| Porcine reproductive and respiratory syndrome (PRRS) is a severe viral disease in pigs, causing great economic losses worldwide each year. The causative agent of the disease, PRRS virus (PRRSV), is a member of the family Arteriviridae. It has been reported that the unparalleled large-scale outbreaks of an originally unknown, but so-called "high fever" disease in China in 2006 with the essence of PRRS, which spread to more than 10 provinces (autonomous cities or regions) and affected over 2,000,000 pigs with about 400,000 fatal cases. Different from the typical PRRS, numerous adult sows were also infected by the "high fever" disease. This atypical PRRS pandemic was initially identified as a hog cholera-like disease manifesting neurological symptoms (e.g., shivering), high fever (40～42℃), erythematous blanching rash, etc.The aim of this work was to characterize the lesions and agents present in clinically affected pigs in hogpens of Hunan during an outbreak of atypical PRRS, and to establish the indirect immunofluorescence(IIF) techniques for detecting fatal PRRSV variants. Autopsies combined with immunological analyses clearly showed that multiple organs were infected by highly pathogenic PRRSVs with severe pathological changes observed. The lung showed interstitial pneumonia change, spleen infarct and bladder dilatation filled with reddish brown urine, obvious haemorrhagic spots in lymph node. IIF revealed that PRRSV were mostly present in the center of lymphatic nodule in spleen and lymph node, and present in cryptae tonsillares or around it. In a word , the fatal PRRSV variants could be make disoperation of monocaryon- macrophage system, so the power of resistance was decreasing which made more sensibility to catch other diseases. Measures must be taken to control the outblreak of Fatal PRRS.
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