For SNT, the sera was diluted in DMEM as two-fold in a 96well flat-bottomed tissue culture plates (Castor, USA). Virus suspension with a titer of 100 TCID 50 in 50 ul was added to each sera well and the mixture was incubated for 1 h at 37uC and 5% CO2. 50 ul of BHK-21 cell suspension (1.5610 6 ml 21 ) was added to each well and incubated for 4-5 days. Appropriate serum, virus and cell control were included in this test. The plates were observed via microscope for cytopathic effect.