Pathogenic ecological characteristics of PCV2 in large-scale pig farms in China affected by African swine fever in the surroundings from 2018 to 2021

Abstract

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 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 both 100%. Through the systematic and clinical sampling, 15,130 samples collected from 30 large-scale pig farms in eight provinces in China (including Hubei, Hunan, Henan, Jiangxi, Shanxi, Guangdong, Hainan, and Heilongjiang) were subjected to early warning surveillance and/or clinical diagnosis. These results revealed that the overall positive rates of PCV3 and PCV2 were 0 and 28.29%, respectively, with the lowest level recorded in Jiangxi province. The highest carrying rate was observed in Hainan province. Pigs at different ages displayed varying carrying rates for PCV: fattening pigs and gilts had the highest and the lowest carrying rates for PCV, respectively. In addition, the excretion rates for PCV of colostrum, semen, and nasal, anal, and vulval swabs were tested. The colostrum, anal swabs, and semen had higher excretion rates for PCV; these were followed by the vulval and nasal swabs that had excretion rates for PCV. 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 to reveal the molecular epidemic dynamics of PCV2. The results indicated four major branches, namely, PCV2a, PCV2b, PCV2c, and PCV2d, concerning PCV2 molecular epidemiology in China, with PCV2a, PCV2b, and PCV2d dominating. In conclusion, the results obtained in this study elucidated the molecular epidemiology, transmission, and positive blood samples of PCV and provided new ideas for developing comprehensive PCV control technologies to begin eliminating the disease caused by PCV by cleaning pig farms.

Keywords: PCV2; PCV3; dual nested PCR; excretion rate for PCV; molecular epidemiology; positive rate of blood samples.

Figure 1

Geographical distribution of sample collection from 30 pig farms in eight provinces in China. The numbers show the distribution of pigs on the farms within the provinces in China.

Figure 2

Sampling scheme diagram. (A) Clinically healthy pigs. (B) Piglets at different ages.

Figure 3

PCV2 infection in pigs at different ages in eight provinces. The PCV2 infection in pigs at different ages indicated that PCV2 was able to infect pigs of all ages. The positive rate of PCV2 in finishing pigs (65.01%) was the highest, followed by nursing pigs (58.26%) and lactating pigs (12.05%). The positive rates of economic sows, gilts, and primiparous sows were 10.42, 6.87, and 7.91%, respectively. The test results for PCV2 infection in pigs at different ages in the eight provinces were analyzed using the SPSS 24.0 chi-square test. p < 0.05 was considered significant.

Figure 4

Overall status of porcine circovirus type 2 excretion. (A) Overall elimination of PCV2 in different organs of pigs in eight provinces. PCV2 is detoxified through colostrum, semen, nose, anus, and vulva and still showed a high positive rate following immunization with the ring vaccine. The positive rates of colostrum, semen, and anal swabs were similar, 26.65, 22.71, and 25.30%, respectively. Nasal swabs and vulvar swabs were recorded as 12.81 and 15.47%, respectively. (B) Different excretion pathways of pigs in the eight provinces. Among them, the positive range of colostrum was 7.35 to 56.69%, and the positive rate of colostrum from Jiangxi province was the lowest at 7.35%. The positive colostrum rate in Hainan province was the highest at 56.69%. The lowest semen-positive rate was 0 in Henan province, while the highest was observed in Guangdong province at 46.35%. The detection rates in the anus, nose, and vulva were 9.26 to 47.79%, 7.74 to 22.02%, and 2.67 to 36.84%, and the lowest detection rates were in the Henan, Shanxi, and Jiangxi provinces. The provinces with the highest detection rates were Guangdong, Hainan, and Hunan.