Diagnosis: Orf (Contagious Ecthyma) - Parapoxvirus Infection
The correct answer is D. Parapoxvirus - this patient has orf (contagious ecthyma), a zoonotic infection acquired from direct contact with infected goats during ear tagging without protective equipment.
Clinical Reasoning
The clinical presentation is pathognomonic for orf virus infection:
- Occupational exposure: Direct contact with goats showing crusted lesions during ear tagging without PPE 1
- Characteristic lesion: Solitary, small, firm nodule at the site of contact 2, 3
- Animal reservoir: Goats and sheep are the primary hosts, with crusted lesions representing active viral shedding 1
- Transmission mechanism: Orf virus is specifically documented to occur following ear-tagging procedures in goats 4
Why Other Options Are Incorrect
Streptococcus pyogenes (A) would present with spreading erythema, warmth, and systemic symptoms of cellulitis or erysipelas, not a solitary firm nodule 1.
Bartonella henselae (B) causes cat-scratch disease with regional lymphadenopathy following cat scratches or bites, not goat contact 1.
Human papillomavirus (C) causes warts but has no association with animal contact and develops over weeks to months, not acutely after a specific exposure event.
Disease Characteristics
Orf is caused by a parapoxvirus that primarily affects sheep and goats 2, 3. Key features include:
- Self-limited course: Resolves spontaneously within 6-8 weeks without specific treatment 2, 3
- High environmental resilience: The virus persists in the environment and on fomites, contributing to transmission 3
- Zoonotic transmission: Occurs through direct contact with infected animals or contaminated equipment 1
- Occupational hazard: Particularly affects farmers, veterinarians, and those handling sheep and goats 3, 5
Management Approach
Treatment is primarily supportive, as orf is self-limiting 2:
- No specific antiviral therapy is required for uncomplicated cases 2
- Various interventions (imiquimod, cidofovir, cryotherapy, curettage) have been reported but lack controlled trial evidence 3
- Monitor for complications such as secondary bacterial infection or rare erythema multiforme 6
- Reassure the patient about the benign, self-resolving nature of the infection 2, 3
Prevention Considerations
This case highlights the critical importance of PPE when handling animals with visible lesions 1:
- Wear gloves when tagging ears or handling animals with crusted lesions 1
- Orf virus is highly contagious and can be transmitted through fomites 3
- Prior smallpox vaccination provides no cross-protection against orf virus 5
Common Pitfall
Do not mistake this for a bacterial infection requiring antibiotics. The firm, nodular character at a site of animal contact with visible crusted lesions on the animals is classic for orf, not bacterial cellulitis or abscess 1, 2.