Microbiology & infectious diseases



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

Last author update: 1 April 2018
Last staff update: 29 March 2021

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PubMed Search: Orf [title] skin

Margaret Redpath, M.D.
Ayman Al Habeeb, M.D.
Page views in 2024 to date: 873
Cite this page: Redpath M, Al Habeeb A. Orf. PathologyOutlines.com website. https://www.pathologyoutlines.com/topic/microbiologyorf.html. Accessed April 14th, 2024.
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

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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Balloon degeneration and spongiosis

  • 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 style 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 style answer #1
D. Orf. The other diseases are caused by RNA viruses.

Comment Here

Reference: Orf
Board review style 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 style answer #2
D. True, inclusion bodies are typically present.

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

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Reference: Orf
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