Skin inflammatory (nontumor)
Infectious disorders
Orf


Senior Author: Ayman Al Habeeb, M.D.
Editorial Board Member: Hillary Rose Elwood, M.D.
Editor-in-Chief: Debra Zynger, M.D.
Margaret Redpath, M.D.
Ayman Al Habeeb, M.D.

Topic Completed: 1 April 2018

Revised: 1 April 2019

Copyright: 2003-2019, PathologyOutlines.com, Inc.

PubMed Search: Orf [title] skin


Margaret Redpath, M.D.
Ayman Al Habeeb, M.D.
Cite this page: Redpath M Orf. PathologyOutlines.com website. http://www.pathologyoutlines.com/topic/skinnontumororf.html. Accessed June 18th, 2019.
Definition / general
Essential features
  • Rare viral infection in humans secondary to exposure to infected sheep or goats
  • Lesions are usually located on forearms and hands
  • Self-limited course, no treatment is required
Clinical features
  • Infection typically occurs on fingers, hands and forearms
  • Lesions typically evolve through several stages:
    • Primary macule develops into erythematous papule
    • Lesion develops targetoid appearance
    • Exudative phase
    • Nodule formation
    • Drying out and crusting
    • Complete regression (JRSM Open 2015;6:2054270415593718)
Clinical images
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Firm, erythematous papule of the right thumb

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Hand infection

Treatment
Case reports
Microscopic (histologic) description
  • Intraepidermal vesiculation (particularly in early lesion)
  • Epidermal hyperplasia with serum crust, variable epidermal vacuolation
  • Eosinophilic inclusions in the nucleus or cytoplasm of keratinocytes
  • Papillary dermal edema with mixed inflammation of lymphocytes, histiocytes, plasma cells
  • Epidermal necrosis in later lesions
Microscopic (histologic) images
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Rare necrotic keratinocytes

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Legend: balloon degeneration and spongiosis

Diagnosis
  • Usually based on history of exposure to infected animals with characteristic clinical and histologic features
  • Electron microscopy and PCR testing can be used to confirm the diagnosis if needed
Electron microscopy description
Electron microscopy images

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Oval viral particles

Differential diagnosis
Board review question #1
    Which of the following diseases is caused by a DNA virus?

  1. Hand foot and mouth disease
  2. Hepatitis C
  3. Measles
  4. Orf
  5. Rabies
Board review answer #1
D. Orf. The other diseases are caused by RNA viruses.
Board review question #2
    Which of the following is true regarding Orf?

  1. It is a common viral infection in humans
  2. Direct human to human transmission is the most common mode of transmission
  3. Lesions should be aggressively treated, with surgical resection and antivirals
  4. Cytoplasmic or intranuclear inclusion bodies are usually present within infected keratinocytes
  5. Infected keratinocytic nuclei show margination of chromatin with multinucleation and nuclear molding
Board review answer #2
    D. True, inclusion bodies are typically present.

  1. Incorrect, Orf is a rare viral infection in humans
  2. Incorrect, as transmission is typically from infected sheep or goats to humans, not human to human
  3. Incorrect, lesions typically resolve on their own
  4. Incorrect, answer is describing herpes virus change


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