What is the likely cause of a solitary small, firm nodule in an adult patient with occupational exposure to livestock, who developed the lesion after tagging the ears of goats with crusted lesions without using personal protective equipment (PPE)?

Orf Virus (Parapox Virus) - Answer: D

The patient's presentation is pathognomonic for orf virus infection (contagious ecthyma), a parapoxvirus transmitted from sheep and goats with crusted lesions during occupational exposure without protective equipment. 1, 2

Clinical Reasoning

The key diagnostic features that definitively point to orf virus include:

• Occupational exposure to goats with crusted lesions - The CDC identifies goats and sheep with crusted lesions as the primary reservoir for orf virus, with these lesions representing active viral shedding 1
• Direct contact during ear tagging without PPE - Orf virus transmits through direct skin contact with infected animals or contaminated fomites, particularly during procedures like ear tagging 1, 3
• Solitary, small, firm nodule - The American Academy of Dermatology notes this presentation is characteristic of orf virus infection at the contact site, distinguishing it from other bacterial infections 1, 4

Why Other Options Are Incorrect

• Streptococcus pyogenes (Option A) would present with spreading erythema, warmth, and systemic symptoms of cellulitis or erysipelas, not a solitary firm nodule 1
• Bartonella henselae (Option B) is associated with cat scratches or bites, not goat contact 5, 2
• Human papillomavirus (Option C) does not transmit from animals and would not present acutely after a single exposure event

Epidemiology and Transmission

• Orf is a zoonotic parapoxvirus endemic worldwide, principally associated with small ruminants 3, 6
• Human infections appear as ulcerative skin lesions after contact with infected animals or contaminated fomites 3
• This is an occupational disease affecting breeders, veterinarians, and farm workers who have close contact with sheep and goats 6, 7
• Breaches in the integument (such as from handling equipment during ear tagging) are important risk factors for transmission 8

Clinical Course and Management

• The infection is self-limiting and typically resolves without specific treatment within 6-8 weeks 1
• The Infectious Diseases Society of America recommends supportive treatment only, as no specific antiviral therapy is required for uncomplicated cases 1
• Critical caveat: Orf shares clinical manifestations with potentially life-threatening zoonoses like cutaneous anthrax and is likely under-recognized due to lack of clinical suspicion 3

Prevention

• The CDC emphasizes wearing personal protective equipment (gloves) when handling animals with crusted lesions to prevent transmission 1, 2
• Barrier precautions and proper hand hygiene are the primary preventive measures 3