Poxviruses

Poxvirus Basics - Big Brick Viruses

• Largest & most complex animal viruses; brick-shaped virion.
• Linear, dsDNA genome.
• Unique among DNA viruses for replicating entirely within the cytoplasm.
  • Carries its own DNA-dependent RNA polymerase & transcription factors.
  • Forms intracytoplasmic inclusions (Guarnieri bodies).
• 📌 Mnemonic: "Pox is out of the box" (i.e., out of the nucleus).

⭐ High-Yield: Poxviruses are the only DNA viruses that replicate fully in the cytoplasm. This is a frequent exam question distinguishing them from all other DNA viruses (e.g., Herpes, Adeno, Papova) which replicate in the nucleus.

Key Pathogens - The Pox Lineup

• Variola (Smallpox)

  • Eradicated globally; bioterrorism concern.
  • Lesions are synchronous (all in the same stage).
  • High mortality (~30%).

• Vaccinia

  • Used in the smallpox vaccine.
  • Can cause localized or disseminated disease in immunocompromised individuals.

• Cowpox

  • Zoonotic, classically from cats (rodent reservoir).
  • Causes localized, painful pustular lesions.

• Molluscum Contagiosum Virus (MCV)

  • Common in children, sexually active adults, and HIV patients.
  • Causes flesh-colored, dome-shaped, umbilicated papules.

⭐ High-Yield: Intracytoplasmic eosinophilic inclusions, known as Molluscum bodies (or Henderson-Paterson bodies), are pathognomonic for MCV.

Pathogenesis & Lesions - The Pustule Story

• Entry & Spread: Inhalation of respiratory droplets → replication in upper respiratory tract & regional lymph nodes.
• Viremia:
  • Primary viremia: Spreads to reticuloendothelial system (liver, spleen, bone marrow).
  • Secondary viremia: High fever, toxemia; virus disseminates to the skin.
• Replication Site: Virus replicates in the cytoplasm, forming characteristic eosinophilic intracytoplasmic inclusions (Guarnieri bodies).

⭐ Key Differentiator: Poxvirus lesions are synchronous-all lesions are in the same stage of development. This contrasts with the asynchronous (crops of) lesions seen in Varicella (chickenpox).

Lesion Evolution:

Diagnosis & Management - Taming the Pox

• Diagnosis:
  • Clinical: Rash with synchronous lesions, often centrifugal (face/extremities > trunk).
  • Lab: PCR of lesion fluid/crusts is the gold standard.
  • Microscopy: Electron microscopy reveals characteristic brick-shaped virions. Light microscopy shows Guarnieri bodies (intracytoplasmic eosinophilic inclusions).
• Management & Prevention:
  • Primary: Supportive care.
  • Antivirals: Tecovirimat for severe disease; Cidofovir is an alternative.
  • Vaccination: JYNNEOS (live, non-replicating) for pre/post-exposure prophylaxis.

⭐ Mpox vs. Smallpox: Mpox is distinguished clinically by the presence of significant lymphadenopathy, which is typically absent in smallpox.

High‑Yield Points - ⚡ Biggest Takeaways

• Poxviruses are the largest DNA viruses and are unique for replicating entirely within the cytoplasm.
• They carry their own DNA-dependent RNA polymerase, allowing for cytoplasmic replication.
• Smallpox (variola), now eradicated, caused synchronous, deep-seated pustules.
• Molluscum contagiosum causes characteristic flesh-colored, umbilicated papules.
• Monkeypox presents with fever and a progressive rash, often with lymphadenopathy.
• Look for eosinophilic intracytoplasmic inclusions (Guarnieri bodies).