Poxviruses Overview - Brick-Shaped Invaders
- General: Largest DNA viruses; brick-shaped with complex internal structure.
- Cytoplasmic Replication:
⭐ Poxviruses are the only DNA viruses that replicate entirely within the cytoplasm, a unique trait.
- Inclusions: Produce characteristic intracytoplasmic inclusion bodies.
- Key Genera (Chordopoxvirinae):
- Orthopoxvirus: Variola (smallpox), vaccinia, cowpox, mpox.
- Parapoxvirus: Orf, milker's nodules.
- Molluscipoxvirus: Molluscum contagiosum.
- Yatapoxvirus: Tanapox, Yaba monkey tumor virus.
Molluscum Contagiosum - Pearly Pimple Parade
- Etiology: Molluscum Contagiosum Virus (MCV), a Molluscipoxvirus (MCV-1 most common).
- Transmission: Direct skin-to-skin contact, fomites, autoinoculation. Sexual transmission common in adults.
- Clinical Features:
- Firm, pearly, flesh-colored to pink, dome-shaped papules (2-5 mm) with central umbilication.
- Henderson-Paterson bodies: Intracytoplasmic eosinophilic inclusion bodies containing virions.
- Common Sites:
- Children: Trunk, extremities, face.
- Adults: Genitals, lower abdomen, inner thighs.
- Immunocompromised (e.g., HIV): Numerous, larger, persistent, atypical lesions (giant molluscum, facial involvement).
- Diagnosis: Primarily clinical. Biopsy/smear shows Henderson-Paterson bodies.
- Treatment: Often self-limiting. Options: Curettage, cryotherapy, cantharidin, podophyllotoxin, imiquimod, salicylic acid. Treat underlying immunosuppression.
⭐ Henderson-Paterson bodies are pathognomonic intracytoplasmic inclusions seen in Molluscum Contagiosum.
Orf & Milker's Nodule - Farmyard Pox Pals
⭐ Orf: 6 clinical stages; targetoid/iris lesion common in acute phase.
| Feature | Orf (Contagious Ecthyma) | Milker's Nodule (Pseudocowpox) |
|---|---|---|
| Virus | Parapoxvirus | Parapoxvirus |
| Animal Reservoir | Sheep, goats | Cattle (cow udders) |
| Transmission | Direct contact (animals, fomites) | Direct contact (animals, fomites) |
| Clinical Lesion | Single/few; hands/fingers. 6 stages (maculopapular → targetoid → weeping → nodular → papillomatous → regressive). | 1-5 firm, reddish-blue, non-umbilicated papules/nodules; hands. |
| Systemic Symptoms | Usually mild/absent; low-grade fever, malaise. | Usually mild/absent; low-grade fever, malaise. |
| Complications | Secondary infection, Erythema Multiforme (esp. Orf), lymphangitis. | Secondary infection, Erythema Multiforme, lymphangitis. |
| Diagnosis | Clinical, history. EM (ovoid virions, criss-cross). PCR. | Clinical, history. EM (ovoid virions, criss-cross). PCR. |
| Management | Self-limiting (4-6 wks). Symptomatic, wound care. | Self-limiting (4-6 wks). Symptomatic, wound care. |
Mpox (Monkeypox) - Global Alert Pox
- Etiology: Mpox virus (MPXV), Orthopoxvirus. Clades: I (severe), II (milder, 2022+ global outbreak).
- Transmission: Zoonotic (rodents, primates). Human-to-human: direct contact (lesions, fluids), respiratory (prolonged), fomites. Vertical possible.
- Clinical Features:
- Incubation: 5-21 days. Prodrome (1-5 days): fever, headache, lymphadenopathy (key vs smallpox), myalgia.
- Rash (1-3 days post-fever): Evolves: macules → papules → vesicles → pustules → crusts. Centrifugal or genital/perianal/localized. Painful.
- Diagnosis: PCR of lesion material.
- Management: Supportive care. Tecovirimat for severe cases/immunocompromised. Vaccinia Immune Globulin (VIGIV).
- Prevention: Smallpox vaccines (e.g., JYNNEOS) for Pre-Exposure Prophylaxis (PrEP)/Post-Exposure Prophylaxis (PEP). Avoid contact.
⭐ Prominent lymphadenopathy is a characteristic feature of Mpox, helping distinguish it from smallpox where it is typically absent.
Smallpox & Vaccinia - Eradicated & Engineered
- Smallpox (Variola Virus - Orthopoxvirus)
- Eradicated 1980 (WHO).
- Highly contagious; Variola major ~30% mortality.
- Clinical: Synchronous, centrifugal rash (macules → papules → vesicles → pustules → scabs); no significant lymphadenopathy.
- Guarnieri bodies: Intracytoplasmic eosinophilic inclusions.
- Bioterrorism concern.
- Vaccinia Virus (Orthopoxvirus)
- Smallpox vaccine virus.
- Localized 'take' at site.
- Complications: Eczema vaccinatum (atopic dermatitis), progressive vaccinia (immunocompromised), generalized vaccinia, postvaccinial encephalitis.
⭐ Smallpox, caused by Variola virus, was declared eradicated in 1980, a monumental achievement of global vaccination efforts using the Vaccinia virus.
High‑Yield Points - ⚡ Biggest Takeaways
- Molluscum Contagiosum (MCV): Features umbilicated pearly papules, Henderson-Paterson bodies. Extensive in HIV/immunodeficiency.
- Orf: Parapoxvirus from sheep/goats. Presents targetoid lesions on hands; self-limiting. Occupational hazard.
- Milker's Nodule: Parapoxvirus from cattle. Similar to Orf, forms nodules on hands; self-limiting.
- Poxviruses: Are the largest DNA viruses, with unique cytoplasmic replication.
- Cowpox: Orthopoxvirus. Rodent reservoir, cat vector. Historically significant for smallpox vaccine.
- Smallpox (Variola): Eradicated Orthopoxvirus. Known for synchronous centrifugal rash, Guarnieri bodies.
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