Microscopic detection of adventitious viruses in cell cultures

Sixty-five batches of three types of monkey kidney cells were examined by light and electron microscopy for the presence of simian viruses. Cynomolgus cells showed the highest contamination rate (67%), followed by rhesus (18%). Foamy virus was the most common contaminant, being detected in 13 batches, twice in concert with a paramyxovirus. Six batches contained a paramyxovirus; one batch contained a papovavirus. Of the 22 batches of African green monkey kidney cells examined, none showed evidence of virus. "Healthy" established cell lines from three laboratories were also checked by electron microscopy, and several contaminating viruses were found. These were apparently either accidentally introduced during handling, as in the case of SV5, reovirus, and parvovirus, or were latent viruses (hamster and mouse) derived from the host tissue. While negative staining provided the simplest technique for detection of the viral contaminants, thin sectioning was required to locate foamy virus and the latent "tumor" viruses. A rapid method for the preparation of thin sections is described.