Isolation of a herpesvirus from the cell culture of a malignant melanoma of a ground squirrel (Spermophilus tridecemlineatus).

A subcutaneous neoplastic mass in a 13-lined ground squirrel which metastasized to regional lymph nodes and lung was studied. Histopathologically, the tumor architecture and cellular morphology were compatible with that of a malignant amelanotic melanoma. Ultrastructurally, the neoplastic tissue was composed of oval cells, spindle-shaped cells, and spindle-shaped cells with electron-dense cytoplasmic granules. Virus particles were not seen in these cells. Cell cultures from neoplastic tissue grew in complete monolayers and on initial passages contained a few herpesvirus particles. Secondary monolayer cell culture, when exposed to 5-iodo-2'-deoxyuridine or made into several serial subculture passages, caused the appearance of cytopathic effect and the demonstration of many virus particles. The ground squirrel agent, because it contained DNA, was sensitive to chloroform treated and had herpesvirus characteristics on electron microscopy, was considered a herpesvirus. The buoyant density of the virus was 1.298 g/cm3 and the diameter of the enveloped virus particles was 146 nm. This ground squirrel herpesvirus was antigenically distinct from other known herpesviruses.