Orf

Orf is a parapox viral infection of the skin contracted from sheep, especially lambs, and goats. It can be transmitted to humans by direct inoculation from an infected animal or carcass. Orf presents most commonly as a red-blue weeping nodule on the dorsum of a finger. It is self-limiting and generally heals spontaneously within six weeks. 

This chapter is set out as follows:

• Orf is a relatively common infection among sheep farmers, and others in contact with sheep, goats and infected carcasses
• Orf is not transmitted to cattle or from one human to another 

• There may be mild systemic upset
• Lesions are often tender

• Distribution
• Orf lesions are generally solitary or few in number
• They occur most commonly on the fingers, especially the dorsum of the index finger; hands; or forearms but can appear on the face

• Morphology
• After an incubation period of 5 or 6 days a small, firm, red or red-blue papule enlarges to form a target-like nodule with a red centre, a white middle and a red periphery
• The lesion then begins to weep and crust, which is followed by the development of black dots and small papilloma's on the surface. The crust becomes thickened and then falls off
• The fully developed lesion is usually 2-3 cm in diameter but may be as large as 5 cm
• Larger, persistent, fungating lesions are occasionally seen, mainly in immunosuppressed patients

• Systemic features
• There may be a mild fever and mild lymphangitis

• Other cutaneous features
  • Secondary infection may occur
  • Erythema multiforme is occasionally associated, usually arising 10-14 days after the initial infection - refer to the related chapter
  • Rarely a widespread papulovesicular or bullous pemphigoid-like eruption occurs a few weeks after the initial infection, which usually resolves spontaneously

• Orf is usually self-limiting and no specific treatment is necessary, other than occasionally for secondary infection 

• Most cases of orf usually clear within six weeks, with lifelong immunity after the infection

• Occasionally larger, and sometimes fungating lesions may be more recalcitrant to treatment, especially in immunosuppressed patients. Such lesions have been treated with cryotherapy, cidofovir cream, topical imiquimod, or curettage, however not all lesions respond and some may recur after surgery