Contributors: Laurie Good MD, Whitney A. High MD, JD, MEng, Jeffrey D. Bernhard MD, Noah Craft MD, PhD, Lindy P. Fox MD, Lowell A. Goldsmith MD, MPH, Michael D. Tharp MD
Milker's nodules, also known as paravaccinia or pseudocowpox, is a cutaneous disease transmitted from dairy cows to humans. It is a self-limiting infection caused by a parapox virus. In humans, the virus causes lesions on areas of skin that are in direct contact with the cow, classically the hands, wrists, and arms.
The incubation period in humans ranges from 5-28 days, followed by the appearance of violaceous nodules that may be smooth, verrucous, or, less commonly, pustular or bullous. Lesions may be surmounted by a superficial grayish crust. Classically, 1-5 lesions are present on an affected individual, each roughly 0.6-2 cm. Mild constitutional symptoms such as low-grade fever, regional lymphadenitis, and moderate leukocytosis and eosinophilia may be present. The lesions of milker's nodules may be painful on direct pressure. Although rare, milker's nodules may be spread from person to person via direct contact. The condition usually resolves in about 6 weeks.
A detailed patient history will typically elicit exposure to a cow, usually through milking. The cow may actually have papular, pustular, vesicular, or crusted lesions upon the teats and possibly other areas. Lesions in cattle are often accompanied by alopecia. Epidemics affecting many animals within a herd are common. Infections in cows are thought to be spread by contact, either through milking machines or the farmers.
Codes
ICD10CM: B08.03 – Pseudocowpox [milker's node]
SNOMEDCT: 27240009 – Milker's nodule
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Differential Diagnosis & Pitfalls
Ecthyma – A simple bacterial infection caused by Staphylococcus aureus.
Halogenoderma (bromide and iodide eruptions) – Differentiated by a specific exposure to ingestants containing these halides.
Tuberculosis verrucosa cutis – Red papular nodules in the skin that may appear 2-4 weeks after inoculation by Mycobacterium tuberculosis in a previously infected and immunocompetent individual.
Primary or secondary syphilis – Distinguished by a history of sexually transmitted disease and performance of a rapid plasma reagin (RPR) or Venereal Disease Research Laboratory (VDRL) test.
Cowpox – This is a rare condition more often associated with cats and rodents rather than cattle.
Ecthyma contagiosum (orf) – A similar condition caused by a similar parapox virus that typically affects sheep rather than cows.
A thorough history, culture, or biopsy can be used to differentiate milker's nodules from most of the above, although there can be secondary bacterial infections in the lesions of milker's nodules.
