Long periods of viraemia and persistent infection were proved to be one character of porcine reproductive and respiratory syndrome.Using reverse transcription -polymerase chain reaction,viral RNA was detected from serum at different post-infection days of SPF piglets infected only with PRRSV BJ-4 strain or infected 48 hours before hog cholera vaccination.Viral RNA could be detected as early as 24 hours after infection,persisted to 37 days post-infection, and diminished before 50 days post-infection. No effect of HCV vaccines on PRRSV infection was observed.The results of this study provide direct evidence of persistent PRRSV infection and explain field observations of long-term herd infection and transmission via purchase of clinically normal,but PRRSV infected,animals.Effective prevention and control strategies will need to be developed in the context of these results.%猪繁殖与呼吸综合征感染的特点之一是持续性的感染和病毒血症。本研究利用反转录聚合酶链式反应检测了PRRSV BJ-4株单独感染SPF仔猪和接种PRRSV BJ-4后再接种猪瘟疫苗不同时间的血清中病毒的存在。结果显示在感染24 h后的血清样品中就发现有病毒RNA存在，病毒血症至少持续到感染后37天，到第50天时已经消失。PRRSV BJ-4感染后再接种猪瘟疫苗的仔猪的PRRSV病毒血症没有受到影响。这些结果提供了PRRSV持续性感染的直接证据，解释了实际生产中通过引进临床正常但已经感染了猪繁殖与呼吸综合征病毒的猪群造成猪场内病毒的传播和长期感染的存在，为采用合理的措施控制疾病提供了依据。
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