Parapoxvirus is a member of the family Poxviridae, which contains double-stranded DNA viruses known to be the largest viruses. The orf virus is a cylindrical virus measuring 260x160 nm. Its surface tubules form a long crisscross design that is seen on negatively stained preparations by electron microscopy. This virus resists physical damage and persists through winter months on hedges, feeding troughs, and barns. Orf is transmitted by direct contact inoculation.
Humans acquire the infection from contact with infected animals, carcasses, or nonliving material.
It is very common, therefore, among shepherds, veterinary surgeons, farmers' wives who bottle-feed young lambs, and in butchers and meat porters from handling infected carcasses. No serologic tests are routinely done, but there may be a detectable antibody response. Complement-fixing antigen has been proposed to be the most sensitive technique for the diagnosis of orf. Confirmation of the clinical diagnosis is done by electron microscopy with negative staining of the crust or a small biopsy. Electron microscopy also has been done on fluid obtained from the lesion.