Dermatology MCQ - Viral Infections - Cowpox

A young child presents with a tender, erythematous nodule on the hand that has developed a central ulcer and a surrounding zone of edema and induration. The parents report the child was playing with a pet cat a week ago, which has since developed lesions on its paws. A viral culture is performed, which reveals large, brick-shaped virions that grow well on chorioallantoic membrane and produce hemorrhagic pocks. Which of the following is the most likely causative agent?

A) Variola virus
B) Vaccinia virus
C) Cowpox virus
D) Orf virus
E) Herpes simplex virus type 1

Correct Answer: C) Cowpox virus

Explanation

This clinical scenario is a classic description of cowpox infection, a zoonotic orthopoxvirus infection.

Key Clinical Features of Cowpox:

• Transmission: Typically acquired from infected animals, most commonly domestic cats (not cows, despite the name), or occasionally rodents. The inoculation occurs through a break in the skin.

• Clinical Presentation: The lesion begins as a painful, inflamed macule that progresses over 1-2 weeks through stages of papule → vesicle → pustule → ulceration with a central black eschar (necrosis). It is surrounded by significant edema, induration, and erythema, often making it resemble a bacterial cellulitis or anthrax. Lymphangitis and regional lymphadenopathy are common.

• Systemic Symptoms: Patients may experience fever, malaise, and vomiting.

Virologic Identification:

• Viral Culture Characteristics: Cowpox virus is an orthopoxvirus.

  • Morphology: It produces large, brick-shaped virions visible by electron microscopy.

  • Chorioallantoic Membrane (CAM) Assay: This is a classic diagnostic method for orthopoxviruses. Cowpox virus produces hemorrhagic pocks on the CAM of chick embryos, which helps distinguish it from other orthopoxviruses like vaccinia (which produces white pocks).

Why Not the Other Options?

• (A) Variola virus: Causes smallpox, which presents with a widespread, centrifugal, monomorphic vesiculopustular rash, not a solitary necrotic ulcer. It was eradicated and is not acquired from cats.

• (B) Vaccinia virus: The virus used in the smallpox vaccine. It can cause a localized "take" reaction (similar to the described lesion) but is not typically acquired zoonotically from cats. Its CAM pocks are non-hemorrhagic (white).

• (D) Orf virus: A parapoxvirus that causes "contagious ecthyma" in sheep and goats. Human lesions can be similar (a solitary necrotic nodule) but are often less inflamed. Parapoxviruses produce ovoid virions (not brick-shaped) and a different CAM culture pattern.

• (E) Herpes simplex virus type 1: Causes herpetic whitlow, which presents as grouped vesicles on an erythematous base. It does not form a large, necrotic eschar and is not associated with contact with cats.

Management:

• Supportive care is primary, as the infection is usually self-limiting in immunocompetent hosts.

• Wound care to prevent secondary bacterial infection.

• In severe or immunocompromised cases, tecovirimat or vaccinia immune globulin (VIG) may be considered, though their efficacy for cowpox is less established than for smallpox or vaccinia.

Prognosis: Excellent, with resolution in 4-8 weeks, often leaving a scar.