GP Quiz 8 - Case 6.
A sheep farmer presents with an irritable rash predominantly on hands and feet. This has arisen three weeks after the appearance of a blistered nodule on the thumb. She is mildly unwell with regional lymphadenopathy.
What is the cause of the crusted bulla?
The lesion on the thumb is Orf. Orf is a paravaccinia viral infection, which presents as one or more 10-20mm bullae or nodules on an exposed site. Human orf arises a few days after contact with an infected sheep or goat. The animal suffers from "scabby mouth" with weeping or crusted lesions around the mouth, nares, teats and udders. Characteristically, the human infection goes through several stages over about six weeks; maculopapular, target (iridoform bullae), acute (weeping), regenerative (firm, thin-crusted nodule), papillomatous and regressive (thick crust with resolution). There may be regional lymphadenopathy, malaise and fever.
What is the secondary papular rash?
The patient has developed erythema multiforme minor, an occasional complication of orf. Erythematous papules, round plaques and target lesions erupt over a few days, mainly in acral locations (especially hands, feets, elbows and knees). The lesions are itchy and sometimes blister. Mucosal erosions may occur.
Advise on management.
Both skin conditions are self-limited without serious sequelae.
The diagnosis of orf is based on the clinical presentation but can be confirmed by electron microscopic identification of paravaccinia viruses. Treatment is not usually needed. However, persistent lesions can be removed by curettage and electrodessication or shave excision. Infected animals should be isolated but the human patient is not contagious.
There is no specific therapy for erythema multiforme minor, which resolves within three weeks. Systemic steroids should be avoided. Wet compresses should be used for exudative skin lesions, and topical steroids or antipruritic lotions can reduce itching.