2.5 Identification of PRV isolate
Physicochemical properties of NY isolate as a representative PRV strain were assayed as described previously (Binn et al., 1970). Briefly, the NY isolate was added into eight eppendorf tubes. Tubes 1-2 were treated by water bath at 56℃ for 60 min and chloroform at 4℃ for 30 min, respectively. Tubes 3-4 were treated by adjusting the culture medium pH to 3.0 and 11.0 with 0.1 M HCl/NaOH solutions, and after incubation at 37℃ for 1 h, and the pH values were then adjusted back to 7.0. Tube 5 was digested with trypsin at 37℃ water bath for 1.5 h, and then added 4 mL of the inactivated fetal bovine serum to terminate the reaction. Tube 6 was incubated with formaldehyde at 37℃ for 2 d. Tube 7 was treated under ultraviolet ray for 30 min. Tube 8 was used as a negative control. Viruses of the eight tubes were inoculated on ST monolayers respectively and TCID50 were determined. In addition, the 7th passage of NY isolate was stained with uranyl acetate and examined using a Hitachi TEM transmission electron microscope (Hitachi, Japan).