Abstract:

Porcine circovirus type 2 (PCV2) is the primary pathogen causing porcine circovirus-associated diseases (PCVAD), which manifest in pigs as poor growth, respiratory difficulties, reproductive disorders, and other symptoms. These diseases have caused significant economic losses to the global pig industry. Currently, multiple subtypes of PCV are prevalent in China, resulting in tremendous economic losses to the pig industry. To identify the tissue sample of a pig suspected of dying from a novel PCV2 infection, we collected tissue samples from pigs suspected of PCV infection and isolated and cultured the virus. The virus was identified through PCR, immunofluorescence staining, and sequencing analysis. Six week old BALB/c mice were used to observe the pathological sections of different tissues after infection with the virus strain. The positive samples underwent whole-genome amplification and sequencing. Homology, phylogenetic trees, and recombination characteristics were analyzed with 28 PCV strains from different countries and regions in GenBank. The fifth-generation virus solution was used to infect healthy 14-week-old pigs through nasal drop administration and intramuscular injection into the neck. The pathogenicity of the virus strain was evaluated by observing and analyzing the pigs' body temperature, weight gain, viremia, and viral load in various tissues. The results showed that a PCV2d subtype strain, named SD02, was isolated in this experiment. The whole genome size of this strain was 1 767 bp (GenBank accession number OQ730503), of which the ORF2 size was 705 bp, encoding a total of 234 amino acids. Compared to other reported PCV2d strains, SD02 has an additional lysine at the carboxyl terminus of the Cap protein encoded by ORF2. The nucleotide homology of the entire gene sequence between SD02 strain and the reference strain ranges from 94.2% to 99.1%, with the highest homology of 99.1% with the 22625-33 strain (PCV2d). The mice infected with this strain showed obvious symptoms such as interstitial pneumonia and necrotizing hepatitis. After infection with this strain, it can cause symptoms such as emaciation and slow growth in pigs. Viremia lasts for more than 21 days and has a wide range of tissue tropism, with higher viral loads in various lymph nodes, which can cause immune suppression in the body. This study provides new references for PCV epidemiological research and vaccine development.

Key words: porcine circovirus type 2, isolation and identification, analysis of genetic variation, pathogenicity