Dermatology MCQ - Viral Infections - Orf

A shepherd presents with a solitary, tender, erythematous papule on the dorsum of his hand. The lesion has evolved over a week into a firm, targetoid nodule with a central red core, a middle white ring, and a peripheral red halo. He reports delivering lambs two weeks prior. Which of the following is the most likely causative agent?

A) Bartonella henselae
B) Cowpox virus
C) Staphylococcus aureus
D) Orf virus
E) Mycobacterium marinum

Correct Answer: D) Orf virus

Explanation

This clinical presentation is pathognomonic for orf, also known as contagious ecthyma or scabby mouth.

Key Clinical Features of Orf:

• Transmission: A zoonotic infection acquired through direct contact with infected sheep or goats (often during lambing, shearing, or slaughtering). The virus enters through cuts or abrasions in the skin.

• Clinical Evolution: The lesion progresses through six distinct clinical stages over 4-6 weeks:

  1. Maculopapular stage

  2. Targetoid stage: This is the classic appearance described in the question—a nodule with a central red core (granulation tissue), a middle white ring (epidermal regeneration), and a peripheral red halo (inflammation).

  3. Acute stage: Weeping, nodular surface.

  4. Regenerative stage: Dry, crusted surface.

  5. Papillomatous stage: Small, finger-like projections.

  6. Regression stage

• Symptoms: The lesion is often tender but may be pruritic. Lymphangitis and mild regional lymphadenopathy can occur.

Virologic and Histopathologic Features:

• Causative Agent: Orf virus, a member of the Parapoxvirus genus.

• Microscopy: Electron microscopy reveals ovoid virions with a characteristic criss-cross pattern of tubular filaments on their surface, distinguishing them from the brick-shaped orthopoxviruses (e.g., vaccinia, cowpox).

• Histopathology: Shows epidermal ballooning degeneration, intraepidermal vesicles, and eosinophilic cytoplasmic inclusions.

Why Not the Other Options?

• (A) Bartonella henselae: Causes cat-scratch disease, which presents as a tender papule at the inoculation site followed by suppurative regional lymphadenopathy. It does not form a targetoid nodule.

• (B) Cowpox virus: Causes a similar solitary necrotic ulcer with eschar and significant surrounding inflammation. However, it is an orthopoxvirus (brick-shaped virions) and is acquired from cats or rodents, not sheep. It does not typically have the classic targetoid appearance.

• (C) Staphylococcus aureus: Causes common bacterial infections like furuncles (boils) or impetigo, which are pustular or crusted but lack the distinctive targetoid morphology and staged evolution of orf.

• (E) Mycobacterium marinum: Causes "swimming pool granuloma"—a chronic, indurated plaque or nodule that may ulcerate. It follows a sporotrichoid distribution (ascending lymphocutaneous nodules) and has a much slower progression than orf.

Management:

• Self-limiting infection: Orf typically resolves spontaneously within 6-8 weeks.

• Treatment is supportive: Wound care to prevent secondary bacterial infection.

• Intervention: May be considered for large or persistent lesions (e.g., cryotherapy, curettage, or imiquimod), but evidence is limited.

• Important: Misdiagnosis can lead to unnecessary treatments like antibiotics or surgery. Recognition of the classic clinical picture and occupational history is key.

Prognosis: Excellent. Immunity is not long-lasting, so re-infection can occur.