Respiratory viruses (influenza, RSV, parainfluenza)

Influenza Virus - The Annual Shifter

• Orthomyxovirus: Enveloped, with a segmented (8 parts) single-stranded, negative-sense RNA genome.
• Key Glycoproteins:
  • Hemagglutinin (HA): Binds to sialic acid on host cells for viral entry.
  • Neuraminidase (NA): Cleaves sialic acid to release progeny virions.
• Antigenic Variation:
  • Drift: Minor changes via point mutations in HA/NA genes → localized outbreaks/epidemics.
  • Shift: Major changes via genetic reassortment of segments (e.g., human + avian) → pandemics.
  • 📌 Mnemonic for segmented viruses: BOAR (Bunyavirus, Orthomyxovirus, Arenavirus, Reovirus).
• Treatment: Neuraminidase inhibitors (Oseltamivir, Zanamivir) effective if given within 48 hours.

⭐ High-Yield Pearl: Unlike most RNA viruses, Influenza virus replicates in the nucleus to splice its mRNA.

RSV - Baby's Breath-Taker

• Pathogen: Enveloped, single-stranded RNA paramyxovirus.
  • Virulence factor: Fusion (F) protein induces syncytia (multinucleated giant cells).
• Epidemiology: #1 cause of bronchiolitis & pneumonia in infants (<1 yr).
  • Seasonality: Winter & Spring.
• Clinical: Low-grade fever, wheezing, cough, tachypnea, ↑ work of breathing. Apnea in premature infants.
• Diagnosis: Clinical; rapid antigen test on nasopharyngeal swab.
• Treatment: Supportive (oxygen, hydration).
• Prevention: Palivizumab (monoclonal Ab against F protein) for high-risk infants (preemies, congenital heart disease).

⭐ The F protein is the target for palivizumab and is essential for viral entry and syncytia formation, a histologic hallmark.

Parainfluenza - Croup's Barking Seal

• Virus: Paramyxovirus (ssRNA, enveloped); lacks neuraminidase activity unlike influenza.

• Transmission: Respiratory droplets, direct contact.

• Pathogenesis: Infects ciliated epithelial cells of the respiratory tract, leading to inflammation and edema.

• Clinical Syndromes:

  • Croup (Laryngotracheobronchitis): Most common cause in children 6 months to 3 years.
    • Presents with hoarseness, inspiratory stridor, and a characteristic "seal-like" barking cough.
  • Can also cause bronchiolitis and pneumonia in infants.

⭐ On frontal neck X-ray, subglottic edema creates the classic "steeple sign."

• Diagnosis: Primarily clinical; RT-PCR is definitive.
• Treatment: Supportive; corticosteroids (dexamethasone) and nebulized epinephrine for severe stridor.

Viral Lineup - Rogues' Gallery

• Influenza Virus (Orthomyxoviridae)

  • Key feature: Segmented (-)ssRNA genome (8 segments). 📌 Orthomyxo = Only one with 8 segments.
  • Virulence: Hemagglutinin (HA) for entry, Neuraminidase (NA) for release.
  • Hallmark: Antigenic shift & drift → epidemics/pandemics.

• Respiratory Syncytial Virus (RSV) (Paramyxoviridae)

  • Key feature: F (fusion) protein → forms syncytia (multinucleated giant cells).
  • Hallmark: Bronchiolitis & pneumonia in infants <1 yr.

• Parainfluenza Virus (Paramyxoviridae)

  • Key feature: Both hemagglutinin & neuraminidase activity.
  • Hallmark: Croup (laryngotracheobronchitis) with "steeple sign" on X-ray.

⭐ RSV is the #1 cause of pneumonia and bronchiolitis in infants.

High‑Yield Points - ⚡ Biggest Takeaways

• Influenza: Antigenic drift (epidemics) & shift (pandemics). Treat with neuraminidase inhibitors.
• A key influenza complication is secondary bacterial pneumonia, especially with S. aureus.
• RSV: The #1 cause of bronchiolitis & pneumonia in infants, using its F protein for cell fusion.
• Palivizumab is a monoclonal antibody used for RSV prophylaxis in high-risk infants.
• Parainfluenza virus: The most common cause of croup (laryngotracheobronchitis).
• Croup is identified by its "seal-bark" cough and steeple sign on X-ray.