Dermatology MCQ - Viral Infections - Milker’s Nodule

A dairy farmer presents with a solitary, purplish, firm nodule on the dorsum of his index finger. The lesion is painless but has a slightly granular surface. He reports that it appeared about two weeks after assisting a cow with a difficult calving. The cow had similar lesions on its udder. Which of the following is the most accurate statement regarding the causative agent of this condition?

A) It is a double-stranded DNA virus from the Orthopoxvirus genus.
B) It is a bacterium commonly found in soil and manure.
C) It is a single-stranded RNA virus from the Parapoxvirus genus.
D) It is a fungus acquired from contact with contaminated hay.
E) It is a double-stranded DNA virus from the Parapoxvirus genus.

Correct Answer: E) It is a double-stranded DNA virus from the Parapoxvirus genus.

Explanation

This clinical scenario is classic for milker's nodules (also known as paravaccinia or pseudocowpox), a common occupational infection among dairy workers.

Key Clinical Features of Milker's Nodules:

• Transmission: Zoonotic infection acquired through direct contact with the teats and udders of infected cows. The virus enters through small cuts or abrasions on the hands.

• Incubation Period: 5-15 days after exposure.

• Clinical Presentation: The lesion begins as a red macule or papule that evolves into a painless, firm, purplish or reddish-blue nodule (often described as a "berry"). It may have a granular or verrucous surface and is typically solitary, though multiple lesions can occur. Unlike orf, it rarely goes through a targetoid stage.

• Systemic Symptoms: Uncommon, but mild fever or lymphangitis can occur.

Virologic Features:

• Causative Agent: Parapoxvirus bovis 2 (Pseudocowpox virus).

• Virus Family: It is a double-stranded DNA virus belonging to the Parapoxvirus genus. This is the key differentiator from other similar infections.

• Microscopy: Electron microscopy reveals the characteristic ovoid virions with a criss-cross tubular pattern unique to parapoxviruses.

Why Not the Other Options?

• (A) Double-stranded DNA virus from the Orthopoxvirus genus: This describes viruses like vaccinia, cowpox, and variola (smallpox). These cause lesions with central eschar and significant surrounding inflammation (e.g., cowpox), not the typical painless, purplish nodule of milker's nodules.

• (B) Bacterium from soil/manure: This would describe infections like Bacillus anthracis (anthrax) or Erysipelothrix rhusiopathiae (erysipeloid), which have different clinical presentations (e.g., black eschar in anthrax, violaceous plaques in erysipeloid).

• (C) Single-stranded RNA virus: Parapoxviruses are DNA viruses. RNA viruses cause conditions like hand, foot, and mouth disease (enteroviruses) or measles, which present with widespread eruptions, not solitary nodules.

• (D) Fungus from contaminated hay: This would describe cutaneous fungal infections like dermatophytosis (ringworm) or sporotrichosis, which present as scaly plaques or ulcerated nodules with a sporotrichoid distribution, not the specific purplish nodule linked to bovine contact.

Histopathology:
A biopsy would show epidermal hyperplasia (acanthosis), vacuolization of keratinocytes, and eosinophilic cytoplasmic inclusions. However, diagnosis is typically made clinically based on history and lesion morphology.

Management:

• Self-limiting: The infection resolves spontaneously within 4-8 weeks without scarring.

• Supportive care: Wound care to prevent secondary bacterial infection.

• No specific antiviral treatment is required.

Prevention: Use of gloves and proper hand hygiene when handling infected animals.