In recent years, porcine circovirus (PCV) has caused significant economic losses to the swine industry worldwide. Among them, porcine circovirus types 2 (PCV2) and 3 (PCV3) are the two most common viruses in pig farms, each associated with a variety of pig disease symptoms.
To further explore its epidemiology and clinical characteristics, this study utilized diagnostic data from six veterinary diagnostic laboratories in the United States from 2002 to 2023, including information on PCR results, sample types, and pig age groups. The study analyzed the prevalence and clinical associations of porcine circovirus types 2 (PCV2) and 3 (PCV3).
The results showed that PCV2 prevalence decreased after the introduction of the vaccine in 2006 but rebounded after 2018, particularly in breeding farms. PCV3 prevalence has continued to increase since its first report in 2016, peaking in 2023 and becoming more common in adult pigs. Furthermore, analysis of tissue sample data established PCR thresholds for PCV2 (Ct ≤ 22.4) and PCV3 (Ct ≤ 26.7) diagnostics. These findings may help monitor the clinical significance of PCV test results in real time and identify potential disease threats.
Introduction
Porcine circovirus type 2 (PCV2) causes multisystemic wasting syndrome, while type 3 (PCV3) is associated with reproductive failure and dermatitis and nephropathy. The associated disease (PCVAD) costs approximately $3-4 per pig. Diagnosis cannot be based solely on PCR positivity; it requires a combination of lymphoid tissue lesions and clinical manifestations (such as weight loss), and viral load (Ct value) correlates with disease severity. This study systematically reveals the long-term prevalence trends of PCV2/3 in the United States for the first time and establishes a real-time monitoring framework based on big data.
Research Results
1. PCR Results of PCV2 Cases at Different Time Periods
As shown in Figure 1, the PCV2 positivity rate continued to rise between 2002 and 2006, reaching a peak of 75.28% in 2006. However, after the introduction of the vaccine in 2006, the positivity rate dropped sharply from 2007 to 2011, reaching 27.24% in 2011. It then rebounded slightly from 2012 to 2017, and then rose significantly again after 2018, reaching 41.39% in 2023.
To further explore its epidemiology and clinical characteristics, this study utilized diagnostic data from six veterinary diagnostic laboratories in the United States from 2002 to 2023, including information on PCR results, sample types, and pig age groups. The study analyzed the prevalence and clinical associations of porcine circovirus types 2 (PCV2) and 3 (PCV3).
The results showed that PCV2 prevalence decreased after the introduction of the vaccine in 2006 but rebounded after 2018, particularly in breeding farms. PCV3 prevalence has continued to increase since its first report in 2016, peaking in 2023 and becoming more common in adult pigs. Furthermore, analysis of tissue sample data established PCR thresholds for PCV2 (Ct ≤ 22.4) and PCV3 (Ct ≤ 26.7) diagnostics. These findings may help monitor the clinical significance of PCV test results in real time and identify potential disease threats.
Introduction
Porcine circovirus type 2 (PCV2) causes multisystemic wasting syndrome, while type 3 (PCV3) is associated with reproductive failure and dermatitis and nephropathy. The associated disease (PCVAD) costs approximately $3-4 per pig. Diagnosis cannot be based solely on PCR positivity; it requires a combination of lymphoid tissue lesions and clinical manifestations (such as weight loss), and viral load (Ct value) correlates with disease severity. This study systematically reveals the long-term prevalence trends of PCV2/3 in the United States for the first time and establishes a real-time monitoring framework based on big data.
Research Results
1. PCR Results of PCV2 Cases at Different Time Periods
As shown in Figure 1, the PCV2 positivity rate continued to rise between 2002 and 2006, reaching a peak of 75.28% in 2006. However, after the introduction of the vaccine in 2006, the positivity rate dropped sharply from 2007 to 2011, reaching 27.24% in 2011. It then rebounded slightly from 2012 to 2017, and then rose significantly again after 2018, reaching 41.39% in 2023.
Figure 2. Changes in PCV3 PCR Test Results Over Time, 2017-2023
(Green: Negative; Red: Positive; Black broken line: Positive rate;
Purple: Indeterminate; Yellow: Suspected)
Conclusion: PCV3 positivity has continued to rise since 2016, particularly in adult/sow farms.
3. PCR Test Results of PCV2 and PCV3 Co-infection Cases Over the Years
Figure 3 shows that since the introduction of PCV2/PCV3 multiplex PCR testing in veterinary diagnostic laboratories in 2018, the number of cases with co-infection has surged, from only 71 in 2017 to 8,817 in 2023 (accounting for nearly 80% of PCV2 tests that year). The co-positivity rate for PCV2 and PCV3 has also increased from 14.81% in the spring of 2018 to 33.51% in the spring of 2023.
Figure 3. Number of PCV2 and PCV3 Co-detection Cases and Positive Rates, 2017-2023
(Green: Negative; Red: Positive; Black broken line: Positive rate)
Conclusion: The introduction of multiplex PCR testing significantly increased the PCV2/PCV3 co-detection rate, with processed fluid samples accounting for the highest percentage.
4. Detection at Different Production Stages
Figure 4 shows that the PCV2 positive rate was higher in growing pig herds than in sow farms, while the opposite was true for PCV3. Furthermore, since the winter of 2019, the PCV3 positive rate in sow farms has consistently been higher than in growing pig herds.
Figure 4. Percentage of PCV2 and PCV3 Positive Cases in Finishing/Sow Farms and Wean-to-Market Pigs (2017-2023)
(A: Percentage of Positive Cases in Finishing/Sow Farms; B: Percentage of Positive Cases in Wean-to-Market Pigs)
Conclusion: PCV2 is more common in finishing pigs, while PCV3 is more prevalent in sows.
5. Sample Types Used for PCV2 (A) and PCV3 (B) Testing Over the Years
Figure 5 shows that tissues and organs (e.g., lungs, lymph nodes, spleen, etc.) are the most common sample types tested for PCV2/3. Furthermore, the proportion of processed fluid samples has increased significantly since 2018, accounting for 38% of all PCV2 testing in 2023.
Figure 5. Detection Proportions of PCV2 and PCV3 in Different Sample Types
Conclusion: Processed fluid samples are increasingly used for detecting porcine circovirus types 2 and 3 (PCV2/PCV3) and are particularly suitable for simultaneous detection of both viruses.
6. Correlation between PCR Ct Values and Disease Diagnosis
Logistic regression analysis revealed that the Ct cutoff for PCV2 PCR detection reached 81.63% accuracy and 81.15% sensitivity at a Ct cutoff of 22.4; for PCV3 PCR detection, the Ct cutoff of 26.7 corresponded to 70.69% accuracy and 70.99% sensitivity.
Figure 6. PCV2 and PCV3 Ct Values and Diagnostic Accuracy and Sensitivity
Conclusion: PCR Ct values can be used as a reference indicator for the clinical diagnosis of porcine circovirus types 2 and 3 (PCV2/PCV3). When the Ct value of a sample is below the cutoff, it is more likely to be associated with clinical disease.
Summary
Based on data analysis from six veterinary diagnostic laboratories in the United States, significant differences in PCV2/PCV3 detection results were observed across time periods, production stages, and sample types. PCV2 positivity rates decreased after vaccine promotion but have rebounded in recent years. PCV3 positivity rates have continued to rise since 2016, becoming particularly common in adult pigs and sows. The introduction of multiplex PCR assays has significantly increased the rate of PCV2/PCV3 co-detection, with the use of processed fluid samples being the most common. PCR Ct values can serve as a reference indicator for disease diagnosis, helping veterinarians and producers more effectively monitor and manage pig health.
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