Orf Workup

Updated: Jun 06, 2018
  • Author: Aaron Z Hoover, MD, FAAD; Chief Editor: Dirk M Elston, MD  more...
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Laboratory Studies

The diagnosis of orf is usually made based on the history of contact with infected animal and the presence of a typical clinical lesion, but there are laboratory studies that can aid in the diagnosis.

Polymerase chain reaction (PCR) can definitively identify the virus and is available from the US Centers for Disease Control and Prevention (CDC) (telephone 404-639-4129). [41, 42, 43] Clinicians should contact their state health department to coordinate PCR testing.

Conventional histopathology of an orf skin biopsy may be of significant diagnostic value (see below).

No serologic tests are routinely performed for orf, but a detectable antibody response may be present. [44] Serology cannot distinguish orf virus from other parapoxviruses such as paravaccinia (pseudocowpox) virus.

Confirmation of the clinical diagnosis of orf can be made by electron microscopy with negative staining of the crust or a small biopsy specimen. Electron microscopy has also been performed on fluid obtained from the orf lesion. [45] It demonstrates classic ovoid cross-hatched virions but cannot be used to distinguish orf virus from other parapoxviruses.

Tissue culture with passage of the orf virus to sheep cell cultures is an option; however, the growth of the virus is slow and inconstant.


Histologic Findings

The epidermis usually shows marked pseudoepitheliomatous hyperplasia. Necrosis of the epidermis with ulceration occurs in the center of the lesion. The orf viral infection causes intranuclear and intracytoplasmic inclusion bodies with vacuolization and disaggregation of keratinocytes. Pyknosis of individual keratinocytes occurs. A dense inflammatory infiltrate of plasma cells, macrophages, histiocytes, and lymphocytes is also observed in orf.