The procedure for inoculation of clinical samples for measles virus isolation on Vero/hSLAM cells is provided in Annex 5.2. The morphological changes to the Vero/hSLAM cells, or the cytopathic effect (CPE), produced by measles virus replication in cell culture is easily observed. The typical CPE consists of the formation of syncytia, which appear as large multinucleated cells (giant cells), caused by fusion of infected cells (plaques) See Figure 5, below. These plaques can be seen by the naked eye in the monolayer. Light refracts differently through the lysed cells in the plaques compared to the uninfected cells in the monolayer which allows the visualization of the plaques.
Under the microscope, individual syncytia may consist of 50 or more nuclei encapsulated within a single cytoplasmic membrane. Verification of the isolation of measles virus should be carried out by immunofluorescence, immunohistochemistry or RT-PCR (Chapter 6. Detection of viral RNA by RT-PCR for the confirmation of measles and rubella infection). Because the virus titre is often very high after passaging the measles virus-infected cells in cell culture, the manipulation of the cell lysate to confirm measles virus isolation should be conducted in a manner to avoid contamination of clinical specimens or other cell cultures.