Background
Milker's nodule, first described in the literature in 1799, is a localized, cutaneous, and mostly benign infection caused by a DNA virus of the genus Parapoxvirus. [1] See the image below.
Milker’s nodules. Courtesy of Professor Raimo Suhonen and DermNet New Zealand (http://www.dermnetnz.org/assets/Uploads/viral/s/milkers3.jpg).
The disease is a zoonosis endemic to and common in cattle worldwide. Infections in cattle are also known as bovine papular stomatitis. Human disease is contracted through direct transmission (ie, handling of infected cow teats, calf muzzles, or other sites of active bovine infection) or through indirect transmission (ie, handling of virally contaminated objects).
The course of milker's nodule is usually self-limited, running from 14-72 days, with infrequent systemic symptoms and little or no scarring.
Pathophysiology
Viral replication produces intracytoplasmic inclusions and cytopathic changes in epidermal keratinocytes along with epidermal and dermal reactive changes.
The etiologic organism is the milker's nodule virus, also called paravaccinia virus. The milker's nodule virus is a 140 X 310-nm, double-stranded DNA poxvirus, a member of the cylindrical subgroup. Paravaccinia is resistant to desiccation, cold, and heat up to 56°C and can persist in the environment. Under electron microscopy, it is cylindrical with convex ends, and it is covered with a uniform diagonal criss-cross pattern of parallel ropelike structures. On electron microscopy, the DNA core is surrounded by a protein coat, which is wrapped by 2 narrow parallel coats. Mature virus particles are located within keratin fibrils in the stratum corneum.
Some studies suggest that bovine papular stomatitis virus, which may clinically cause an identical lesion in humans, is an organism distinct from milker's nodule virus. A recent study shows a very close DNA homology between papular stomatitis of Finnish reindeer and milker's nodule virus type 1. [2, 3]
Milker's nodule virus can be propagated in tissue culture. It can be distinguished from orf virus by DNA hybridization, though not by ultrastructural studies. [4]
Etiology
Milker's nodules are caused by a double-stranded DNA virus of the genus Parapoxvirus. Milker's nodule is a zoonosis endemic to and common in cattle worldwide.
Human milker's nodules are contracted through direct transmission (ie, handling of infected cow teats, calf muzzles, other sites of active bovine infection) or through indirect transmission (ie, handling of virally contaminated objects).
Evidence suggests that milker's nodule virus (traditionally associated with disease contracted from papulonodular lesions on cow teats) and bovine papular stomatitis virus (traditionally isolated from erosive lesions on calf muzzles) may be different though closely related viruses. It seems that they may both cause milker's nodule in humans. In fact, each may cause both types of lesions in cattle.
Epidemiology
Milker's nodule is an occupational disease, mainly affecting milkers and farm workers caring for dairy cattle, as well as stockyard workers, butchers, and veterinarians. [5] Because infection generally results in immunity, lesions are most commonly seen in persons new to these occupations or in those who have sporadic contact, such as new milkers, pet owners, and veterinary students. [6] The disease is commonly known among agricultural workers to be benign; therefore, it is rarely reported to doctors or other medical personnel in that setting. Sporadic cases and occasional epidemics occur.
Prognosis
The prognosis is excellent in milker’s nodules, a self-limited disease that results in little or no scarring.
Lymphangitis, lymphadenitis, and fever, which may last from a few days to a few weeks, may occur in milker's nodules patients.
No reports exist describing milker's nodule infection during pregnancy; however, two reports exist of orf virus, a related Parapoxvirus, being contracted in the third trimester (33rd and 34th wk), resulting in maternal lesions but normal-term infants. [7]
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Milker’s nodules. Courtesy of Professor Raimo Suhonen and DermNet New Zealand (http://www.dermnetnz.org/assets/Uploads/viral/s/milkers3.jpg).
