Detection Of Classical Swine Fever Virus And The Vaccinated Efficacy In A Pig Farm Of Tai'an

Classical swine fever(CSF), caused by classical swine fever virus(CSFV), is a highly contagious and often fatal disease of swine. CSF has influenced swine production since the19 th century and caused great economic loses in the world. The use of C-strain vaccine has effectively reduced the epidemic range and the loses, and typical CSF has rarely occurred these years. While an epidemic disease characterized by fever, high morbidity and mortality in nursery pigs and temporarily reproductive failure in sows occurred in many swine farms since late 2013. Similar epidemic disease occurred in a swine farm in Tai'an city in November 2014, the nursery pigs were characterized by high fever(40-41?), depressed of spirit, inappetence, the inside of limbs and tip of ear were cyanosed. Routine treatments exhibited limited effects. To identify the infective pathogen, autopsy and histopathology were performed, the virus were detected by PCR and were isolated and identified. The complete E2 gene of CSFV were amplified, cloned, and sequenced. The autopsy and histopathology examination indicated that the silk pigs showed typical CSF-related lesions,including serous-hemorrhagic lymphadenitis, splenic infarction, viral encephalitis, intestinal lymphatic tissue necrosis and solid membrane enteritis and others. The lesions indicated the infection of CSFV, and bacterial infection was excluded. The PCR detection and virus isolation revealed that the detected specimen were only infected by CSFV, and only CSFV(named SDTA2015) were isolated in PK-15 cells. The complete E2 gene of the isolated CSFV strain were cloned and sequenced. The results revealed that the E2 gene of SDTA2015 was 1,119 nt in length and coded 373 amino acids. The similarity analysis of nucleotide and amino acid sequences revealed that SDTA2015 strain shared a nucleotide similarity of 81.3%-97.4% and an amino acid similarity of 87.7%-98.1% with other reference strains. When compared with 8 reference strains of various genotypes, SDTA2015 strain shared the lowest nucleotide and amino acid similarities of 81.8% and 88.7% with TWN strain(3.4 subgenotype). When compared with the subgenotype 2.1 strains,SDTA2015 strain shared higher nucleotide(90.4%-96.2%) and amino acid(93.8%-97.6%)similarities. The newly isolated strain shared the highest nucleotide(96.2%) and amino acid(97.6%) similarity with SDQS strain(subgenotype 2.1d). Moreover, the SDTA2015 sharedhigher similarities of nucleotide and amino acid with subgenotype 2.1b when compared with2.1a or 2.1c, indicating that the 2.1d strains were more similar with 2.1b strains, and these results were in accordance with previous reports. We further constructed a phylogenetic tree using SDTA2015 strain and other 27 reference strains. The results showed that the 28 sequences were divided into three genotypes(1, 2, and 3) including eight subgenotypes(1.1-1.4,2.1-2.3,3.4). Among the subgenotypes, subgenotype 2.1 can be further divided into four subgenotypes including 2.1a, 2.1b, 2.1c and a newly defined 2.1d. The SDTA2015 strain isolated in this study were clustered into subgenotype 2.1, and the strain showed great differences between the classical Shimen strain(subgenotype 1.1) or the attenuated C-strain virus(HCLV, subgenotype 1.1).Blood samples were collected when the pigs in this farm were all returned to health to evaluate the immune status and the immune effects. A total of 131 samples were collected and 106 samples were positive for anti-CSFV antibodies, resulting a positive rate of 80.92%.The antibodies level indicated the pigs of the farm were in good status of anti CSFV infection. The pigs of different production phases showed various positive rates, the rates varied from 60.00% to 86.67%. Among them, the levels of anti-CSFV antibodies of boars were highest, while the fattening pigs shared the lowest positive rate. These results indicated that boars were under great immune status, while the fattening pigs were in great risk to infect CSFV. Moreover, the positive rates of pregnant sows and nursery pigs were both lower that 85.00%, indicating that the anti-CSFV antibodies levels of the pigs of these two phases were not satisfied.In conclusion, the epidemic disease in this farm was caused by CSFV, and the virus were clustered into 2.1d subgenotype. The virulence and pathogenicity of SDTA2015 need to be further analyzed. Moreover, the antibodies results indicated that the pigs in this farm were all in good immune status since the elimination of the epidemic except for the fattening pigs. The fattening pigs should be performed booster immunization of CSFV vaccine in case of the occurrence of CSF cases.