Electron microscopy of vesicle fluid or scab extracts is the most rapid and useful technique to aid in diagnosis. Orthopoxviruses, including cowpox, have characteristic "mulberry" and "capsule" forms that allow them to be differentiated from parapoxviruses and herpes viruses. Electron microscopy does not distinguish between cowpox, smallpox, vaccinia, and molluscum viruses, so clinical information is critical.
Virus may be grown from lesional skin in cell culture and then studied by electron microscopy. Growth on chick chorioallantoic membrane produces characteristic hemorrhagic pocks.
Polymerase chain reaction may be performed on biopsy material or cell culture extracts to amplify the cowpox A36R, thymidine kinase, or hemagglutinin gene and identify it by sequencing or restriction fragment length polymorphism analysis. 
Serologic tests to detect antibodies to cowpox virus are not routinely available in hospital laboratories but may be performed in research laboratories. Previous vaccination with vaccinia may cause false-positive results, but these can be eliminated by the use of specific immunoglobulin M tests.
Skin biopsy for routine histology, electron microscopy, culture, or molecular detection methods may be performed.
Using routine light microscopy, characteristic cytoplasmic inclusions have been observed in biopsies from feline cowpox but not in human material. Immunohistochemistry detects cowpox antigens in feline cases. Using electron microscopy, biopsy material may reveal viral particles.
What would you like to print?