The histopathology and electron microscopy of a human monkeypox lesion.

The histological and electron microscopic findings from a solitary cutaneous monkeypox lesion taken post mortem from a child who died after a five-day illness are reported. This child is 44th in the WHO register of monkeypox cases. The lesion was at the papulonecrotic stage, with early evidence of vesiculation and minimal evidence of pustulation. Necrosis affected the stratum basale, the related basement membrane and adjacent areas of the dermal papillae at the centre of the lesion. Cell necrosis affected the next two or three layers of stratum spinosum above the destroyed stratum basale. Lateral to this zone, marked hyperplasia and intracellular oedema of the stratum spinosum constituted the papule and produced spindle-cell features. In the middle layer of the stratum spinosum, above the necrotic focus, there were minute vesicles and between these were occasional multinuclear giant cells. Bodies similar to Guarnieri bodies (GB) were present in the cytoplasm of sweat duct-lining cells in the epidermis and upper corium. Very scanty similar bodies were evident elsewhere in the papular epidermis but were difficult to distinguish from debris. Granules in the lesion with the same size as mature virions (elementary bodies) have been assessed not to be these because similar granules are present in the normal epidermis. Changes in the dermis apart from those mentioned above were minimal oedema, very mild perivascular infiltration by round cells and an occasional eosinophil. Electron microscopy showed abundant immature and mature orthopoxvirus particles in the cytoplasms of infected epidermal cells. A limited range of histochemical tests is detailed. In general, the features are indistinguishable from the papulonecrotic stage of smallpox (variola) and from tanapox as recorded in man.