Plaque assay
To evaluate viral titers,the plaque assay on Madin–Darby canine kidney(MDCK)cells was performed as described previously. In brief,MDCK cells were grown in Dulbecco's modified Eagle's medium with 10% fetal bovine serum and seeded into six-well plates. Confluent monolayers were washed with phosphate buffered saline (PBS)and infected with serial 10-fold dilutions of viruses or cell culture supernatants.