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Table of Contents
Definition / general | Essential features | Clinical features | Clinical images | Treatment | Case reports | Microscopic (histologic) description | Microscopic (histologic) images | Diagnosis | Electron microscopy description | Electron microscopy images | Differential diagnosis | Board review style question #1 | Board review answer #1 | Board review style question #2 | Board review answer #2Cite this page: Redpath M Orf. PathologyOutlines.com website. http://www.pathologyoutlines.com/topic/skinnontumororf.html. Accessed March 31st, 2020.
Definition / general
- Orf (ecthyma contagiosum) is caused by a double stranded DNA virus of the parapoxvirus genus (Viruses 2015;7:1505)
- Primarily a disease of sheep and goats, humans may become infected after direct exposure to the lesions on infected animals (cdc.gov: Orf Virus (Sore Mouth Infection) [Accessed 10 May 2018])
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
Images hosted on other servers:
Firm, erythematous papule of the right thumb
Hand infection
Treatment
- No specific treatment is required (Johnston: Weedon's Skin Pathology Essentials, 2nd Edition, 2016, cdc.gov: Orf Virus (Sore Mouth Infection) [Accessed 10 May 2018])
- Lesions tend to resolve spontaneously within 6 - 7 weeks
Case reports
- 4 year old girl burn victim (J Infect Dis 2016;214:1171)
- 16 year old girl with rare human to human transmission (Int J Dermatol 2014;53:e63)
- 53 year old woman with transmission from cat scratch (Dermatol Online J 2011;17:9)
- 65 year old woman farmer from Connecticut (Dermatopathology (Basel) 2016;3:55)
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
Images hosted on other servers:
Rare necrotic keratinocytes
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
- Intracytoplasmic poxvirus particles
- Oval shaped with an electron dense core (Johnston: Weedon's Skin Pathology Essentials, 2nd Edition, 2016)
- Laminated capsule
Electron microscopy images
Images hosted on other servers:
Oval viral particles
Differential diagnosis
- Anthrax
- Cowpox
- Herpes simplex
- Milker’s nodule
- Smallpox
- Varicella-Zoster
Board review style question #1
- Which of the following diseases is caused by a DNA virus?
- Hand foot and mouth disease
- Hepatitis C
- Measles
- Orf
- Rabies
Board review answer #1
D. Orf. The other diseases are caused by RNA viruses.
Board review style question #2
- Which of the following is true regarding Orf?
- It is a common viral infection in humans
- Direct human to human transmission is the most common mode of transmission
- Lesions should be aggressively treated, with surgical resection and antivirals
- Cytoplasmic or intranuclear inclusion bodies are usually present within infected keratinocytes
- Infected keratinocytic nuclei show margination of chromatin with multinucleation and nuclear molding
Board review answer #2
- D. True, inclusion bodies are typically present.
- Incorrect, Orf is a rare viral infection in humans
- Incorrect, as transmission is typically from infected sheep or goats to humans, not human to human
- Incorrect, lesions typically resolve on their own
- Incorrect, answer is describing herpes virus change
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