Epidemiology
Etiology
- Family: Orthomyxovirus (Enveloped, negative-sense ssRNA, segmented genome).
- Segmented genome allows for genetic reassortment.
- Key Surface Antigens:
- Hemagglutinin (HA): Binds sialic acid receptors on host cells → viral entry.
- Neuraminidase (NA): Cleaves sialic acid to release progeny virus → viral exit.
- Genetic Variation:
- Antigenic Drift: Minor changes due to random point mutations in HA or NA. Causes seasonal epidemics. Reason for annual vaccine updates.
- Antigenic Shift: Major change due to reassortment of RNA segments between two viruses (e.g., human + swine flu) infecting the same cell. Causes Pandemics (Shift = Segments). t
Pathophysiology
- Replication cycle
- Influenza viruses bind to the respiratory tract epithelium.
- Viral hemagglutinin (H) binds sialic acid residues (neuraminic acid derivatives) on the host cell membrane → virus fusion with the membrane → entry into the cell
- The virus replicates in the nucleus of the cell.
- The new virus particles travel to the cell membrane → formation of a membrane bud around the virus particles (budding).
- Viral neuraminidase (N) cleaves the neuraminic acid → virions exit the cell.
- Host cell dies → cellular breakdown triggers a strong immune response
Clinical features
Complications
- Secondary Bacterial Pneumonia:
- High-yield presentation: Initial improvement of flu symptoms followed by recurrent high fever, worsening cough, and purulent sputum.
- Etiology: Streptococcus pneumoniae (most common overall), Staphylococcus aureus (classic post-flu; causes severe necrotizing pneumonia w/ cavitary lesions/nematoceles).
- Primary Influenza Pneumonia: Severe, rapidly progressive respiratory distress, hypoxia, diffuse bilateral infiltrates on CXR. c
- Myositis & Rhabdomyolysis: Severe muscle pain/tenderness, markedly ↑ CK, dark/tea-colored urine.
- Cardiovascular: Myocarditis, pericarditis, acute coronary syndrome exacerbation.
- Neurologic: Encephalitis, aseptic meningitis, acute transverse myelitis, Guillain-Barré Syndrome (GBS).
- Reye Syndrome: Rapidly progressive encephalopathy + hepatic failure (microvesicular fatty liver) in children given Aspirin during viral (influenza or varicella) illness.
Diagnostics
Treatment
Prevention
Inactivated versions of the influenza vaccine stimulate the formation of neutralizing antibodies against the hemagglutinin antigen of included strains. Subsequent exposure to a strain of influenza included in the vaccine will not result in infection because the antibodies bind to hemagglutinin, thereby preventing hemagglutinin from attaching to the sialic acid receptor on host respiratory epithelial cells (preventing viral entry).