Milker's nodule: an occupational infection and threat to the immunocompromised

Milker's nodule virus, also called paravaccinia virus, is a DNA virus of the parapoxvirus genus transmitted from infected cows to humans. It results from contact with cattle, cattle by-products or fomites. Classified as an occupational disorder, those at risk of exposure include farmers, butchers and agricultural tourists. The viral infection begins 5–15 days after inoculation as an erythematous-purple, round nodule with a clear depressed centre and a surrounding erythematous ring. While familiar to those in farming communities, the presence of the nodule may be concerning to others, particularly the immunosuppressed. Milker's nodules are self-limited in immunocompetent individuals and heal without scarring within 8 weeks. Another member of the Parapoxvirus genus, the orf virus, is also transmitted from animals to humans by direct contact. While complications are rare, haematopoietic stem cell transplant recipients are at risk of graft-versus-host disease, as the parapoxvirus may trigger these complications in immunocompromised individuals. In addition, paravaccinia may serve as the antigen source for the development of erythema multiforme. The unique structure and replication process of viruses in the Poxvirus family, while includes the Parapoxvirus genus, have been a focus for treatment of infections and cancer. Manipulation of these viruses has demonstrated promising therapeutic possibilities as vectors for vaccines and oncologic therapy.