Using diagnostic data from veterinary diagnostic laboratories to unravel macroepidemiological aspects of porcine circoviruses 2 and 3 in the United States from 2002-2023

Abstract

Porcine circoviruses (PCVs), including porcine circovirus 2 (PCV2) and porcine circovirus 3 (PCV3), 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, this study analyzed a dataset comprising diagnostic data from six veterinary diagnostic laboratories (VDLs) in the United States of America. The data comprised of polymerase chain reaction (PCR) test results, sample type, and age group for PCV2 and PCV3 submissions from 2002-2023. Findings indicated a decrease in the percentage of PCV2-positive submissions after introducing a commercial PCV2 vaccine in 2006 and a resurgence in positivity after 2018, particularly in breeding herds, associated with an increased number of submissions using processing fluid samples. After its first report in the U.S. in 2016, PCV3 detection had an upward trend in the percentage of positive cases, peaking in spring 2023. PCV3 detection was more frequent in adult/sow farms, while PCV2 was more frequently detected in the wean-to-market category. An additional analysis used results from tissue diagnostic data from 2019-2023 from one VDL to associate PCR cycle threshold (Ct) values with the probability of confirming a PCV2 or PCV3 disease diagnosis confirmation. An interpretative PCR Ct cutoff for PCV2 and PCV3 diagnoses was assessed based on the logistic regression model associating Ct values with the presence of tissue lesions. The analysis considered only cases tested for PCV2 and PCV3 by PCR with tissue evaluations by diagnosticians. An interpretative Ct cutoff of 22.4 for PCV2 was associated with a high probability of confirming a diagnosis of PCV2 clinical disease through histopathology. For PCV3, the interpretative cutoff with the highest performance was 26.7. These findings contribute to the ongoing efforts to monitor and understand the clinical relevance of PCV2 and PCV3 PCR results, identifying potential disease challenges.

Fig 1. Results of PCV2 cases tested by PCR over time (2002–2023).

Each point (x-axis) represents a year. The bars represent the number of cases tested and color-coded by testing results where red was positive, green negative, yellow suspect, and purple inconclusive (Y-axis). The black line represents the percentage of positive cases (secondary Y-axis).

Fig 2. Results of PCV3 cases tested by PCR over time (2017–2023).

Each point (x-axis) represents a season (1-Wi: Winter; 2-Sp: Spring; 3-Su: Summer; 4-Fa: Fall) within a year. The bars represent the number of cases tested and color-coded by testing results where red was positive, green negative, yellow suspect, and purple inconclusive (Y-axis). The black line represents the percentage of positive submissions (secondary Y-axis).

Fig 3. Number of cases tested for both PCV2 and PCV3 and number of cases and percentage of positive cases for both analytes over time (2017–2023).

Each point (x-axis) represents a season (1-Wi: Winter; 2-Sp: Spring; 3-Su: Summer; 4-Fa: Fall) within a year. The green bars represent the total number of cases tested for PCV2 and PCV3, and the red bars represent the total positive cases for both PCV2 and PCV3 on the same case (Y-axis). The black line represents the percentage of positive submissions for both PCV2 and PCV3 on the same case (secondary Y-axis).

Fig 4. Percentage of positive cases tested for PCV2 and PCV3 in adult/sow farm and wean-to-market categories over time (2017–2023).

Each point represents a season (1-Wi: Winter; 2-Sp: Spring; 3-Su: Summer; 4-Fa: Fall) within a year (x-axis). A: Percentage of positive cases in Adult/Sow farm. B: Percentage of positive cases in wean-to-market.

Fig 5. Proportion of specimens tested for PCV2 and PCV3 by PCR over time.

A: Each bar represents a year, and each color is a specimen type tested for PCV2 PCR. B: Each bar represents a year, and each color is a specimen type tested for PCV3 PCR.

Fig 6. Optimal Ct cutoff curve for PCV2 and PCV3 cases with diagnosis confirmed on tissue evaluation.

A: Accuracy and recall of each average PCR Ct value of PCV2 cases with/without diagnosis confirmed on tissue evaluation. B: Accuracy and recall of each average PCR Ct value of PCV3 cases with/without diagnosis confirmed on tissue evaluation.