Porcine circoviruses, including porcine circovirus 2 and porcine circovirus 3, have been associated with clinical syndromes in swine, resulting in significant economic losses. To better understand the epidemiology and clinical relevance of PCV2 and PCV3, the Swine Health Information Center helped fund a study to analyze diagnostic data collected between 2002 – 2023 for PCV2 and PCV3 from six US veterinary diagnostic laboratories. The research, led by Drs. Giovani Trevisan and Daniel Linhares at Iowa State University, aimed to evaluate the macroepidemiological trends of aggregated PCV2 and PCV3 PCR data over time, establish the real-time capacity to rapidly identify changes in PCV2 and PCV3 detection patterns, and investigate the association between PCV-positive PCR cycle threshold (Ct) values and confirmed PCV disease diagnosis in tissues.
Guilherme Cezar, the graduate student working with the team, reported a decrease in the percentage of PCV2-positive submissions after introducing a commercial PCV2 vaccine in 2006 and a resurgence in positivity after 2018. The 2018 resurgence was primarily in breeding herds associated with an increased number of processing fluid sample submissions. PCV3 detection was more frequent in adult/sow farms, while PCV2 was more frequently detected in the wean-to-market category. An interpretative Ct cutoff of 22.4 for PCV2 was associated with a high probability of confirming a PCV2 disease diagnosis through histopathology. For PCV3, the interpretative Ct cutoff with the highest performance was 26.7.
See the published study with references here. Visit the SDRS site here.
Porcine circovirus-associated disease (PCVAD) can cost producers an average of $3–4 per pig in economic losses, demonstrating the importance of monitoring and controlling these pathogens in swine farms. Due to the complexity of monitoring PCV clinical cases, there is a need to develop tools to reveal and monitor changing patterns of PCV2 and PCV3 detection in swine farms. This study used aggregated PCR cases reported by VDLs for PCV2 and PCV3 detection to unravel the megatrends of these viruses in the US over the last two decades.
PCV2 and PCV3 submissions from 2002 to 2023 were collated using distinct accession IDs. PCR results reported by the VDLs (positive, negative, suspect, or inconclusive) were used to establish the final case result in the database. For cases to be considered positive, at least one sample within the case was required to be PCR-positive. Alternatively, negative cases had to have negative PCR testing results across all samples. Suspect and inconclusive cases were reported according to each laboratory criteria.
A variable age category was created based on the farm type, age unit, and age variables provided by VDL submissions. If the farm type was provided, the age category was assigned based on this variable (e.g., suckling piglets, breeding herd, nursery, grow-finish, replacement, boar stud). When the farm type was not provided, the age category was established based on the age of the animals (0–21 days as suckling piglets; 22–63 days as nursery; 64–200 days as grow-finish; >200 days as adults). Then, the age categories were aggregated into phases: adult/sow farm (breeding herds, replacement, boar stud, suckling piglets, and adults) and wean-to-market (nursery and grow-finish). Submissions lacking information regarding farm type, age, or age unit were categorized as unknown.
The Ct values of the PCR results were aggregated using the positive samples within a laboratory submission to calculate the average, minimum, and maximum PCR-positive Ct values. For example, a submission with three positive samples for PCV2 with Ct values of 34, 33, and 23 had an average, minimum, and maximum Ct value assigned as 30, 23, and 34, respectively. The number of positive samples was assigned based on distinct sample IDs within a case. In cases where the sample ID had more than one result, such as retesting the same sample, the most recent reported result date was retained.
The final PCR database comprised 154,984 PCV2 cases from 2002 to 2023. The first PCV3 PCR data was recovered in 2016 and had a total of 49,975 cases tested up to December 2023. The generated and aggregated information was made publicly available in an online visualization platform at the SDRS website.Data analysis of PCR test results, sample type, and age group revealed several key findings. PCV2 cases averaged nearly 6,000 annually from 2002 to 2017, rising to over 10,800 annually from 2018 to 2023. While the percentage of PCV2 positive cases peaked in 2006 (75%), it dropped significantly by 2011 (27%) and then rose again, averaging around 41% from 2018 to 2023. PCV3 testing became more frequent after 2018, with positive case percentages averaging around 50% per season. The study also noted an increasing trend of concurrent PCV2 and PCV3 detection since the introduction of multiplex PCR testing, which also contributed to the increase in the frequency of PCV3 cases reported after 2018.
Sample types submitted for testing also changed over time. While tissue samples were initially the most common for PCV2, processing fluids became the most frequently submitted sample type for both PCV2 and PCV3 after 2018. The age category unknown decreased significantly over time due to improved data capture.
Further, tissue diagnostic data from one VDL between 2019 and 2023 was used to correlate PCR Ct values with confirmed PCV2 and PCV3 disease diagnoses. The analysis, which considered only cases with tissue evaluations by diagnosticians, aimed to establish interpretative Ct cutoffs. Using a logistic regression model, a Ct cutoff of 22.4 for PCV2 and 26.7 for PCV3 was associated with a high probability of confirming a disease diagnosis through histopathology. These cutoffs represent the point where the likelihood of disease, despite a positive PCR result, decreases significantly. Specifically, Ct values above these cutoffs suggest that while the virus may be present, it is less likely to be contributing to clinical disease.
This study unraveled the macroepidemiological aspects of PCV2 and PCV3 in the US
swine population. Additionally, a PCV2 monitoring tool based on the interpretative Ct cutoff
results was implemented into the SDRS project and added to monthly PDF reports for continuous updates. Monitoring average Ct values of PCV2 tissue cases in the US helps to alert the industry when the average Ct of submissions is below 22.4 and can aid producers and veterinarians in recognizing higher PCV2 activity in the field. The study also sheds light on PCV3 detection trends, contributing to further investigations regarding virus dynamics. Detection megatrends revealed under this work can be used to further guide research questions and design experimental or field-based trials to explore and clarify root causes of secular trends in PCV2 or PCV3 detection.
This information will help pork producers and their herd veterinarians understand these pathogens and aid in disease management decision making. This research provides valuable insights into the evolving epidemiology of PCV2 and PCV3 in US swine herds. The established Ct cutoffs offer a practical tool for interpreting PCR results and may aid in disease management decisions. The study also highlights the importance of ongoing surveillance and data analysis to understand the complex dynamics of these economically significant swine pathogens.
The Swine Health Information Center, launched in 2015 with Pork Checkoff funding, protects and enhances the health of the US swine herd by minimizing the impact of emerging disease threats through preparedness, coordinated communications, global disease monitoring, analysis of swine health data, and targeted research investments. As a conduit of information and research, SHIC encourages sharing of its publications and research. Forward, reprint, and quote SHIC material freely. For more information, visit http://www.swinehealth.org or contact Dr. Megan Niederwerder at [email protected] or Dr. Lisa Becton at [email protected].
Copyright 2025 | Swinehealth.org | Website by Heartland Marketing Group
