Virus titration by plaque assay. Confluent monolayers of VeroE6 cells were infected with serial tenfold dilutions of virus supernatants for 2 h at 37 °C. The inoculum was removed and replaced with serum-free MEM (Gibco, LifeTechnologies) containing 1.5% carboxymethylcellulose (Sigma-Aldrich). Four days post-infection, cells were fixed for 2 h at room temperature with formaldehyde directly added to the medium to a final concentration of 5%. Fixed cells were washed extensively with water before staining with H2O containing 1% crystalviolet and 10% ethanol for 30 min. After rinsing with water, the number of plaques was counted and virus titres were calculated.