Josepha DeLay, Davor Ojkic, Jim Fairles

Animal Health Laboratory, University of Guelph, Guelph, ON

AHL Newsletter 2021;25(2):14.

Porcine circovirus 3 (PCV3) was first identified in 2015 in association with reproductive loss and maternal systemic disease in a sow herd in the United States.  Systemic disease in growing pigs has also been linked to PCV3 infection.  Retrospective studies have confirmed that PCV3 has been present but undetected in swine herds for many years, and association with clinical disease is a recent event.  Direct disease causation by the virus remains to be proven, and Koch’s postulates have not yet been fulfilled, although ongoing research has provided evidence of disease in swine that is linked to presence of the virus.

In sow herds, PCV3 has been associated with an increase in mummified and stillborn fetuses, and with weak neonates.  Litters from lower parity sows are most commonly affected.  Poor growth in older pigs has also been associated with PCV3.  Histologic lesions in in PCV3 infected fetuses and pigs include lymphocytic myocarditis and myocardial fibrosis, and lymphocytic perivasculitis in various organs, especially in heart and kidney.  Using in situ hybridization (ISH) methods, PCV3 nucleic acid can be detected in tissues, co-localizing with histologic lesions.  Immunohistochemical (IHC) assays for PCV3 currently have very limited availability and are restricted to research use.

Polymerase chain reaction (PCR) testing for PCV3 began at the AHL in 2018.  The cycle threshold (Ct) value for PCR is important in interpreting the significance of the results and provides an indication of the viral load in the sample.  Subjectively, PCV3 Ct values of approximately 20 or less are considered significant and potentially associated with clinical disease, if the clinical scenario and histopathologic findings are compatible with PCV3-associated disease.  Importantly, a PCV3 Ct value of ≤20 is not a validated cutoff value; rather, this is a guide for interpreting the potential significance of the results.  Higher Ct values may be clinically significant in some situations, depending on microscopic lesions, clinical history, and the stage of infection.   Alternatively, higher PCV3 Ct values (representative of lower viral load) may not be directly associated with disease and may only indicate PCV3 presence in the herd.  Consideration of clinical and pathologic context is important in the interpretation of PCV3 PCR test results. 

Between December 2018 and May 2021, the AHL carried out PCV3 PCR tests on a total of 845 samples, representing 278 cases (Fig. 1).  PCV3 nucleic acid was detected in 261 samples (31%) and in 84 cases (30%).  PCV3 Ct values were ≤20 and considered likely clinically significant in 13 / 845 (1.5%) samples and 12 / 278 (4.3%) cases.  Significant individual PCV3 PCR Ct values ranged from 16.63 to 20.97, and represented 10 farms.  Although the overall proportion of cases in which PCV3 was detected was high (30%), the viral load was considered to be potentially clinically significant (low Ct) in relatively few cases (4.3%).

Useful samples for PCV3 PCR testing include pooled tissues (lung, heart, kidney) from fetuses or older pigs, fetal thoracic fluid, and processing or castration fluids. 

Among AHL pathology cases between 2018 and May 2021, correlation of PCR results with histologic lesions led to a diagnosis of PCV3-associated disease in the following clinical scenarios:

• reproductive loss due to increased stillborn and mummified fetuses and / or weak neonates (6 cases)
• neonates with a complaint of failure to thrive (3 cases)
• nursery (1 case) or  grow-finish pigs (2 cases) with poor growth and ill-thrift 

Significant gross lesions were not detected in any of the cases.  Demonstration of specific histologic lesions was important in making an association with PCV3 infection.  These lesions included lymphocytic myocarditis and myocardial interstitial fibrosis in fetuses and neonates (Fig. 2), and lymphocytic periarteritis in older pigs.  PCV3 ISH was pursued in 2 related cases, and PCV3 nucleic acid co-localized with myocardial lesions.  PCV3 PCR Ct values for at least 1 tissue sample in all cases were ≤20.  Importantly, in cases involving fetuses, PCV3 was not detected in tissue pools from all litters, illustrating the importance of testing fetuses from multiple litters in order to reach a diagnosis involving PCV3 or other viral pathogens.  The heterogeneity of PCV3 results also suggests that other pathogens may also be contributing to fetal loss in some cases.

Other lesions that have been described in association with PCV3 infection include non-suppurative encephalitis in neonates, and lesions compatible with porcine dermatopathy-nephropathy syndrome (PDNS).  These manifestations of PCV3-associated disease have not been identified to date in Ontario swine.

Although PCV3 belongs to the same virus family as PCV2, vaccines do not provide cross-protection between the 2 viruses, and tests for each virus are distinct and will not cross-react.  AHL

Figure 1.  PCV3 PCR results and distribution of crossing threshold (Ct) values for positive cases, 2018-2021.  Notably, PCR-positive cases with a low (subjectively significant) Ct value of ≤20 involve a small proportion of cases.

Figure 2. Fetal myocardium, stillbirth associated with PCV3: Cardiomyocyte loss and replacement by fibrosis and lymphocyte clusters.  Hematoxylin and eosin stain (10x).

References

1. Arruda B, et al. PCV3-asociated disease in the United States swine herd. Emerging Microbes and Infections. 2019;8:684-698.

2. Mora-Diaz J, et al. Isolation of PCV3 from perinatal and reproductive cases of PCV3-associated disease and in vivo characterization of PCV3 replication in CD/CD growing pigs. Viruses 2020;12:219.

3. Palinski R, et al. A novel porcine corcovirus distantly related to known circoviruses is associated with porcine dermatitis and nephropathy syndrome and reproductive failure. J Virol 2017;91:e01879-16.

4. Phan TG, et al. Detection of a novel circovirus PCV3 in pigs with cardiac and multi-systemic inflammation. Virology J 2016;13:184.

5. Saporiti V, et al. Porcine circovirus 3 detection in aborted fetuses and stillborn piglets from swine reproductive failure cases.  Viruses 2021;13:264.