Viral RNA was extracted from cultured viruses using a QIAamp Viral RNA Extraction Mini Kit (Qiagen, Valencia, CA) according to the manufacturer instructions. Viral RNA was detected by real-time reverse transcription-polymerase chain reaction (real-time RT-PCR) as previously described3 (link)13 (link). Briefly, a 185-bp amplicon was generated and detect by a TaqMan probe labeled with FAM and Blackhole Quencher. The amplification was carried out using an ABI 7500 real-time system (Life Technologies, NY) and the results analyzed with the instrument SDS software (version 2.0.1).