eMedicine Specialties > Dermatology > Viral Infections

Milker's Nodules

Author: Lily L Tinkle, MD, PhD, Staff Physician, Department of Dermatology, University of California Irvine Medical Center
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Updated: Dec 2, 2009

Introduction

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. The disease is a zoonosis endemic to and common in cattle worldwide. Human disease is 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).

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 (MNV), 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 rather than rectangular 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 1.¹

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.²

Frequency

United States

Milker's nodule is an occupational disease, mainly affecting milkers and farm workers caring for dairy cattle, as well as stockyard workers, slaughterhouse workers, and veterinary surgeons. Because immunity seems to be conferred by infection, lesions arise in persons new to these occupations or in those who have sporadic contact, such as pet owners. 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.

International

The occurrence of milker's nodules internationally is similar to the occurrence of milker's nodules in the United States.

Mortality/Morbidity

• 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, 2 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.³

Clinical

History

• Patients typically have no previous history of milker's nodule.
• Patients with milker's nodules have had recent contact with infected cows, calves, or viral fomites. (Milker's nodules are contracted from cows; orf nodules are contracted from sheep or goats.)
• The incubation period for milker's nodules may be as brief as 4 days or as long as several weeks.
• Lesions of milker's nodules are often solitary, and they may be pruritic or painful.
• Some authorities divide the clinical course of milker's nodules into 6 stages, each lasting roughly 1 week.
  • Erythematous maculopapular
  • Target (a papulovesicular lesion with a red center, white ring, and red periphery)
  • Acute weeping nodule (characterized by loss of epidermis over the center)
  • Dry, crusted nodular
  • Papillomatous
  • Regressive

Physical

• Lesions of milker's nodules are usually found on the fingers, the hands, and the forearms.
  • Usually, only a few or even a single lesion is present. Many lesions are occasionally distributed in a larger area, such as a burn site.^4,5
  • Classic milker's nodules lesions are 0.5-1.5 cm in diameter, firm, movable, dome-shaped papules or nodules.
  • Milker's nodules may be red or purplish red in color, or they may have a targetlike appearance.
  • Central ulceration or crust may occur.
  • Lesions typically have a grayish coating in the target stage and a verrucous surface in the papillomatous stage.
  • Milker's nodules often present with a vascular appearance resembling pyogenic granuloma.
  • Milker's nodules are on average smaller than orf lesions, but they may not be distinguishable on a clinical basis.
  • Variant lesions may include vesicles, scaly patches, and erosions. (The patient's history guides the differential diagnosis in these cases.)
• A focused physical examination should be performed. The following findings have been described in patients with milker's nodules:
  • Lymphangitis
  • Regional lymphadenopathy
  • Fever
  • Diarrhea
  • Abdominal cramping

Causes

• Milker's nodules are caused by a 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.

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