3.4 IFA and Electron microscopy observation
The presence of FPV in the cell culture fluids at the fifth passage level was confirmed by PCR assay using the FPV speculative primer pair. IFA results showed green fluorescence in infected CRFK cells probed with an anti-FPV monoclonal antibody (Figure 4A), in contrast, there was no fluorescence in normal CRFK cells (Figure 4B). The diameter of the virion was approximately 20-25 nm which was observed under the electron microscope and revealed no envelope, spherical shape, and the icosahedral structure, and these were consistent with the structural characteristics of panleukopenia virus (Figure 5).