Pathogenic Ecological Characteristics of PCV2 in Large-Scale Pig Farms in China Affected by African Swine Fever from 2018-2021
Authors: Liu Guoping , Li Chunqi , Tong Ke , Li Tong , Tan Xu , Wang Yan , Yang Jie , Yang Xiaolin , Guo Liwei , Zeng Jianguo 
Published: July 2022

Porcine circovirus type 2 (PCV2) has been identified as the causal agent
of postweaning multisystemic wasting syndrome (PMWS), an economically
important multifactorial disease of the swine industry worldwide. This
research designed a dual nested polymerase chain reaction (PCR)
detection method to simultaneously monitor porcine circovirus type 2
(PCV2) and PCV3. The main applications for this protocol focused on
technical support for its use during early warning surveillance of PCV
and its application for the investigation of pathogenic ecology for
large-scale pig farms in some areas of China. Two pairs of primers were
designed based on the conserved regions of both PCV2 and PCV3 strain
genes included in GenBank. This resulted in a highly sensitive detection
method with good specificity, which was constructed by optimizing the
reaction conditions and testing for specificity, sensitivity, and
coincidence rate. Next, 15,130 systematic early warning and clinical
samples were assessed using the developed methods. The limit of
detection (LoD) of sensitivity for PCV2 and PCV3 was ten copies/mL for
both viruses. There was no cross-reaction with any other porcine
pathogens tested and no non-specific amplification. The coincidence and
repetition rates were 100%. Through the systematic and clinical
sampling, 15,130 samples from 30 large-scale pig farms in eight
provinces (Hubei, Hunan, Henan, Jiangxi, Shanxi, Guangdong, Hainan, and
Heilongjiang) were subjected to early warning surveillance and/or
clinical diagnosis. The results revealed that the overall positive rates
of PCV3 and PCV2 were 0% and 28.29%, respectively, with the lowest
level [8.31% (133/1,600)] recorded in Jiangxi province. The highest
carrying rate [42.36% (762/1,799)] was observed in Hainan province.
Pigs at different ages displayed varying carrying rates: fattening pigs
and reserve pigs had the highest and the lowest carrying rates at
65.01% (901/1,386) and 6.87% (98/1,426), respectively. In addition,
the detoxification rates of colostrum, semen, and nasal, anal, and
vulval swabs were tested. The colostrum, anal swabs, and semen had
higher toxicity rates of 26.65%, 25.30%, and 22.71%, respectively;
these were followed by the vulval and nasal swabs that had toxicity
rates of 15.47% and 12.81%, respectively. Furthermore, a high blood
virus-carrying rate was detected in moribund pigs, especially in pigs
with fever and red skin. As to the virus-carrying rate in the pig organs
received from clinical necropsy, the highest rate was found in placental
tissue, followed by the kidneys, and the virus also was detected in
lymphoid organs, liver, stomach, and intestines. The PCV2-positive
samples were sequenced and compared to reveal the molecular epidemic
dynamics of PCV2 and disclose the genetic backgrounds of these epidemic
strains in China. These results provided support for tracing the sources
of virus transmission in future outbreaks. The results showed that the
gene sequences of the 24 PCV2 strains tested showed 95.36 to 100%
homology among the detected strains and 95.46 to 99.98% among the
detected strains and the reference strains. Nine strains belonged to
PCV2a, seven belonged to PCV2b, two belonged to PCV2c, and six strains
belonged to PCV2d.