Laboratory Studies
The diagnosis of milker’s nodule primarily rests on a history of contact with infected animal and presence of a typical clinic lesion. Although no laboratory tests are diagnostic of milker's nodule, the clinician may wish to perform tests to help rule out other entities on the list of differential diagnoses. Specific tests would be suggested by the clinical presentation of each patient as well as the expected time until results from definitive tests can be returned. Note the following:
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Viral culture in milker's nodules: Viral tissue culture of milker's nodule virus on bovine or human cells may be available. This procedure takes several weeks, and tissue culture differences between orf and milker's nodule are subtle.
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DNA hybridization: Milker's nodule virus can be distinguished from orf virus by DNA hybridization to the relatively less conserved terminal regions.
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Bacterial culture in milker's nodules: Culture and sensitivity can direct therapy if suspicion exists of bacterial superinfection.
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Polymerase chain reaction (PCR) in milker's nodules: The “pan-pox” assay is a recently developed PCR assay that can be used for screening and diagnosis of human and animal poxvirus infections. Many laboratories worldwide have access to the technology used in this assay. [13] PCR can differentiate milker’s nodule and orf. [14]
Other Tests
Electron microscopy in milker's nodules [15, 16] can be rapidly performed on either crust or biopsy material without refrigeration or special preservative solutions. Particles may stay infectious for extended periods. Electron microscopy can be useful in demonstrating virus particles; however, it cannot distinguish between milker's nodule, orf, and bovine papular stomatitis.
Procedures
A shave biopsy may be performed for hematoxylin and eosin (H&E) tissue histology. A biopsy plus electrocautery of the base handily removes the lesion.
Histologic Findings
The histologic appearance of milker’s nodule varies with the stage of the disease and is similar to orf. H&E stain of early lesions shows ballooning of keratinocytes, spongiform appearance of vacuolated cells with wispy eosinophilic cytoplasm, vesicle formation, prominent cell membranes, and pyknotic nuclei. There may be eosinophilic intracytoplasmic viral inclusions and rarely intranuclear inclusions. [17] Later in the course of disease, milker’s nodules demonstrate irregular acanthosis with pronounced deep extensions of thin rete ridges. Epidermal necrosis is variable. [18]
In the dermis, a dense infiltrate of lymphocytes and histiocytes, a marked increase in dermal capillaries, and epidermal and dermal edema are observed. Other authors report a mixed infiltrate also including eosinophils, neutrophils, and plasma cells.
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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).
