Determination of focus-forming units (FFU)
The viruses were titrated 10-fold from 1:10 to 1: 1010 in MEM and 100 μl of each dilution was transferred to confluent Vero cells in 96 well tissue culture plates, followed by incubation for 24-48 hours at 37°C and 5%CO2.
Subsequently the supernatants were removed, and the infected cells were washed 3 times with PBS, before fixation by cold acetone (80%) in distilled water for 60 minutes at -20°C. After fixation, fluorescent focus-forming assays were performed by incubation of specific antibodies on the infected cells for 30 minutes at 37°C, followed by incubation with secondary fluorescein-conjugated antibodies for additionally 30 minutes at 37°C (table 1). End point titres (FFU) were set at the last dilution giving unequivocal fluorescence.