Study On Pathology Of High Swine Fever And Its Pathogen Identification In Shandong Province In 2006

Porcine reproductive and respiratory syndrome(PRRS) is a highly infectious respiratory disease in pigs caused by porcine reproductive and respiratory syndrome virus(PRRSV) affecting swine of different stages, sexes and breeds. It is recognized as a serious swine disease and is characterized with either reproductive failure in pregnant sows,or respiratory tract distress particularly in sucking pigs. Initially outbreak and described in the USA in 1987, at present, it widely takes place in many swine-producing nations and regions in the world. The research of PRRS has become a hot topic nowadays.In 2006, an emerged highly pathogenic disease, characterized by high and continued fever high-mortality,occurred in the pigs in main swine industry areas of Shandong province. Different from the typical PRRS, numerous adult sows were also infected by the"high fever"disease. This atypical pandemic PRRS was initially identified as a hog cholera-like disease manifesting neurological symptoms(e.g. , shivering) , high fever , erythematous blanching rash, etc.To investigate the causative agent of the pandemic disease, 19 clinical suspected samples were collected from 15 pig farms of Shandong province where suffering the disease for virus isolation and epidemiological analysis.The pathogenic detection of classical swine fever virus (CSFV), PRRSV, porcine circovirus type 2 (PCV-2),pseudorabies virus(PRV) and toxoplasma in 19 clinical suspected samples were carried out by PCR or RT-PCR methods. The detection results revealed that the infection rate of PRRSV, CSFV, PCV-2, PRV and toxoplasma were 100%,10.5%,21.2% and 0 respectively . Subsequently,the most possible cause to the high fever disease was belived to be the virus infection, although different kinds of pathogens including .E.coli , Stretococcus , Staphloccus , Mycoplasma, etc were detected in the 73.7% samples. Immunohistochemistry experiments showed that some tissues of the pigs especially the lung and the tonsil were positive of the PRRSV. From the clinical suspected lung material, with the Marc-145 cell, a PRRSV strain was separated, named SD-14 strain. In order to research the characters of the strain, we passaged it in Marc-145 cell for many generations. The character of the CPE in Marc-145 cell was that the infected cells becomed rounding, gathering and shedding. While, there is no any CPE appearing when the virus was inoculated in Vero cells and blindly passaged for four generations.The result of the immunofluorescence assay (IFA) with the monocloned antibody of the PRRSV was positive. And, a typical PRRSV virion was observed in the picture taken by the Transmission Electron Microscope (TEM). The results of RT-PCR, IFA and immunoflurescent test showed that the separated virus was PRRSV and the PRRSV was the most important cause to the high fever disease occurred in the Shandong province since 2006.Based on the sequence of ATCC-2332 and RT-PCR, the ORF5 and Nsp2 fragments were amplified following the templet of the total RNA extract of the extract of the infected cells by SD-14. Sequence analysis revealed that the PRRSV strain of Nsp2 and ORF5 shared 98.3%--99.1% and 95.0%--99.8% nucleotide identitieds to JXA1,HEB1,HnNh1,GD, LN, SHH strains that isolated from other districts in 2006,while, compared with the North American genotype VR-2332,CH-1a,S1,HB-1,HB-2 strains, the homologies of Nsp2 and ORF5 gene were only 79.6%--80.5% and 87.2%--87.6% respectively.More importantly, we observed a unique molecular hallmark in the viral isolates, namely a discontinuous deletion of 30 amino acids in Nsp2. In conclusion, the emerged fatlal PRRSV featured by deletions in Nsp2 is possibly the causative agent of the new wave of PRRS in Shandong province since 2006.