Abstract: 【Objective】 This study was conducted to determine whether the morbid pigs in a pig farm in Guangdong were infected with Porcine circovirus type 2 (PCV2).It also evaluated the genetic characteristics of the virus and its prevalence in swine herds,so as to provide scientific data to support the prevention and control of epidemics.【Method】 Real-time quantitative PCR and ELISA were used to detect and analyze the visceral tissues such as kidneys,lymph nodes and spleens,as well as serum samples of sick pigs in the farm.This study further amplified and sequenced the ORF2 gene of PCV2-positive samples by PCR.Using the MegAlign module in the DNAStar software package,the resulting ORF2 gene sequences were comparatively analyzed for similarity with the 23 PCV2 reference strains in GenBank database.In addition,a genetic evolution tree based on the ORF2 gene sequences was constructed using Mega 7.0 software to clarify the genetic relationship between the identified and reference strains.【Result】 According to the epidemiological investigation,clinical symptoms and pathological autopsy results,the sick pigs in this farm were consistent with the characteristics of PCV2 infection.Real-time quantitative PCR test revealed that the Ct values of three samples were 14.42,15.13 and 16.08,respectively,indicating a high viral load and a positive PCV2 test result.ELISA results showed that the Cap protein antibody positivity rate ranged from 60% to 100% and that of Rep protein ranged from 0 to 100% in different age groups of pigs,especially in 150-day-old pigs,the antibody positivity rate of both proteins reached 100%,indicating a high intensity of PCV2 infection.Sequencing of the ORF2 gene yielded three sequences named LZC-2305-1,LZC-2305-2 and LZC-2305-3,respectively.Similarity analysis showed that these three sequences shared 99.9% to 100% similarity with each other and ranged from 82.3% to 99.7% similarity to the sequences of the 23 PCV2 reference strains in GenBank,with the highest similarity of 99.6% to 99.7% to CZ246 sequence.Genetic evolutionary tree analysis further confirmed that these sequences had the closest affinity to CZ246 sequences attributed to the 2d branch of PCV2.【Conclusion】 This study confirmed that the infection of PCV2d branch occurred in pigs of this farm.This study not only provided a scientific basis for the prevention and control of PCV2 in this farm,but also provided a reference for disease monitoring and control in other large-scale pig farms.

Key words: Porcine circovirus type 2 (PCV2); diagnosis; Real-time quantitative PCR; ELISA; genetic evolutionary analysis