Author: Assoc Prof Patrick Emanuel, Dermatopathologist, Auckland, New Zealand, 2013.
Virus of the genus Parapoxvirus is transmitted to humans from infected sheep or goats (orf), cows (milker's nodule), and oral mucosa of cattle (bovine papular stomatitis).
Histology of farmyard pox
Biopsy of a classic fully developed farmyard pox papulovesicle exhibits an intraepidermal blister secondary to massive ballooning of infected keratocytes and subsequent necrosis (figure 1). Necrosis and acutespongiosis often accompanies ballooning (figure 2). The intranuclear inclusions are eosinophilic and may be exceedingly difficult to find (figure 3, arrow). Often, the reaction pattern, necrosis and ballooning keratinocyte degeneration are the only clues and a clinical history of exposure to farm animals together with ancillary studies may lead to the correct diagnosis.
The farmyard pox inclusion bodies may be highlighted by Lendrum’s phloxine tartazine.
Culture, fluorescent antibody test, and electron microscopy may be used to demonstrate the causative pox virus.
Differential diagnosis of farmyard pox pathology
Cowpox, horsepox and smallpox – These also exhibit impressive ballooning of keratocytes, necrosis and inclusion bodies which may resemble those seen in farmyard pox. Clinical history and – if necessary – ancillary microbiologic investigations may be required for precise diagnosis.