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Q1: Definition, classification and the surface antigens (3 min)
- Influenza = acute contagious respiratory infection by influenza A, B (and rarely C) of the Orthomyxoviridae.
- Genome: 8-segment, negative-sense, single-stranded RNA; polymerase lacks proof-reading (basis of drift).
- Two surface glycoproteins:
- Haemagglutinin (HA) — binds alpha-2,6 sialic acid receptors; mediates entry; 18 subtypes; antibody to HA is neutralising.
- Neuraminidase (NA) — cleaves sialic acid to release virions; 11 subtypes; drug target of oseltamivir/zanamivir/peramivir.
- Human A subtypes: H1N1pdm09, H3N2. Influenza B: Victoria and Yamagata lineages.
Examiner follow-up — drift vs shift:
- Drift = point mutations in HA/NA; A AND B; annual epidemics; no pandemic.
- Shift = reassortment of genome segments in co-infected cell (swine mixing vessel has both alpha-2,3 and alpha-2,6 receptors); A only; pandemics.
- Pandemics: 1918 H1N1 (Spanish), 1957 H2N2 (Asian), 1968 H3N2 (Hong Kong), 2009 H1N1pdm09 (Swine).
Q2: Clinical features and bedside differentiation from common cold (3 min)
- Abrupt onset (date to the hour); fever 38–40 degC with rigors; severe myalgia (back/legs); severe frontal headache; dry cough; sore throat; extreme prostration.
- Triad: fever + cough + acute onset.
- Bedside vs common cold: cold is gradual, nasal-dominant, little/no fever, patient remains functional.
- Atypical: elderly — no fever; confusion, falls, functional decline. Pregnant/young child/immunocompromised — higher severity, atypical features.
Q3: Diagnosis (2 min)
- Gold standard = RT-PCR on nasopharyngeal swab (95–98% sensitive, over 99% specific).
- RIDT: 50–70% sensitive, 90–95% specific, 10–15 min. A negative RIDT does NOT exclude influenza — confirm with PCR.
- Sampling: NP swab better than throat swab; BAL if intubated.
- Serology: 4-fold rise in HI antibody between acute and convalescent sera (retrospective).
- Admission bloods: FBC (leukopenia, lymphopenia; neutrophilia suggests bacterial co-infection), CRP, U&E, LFTs, lactate, CXR (bilateral interstitial = viral pneumonitis; lobar = bacterial).
Q4: Antiviral therapy (4 min)
- Oseltamivir 75 mg PO twice daily x 5 days; weight-based in children (under 1 yr 3 mg/kg BD; 1–15 kg 30 mg BD, etc.).
- Within 48 h for uncomplicated; any time for hospitalised, pregnant, immunocompromised, severe.
- Prophylaxis: oseltamivir 75 mg PO once daily x 7 days (10 days in outbreaks).
- Baloxavir marboxil — single-dose cap-dependent endonuclease inhibitor (40 mg if 40–80 kg; 80 mg if over 80 kg); blocks PA subunit.
- Zanamivir — inhaled 10 mg BD x 5 d (avoid in asthma/COPD). Peramivir — IV single 600 mg.
- M2 inhibitors (amantadine, rimantadine) — universally resistant; NOT used.
- Corticosteroids — NOT routine; harm; only for septic shock (hydrocortisone 200 mg/day) or asthma/COPD.
Q5: Complications (3 min)
- Pulmonary: primary viral pneumonia, secondary bacterial pneumonia (Strep pneumoniae, Staph aureus classically post-influenza — cavitation), bronchiolitis, otitis media.
- Cardiac: myocarditis, pericarditis, heart failure exacerbation, acute coronary syndrome.
- Neurological: influenza-associated encephalopathy, Guillain-Barre syndrome (ascending paralysis 1–6 weeks after), transverse myelitis, febrile seizures.
- Musculoskeletal: myositis, rhabdomyolysis (raised CK, dark urine, AKI).
- Reye syndrome in children given aspirin (mitochondrial hepatopathy, raised ammonia, microvesicular steatosis) — NEVER give aspirin to a febrile child.
Q6: Prevention — vaccination (2 min)
- Annual quadrivalent inactivated vaccine (QIV) = 2 A + 2 B strains; updated twice yearly by WHO GISRS.
- Indicated for ALL over 6 months; high-priority: over 65, pregnant, chronic disease, immunocompromise, healthcare workers, children under 5.
- Children 6 months–8 yr first time need TWO doses, 4 weeks apart.
- Over 65: preferentially high-dose, adjuvanted (MF59), or recombinant vaccines.
- Live attenuated intranasal (LAIV) for 2–17 yr in some countries; contraindicated in immunocompromise, severe asthma, salicylate therapy, pregnancy.
- Contraindications: severe allergic reaction to previous dose/component; severe egg allergy (use egg-free recombinant); history of GBS within 6 weeks of previous influenza vaccine.
- Pregnancy: inactivated vaccine at ANY gestation — protects mother and infant for first 6 months via transplacental IgG.