Influenza Virus
One-Sentence Definition
Influenza viruses (Orthomyxoviridae; types A/B/C) are enveloped, segmented negative-sense RNA viruses that cause seasonal epidemics and occasional pandemics through antigenic drift and — for influenza A — antigenic shift.
Taxonomy & Structure
- Types: A (pandemic potential; HA/NA subtypes), B (human), C (milder), D (mainly cattle)
- Genome: 8 segments (A/B) of (−)ssRNA; RNA-dependent RNA polymerase
- Envelope glycoproteins: hemagglutinin (HA) — entry; neuraminidase (NA) — release
- Matrix (M1), M2 ion channel (A), NP, NS1 (interferon antagonism)
Replication & Pathogenesis Highlights
- Respiratory epithelial tropism; cytokine storm in severe disease
- Antigenic drift — gradual HA/NA mutations → seasonal vaccine updates (Antigenic Variation)
- Antigenic shift — segment reassortment (often swine/avian interfaces) → pandemic strains
- Secondary bacterial pneumonia (S. aureus, pneumococcus) important complication
Clinical Syndromes
- Abrupt fever, myalgia, cough, headache; otitis/pneumonia in children/elderly
- Complications: myocarditis, encephalitis (rare), exacerbation of COPD/CHF
- Avian zoonoses (H5N1, H7N9…) — severe pneumonia, high mortality when they infect humans
Diagnosis
- Rapid antigen (lower sensitivity) · RT-PCR / qPCR preferred · syndromic respiratory panels (Syndromic Molecular Panels)
- Specimen: NP swab / aspirate early; RNA Extraction critical
- Subtyping and clade assignment via sequencing for surveillance (Viral Genomics and Surveillance)
Treatment & Prevention (conceptual)
- Neuraminidase inhibitors (oseltamivir, zanamivir…); baloxavir (cap-dependent endonuclease inhibitor) — start early
- Resistance mutations (e.g., H275Y in NA) monitored genotypically
- Annual inactivated / live attenuated / recombinant vaccines; composition updated by WHO (Vaccination)
- Infection control: droplet precautions; antivirals for prophylaxis in outbreaks
Genomics & Computational Notes
- HA/NA phylogenies for vaccine strain selection (Phylodynamics)
- Reassortment detection across segments
- Antiviral resistance SNP calling from clinical sequences
Related Notes
- SARS-CoV-2 · Community-Acquired Pneumonia · RNA Extraction · RT-PCR
- MOCs: MOC - Virology · MOC - Public Health & Epidemiology · MOC - Diseases by System
Active Recall
- Drift vs shift — which needs reassortment?
- Why update influenza vaccines most years?
- Preferred acute diagnostic method in hospitals?