >38°C.
    • Myalgias: Severe muscle aches, especially in the back and legs.
    • Headache: Often severe and frontal.
    • Fatigue and malaise: Profound weakness, often lasting for days to weeks.
  • Respiratory Symptoms:
    • Cough: Usually dry, persistent, and worsens with time.
    • Sore throat and rhinorrhea: Common but less prominent than systemic symptoms.
    • Dyspnea: May occur in severe cases or with pneumonia.
  • Gastrointestinal Symptoms:
    • More common in children and include nausea, vomiting, and diarrhea, although not typical in adults.
Complications
  • Primary Influenza Pneumonia:
    • Characterized by worsening dyspnea, high fever, and hypoxia during the course of influenza infection. Chest X-ray shows diffuse bilateral infiltrates. Risk factors include underlying lung disease, pregnancy, and immunosuppression.
  • Secondary Bacterial Pneumonia:
    • Superinfection by bacteria, particularly Streptococcus pneumoniae, Staphylococcus aureus, and Haemophilus influenzae, occurs after initial improvement of influenza symptoms, with recurrence of fever and productive cough. Chest X-ray shows lobar consolidation.
  • Other Complications:
    • Exacerbation of chronic conditions: Such as asthma or chronic obstructive pulmonary disease (COPD).
    • Myocarditis and pericarditis: Rare but serious cardiac complications.
    • Rhabdomyolysis and acute kidney injury.
    • Guillain-Barré syndrome: A rare post-infectious complication.
Diagnosis
  • Clinical Diagnosis:
    • During influenza season, diagnosis is often made clinically based on the sudden onset of fever, myalgias, and respiratory symptoms.
  • Laboratory Testing:
    • Rapid influenza diagnostic tests (RIDTs): Provide results in 15-30 minutes by detecting viral antigens, but have lower sensitivity.
    • Reverse transcription-polymerase chain reaction (RT-PCR): The most sensitive and specific test, used to confirm infection and differentiate between influenza A and B.
    • Viral cultures: Less commonly used due to the longer turnaround time.
Management
Antiviral Therapy
  • Neuraminidase Inhibitors:
    • Oseltamivir (oral) and zanamivir (inhaled) inhibit neuraminidase, preventing viral release from infected cells. These are effective against both influenza A and B.
    • Antivirals are most effective when started within 48 hours of symptom onset and are recommended for patients at high risk of complications (e.g., elderly, pregnant, immunocompromised). They can also reduce illness duration by 1-2 days in healthy adults.
    • Peramivir (IV): Used for hospitalized patients unable to take oral or inhaled medications.
  • Baloxavir marboxil:
    • A newer antiviral that inhibits cap-dependent endonuclease, necessary for viral replication. It is effective against influenza A and B and requires only a single oral dose.
Supportive Care
  • Symptomatic Treatment:
    • Includes antipyretics (e.g., acetaminophen, ibuprofen), adequate hydration, and rest. Cough suppressants may be used for relief of cough.
  • Hospitalization:
    • Required for patients with severe influenza (e.g., pneumonia, respiratory failure) or those with significant comorbidities at risk of complications.
Prevention
  • Influenza Vaccination:
    • Annual vaccination is the most effective preventive measure. Inactivated influenza vaccines (IIV) and live attenuated influenza vaccines (LAIV) are available. The vaccine composition is updated yearly to include circulating strains of influenza A and B.
    • Vaccination is recommended for all individuals ≥6 months, particularly high-risk populations (e.g., elderly, pregnant women, healthcare workers, and those with chronic diseases).
  • Hand Hygiene:
    • Regular handwashing and the use of alcohol-based hand sanitizers can reduce transmission.
  • Antiviral Prophylaxis:
    • Neuraminidase inhibitors may be used as prophylaxis in close contacts of infected individuals, especially in high-risk groups (e.g., nursing home residents).
Key Points
  • Influenza is a viral illness primarily caused by influenza A and B viruses, transmitted via respiratory droplets and presenting with sudden onset of fever, myalgias, and respiratory symptoms.
  • Diagnosis is often clinical during influenza season but can be confirmed with RT-PCR or RIDTs in certain cases.
  • Complications include primary viral pneumonia, secondary bacterial pneumonia, and exacerbation of chronic diseases.
  • Antiviral treatment with neuraminidase inhibitors (oseltamivir, zanamivir) or baloxavir is most effective when started within 48 hours of symptom onset.
  • Annual vaccination is the cornerstone of prevention, and it is recommended for all individuals ≥6 months, especially those at high risk for complications.

Related Tutorials