Epidemiological Investigation Of Classical Swine Fever In Large Scale Pig Farms In Ningxiang Cities Of Hunan Province

The pig breeding level in China has been gradually improved in recent years,while pigs are always easy to be infected with various diseases in the process of pig breeding.Classical swine fever(CSF)is an infectious disease which causes serious epidemic in pig farms,and the disease(CSF)is caused by classical swine fever virus(CSFV)infection.The pathogenicity of CSFV strains of different genotypes to pigs varies greatly,and pigs infected with low virulence strains do show serious clinical symptoms,which is very easy to be ignored by farmers,but the pigs can carry virus for a long time,which is not conducive to the eradication of this disease,and the prevalence of the disease still causes huge economic losses to China’s pig industry.In order to investigate the current prevention and control of CSF in large-scale pig farms in Hunan Province and the genetic variation of CSFV epidemic strains,We collected2068 blood samples from some large-scale pig farms in Ningxiang city of Hunan Province from 2017 to 2019,and detected the CSFV-specific antibody in serum by ELISA technology to understand the immune effect of CSFV vaccine in pigs.At the same time,140 samples of suspected CSFV infected samples were collected from the investigated pig farms to detect the presence of CSFV nucleic acid in the samples by RT-PCR.Meanwhile,the partial sequences of E2 and NS5 B gene of the positive samples were amplified,sequenced and analyzed to understand the genetic variation of CSFV epidemic strains.Finally,we set the diagnosis and prevention and control of CSF in a standardized pig farm as the starting point,after the diagnosis of CSFV infection through clinical and laboratory diagnosis,the disease was gradually purified by taking active vaccination and other prevention and control measures,which effectively reduced the economic losses caused by CSFV infection.The specific research results are as follows:From March 2017 to March 2019,2068 pig serum samples were collected from 11large-scale pig farms in Ningxiang city,Hunan Province.The existence of CSFV antibody in serum was detected by ELISA to evaluate the immune effect of CSFV vaccine.The results showed that 1754 samples were positive for CSFV antibody,and the average positive rate was 84.82%;The positive rates of which in suckling piglets,nursing pigs,fattening pigs,backup sows,breeding sows and breeding boars were 76.69%,55.11%,71.91%,90.14%,94.07% and 86.54%,respectively.The positive rate of CSFV antibody in serum samples collected from 2017 to 2018 was lower than that in samples collected from 2018 to 2019,being 81.23% and 87.82%,respectively.The antibody positive rate of samples from different seasons was the highest in autumn(90.34%),followed by which in summer and winter,being 83.84% and 85.38%,respectively,but the lowest rate was observed in spring,being77.80%.Subsequently,we used RT-PCR method to detect the presence of CSFV nucleic acid in the tissue samples of sick pigs from different pig farms.A total of 140 tissue samples were sent for examination in the above pig farms.RT-PCR results showed that only 13 samples were CSFV positive,and the field positive rate,batch positive rate and sample positive rate were 54.45%,12.7% and 9.29%,respectively.The detection rates of samples in 2017,2018 and 2019 were 11.54%,12.24%,and 6.15%,respectively.In addition,The partial sequences of E2 gene and NS5 B gene of 8 samples were amplified and sequenced.The results showed that the sequences of E2 and NS5 B genes of 8 samples were 1401 bp and 483 bp in length,respectively.And their nucleotide(amino acid)sequence homology were 90.8%～99.7%(92.1%～99.6%)and 89.5%～99.6%(90.3%～100.0%),respectively.In addition,6 strains had the highest homology with CSFV strains belonging to the sub-genotype 2.1b,and the nucleotide sequence homology with NS5 B and E2 genes were 98.4%～99.6% and98.2%～99.2%,respectively;The other two strains have the highest homology with CSFV strains of the sub-genotype 2.1c,and the homology of corresponding gene sequences are98.8%～99.7% and 98.5%～99.3% respectively.Further genetic evolutionary tree analysis based on the E2 gene sequence of CSFV strain was basically consistent with the homology analysis.The results suggest that CSFV strains with two sub-genotypes are prevalent in some areas of Hunan Province.In early September 2018,a disease suspected with CSF occurred in a large-scale pig farm in Ningxiang city.The clinical signs were mainly characterized by fever,constipation or diarrhea with high mortality.After clinical and laboratory diagnosis,it was finally determined that it was caused by CSFV infection.In order to effectively control the epidemic and even purify the disease,we regularly immunized the pigs with CSFV vaccine,improved the feeding management level,and regularly monitor the CSFV antibody level and pathogen infection in pigs,and finally effectively control the epidemic of the disease.At the same time,there is no similar CSF infection in pigs.If the vaccine is used reasonably and the corresponding monitoring system is established,the CSF can be purified finally.