Histological Lesions and Replication Sites of PCV3 in Naturally-Infected Pigs

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Naturally-Infected Pigs

A team of researchers led by Dr. Fabio Vannucci investigated the lesions associated with a natural infection by Porcine Circovirus 3. They focused their study on histological lesions as well as replication sites to get a better understanding of this virus’ pathology. The article is available in open access on the journal’s website.

 

Methods

Cases selected for this study originated from the University of Minnesota, Veterinary Diagnostic Laboratory cases submitted between 2016 and 2018. They were selected based on the reason for testing and a tissue homogenate PCR test positive for PCV3. Only cases that had at least three formalin-fixed sections of the following organs were selected: heart, spleen, lung, kidney, liver and/or lymph nodes. A total of 25 cases were included in the study.

Hematoxylin and eosin-stained tissues were evaluated for histopathological lesions by two independent blinded pathologists and in-situ hybridization was performed to detect PCV3 RNA.

Results

Ten cases were classified as highly positive for PCV3 (Ct ≤ 20, 40%), 12 cases were classified as moderately positive for PCV3 (20 < Ct ≤ 30, 48%), and three cases were classified as mildly positive for PCV3 (30 < Ct < 40, 12%) based on the real-time PCR results for PCV3 on the tissue homogenate samples.

In this study, almost all cases (20/21) showed positive and marked signals for in-situ hybridization in the heart tissue. This result was associated with histological lesions characterized as lymphoplasmacytic myocarditis.

Lymphoplasmacytic interstitial pneumonia was noted in a majority of the lung samples and PCV3 RNA was found in 18 out of the 21 samples.

Interestingly, lymphoid depletion, one of the most remarkable lesions associated with PCV2 infection, was not a frequent finding associated with PCV3 detection in the present study.

Histopathological lesions associated with mRNA in situ hybridization (ISH-RNA) positive signals for porcine circovirus type 3. (a) Lymph node (20×), apparently normal lymphoid follicle, HE. (b). Lymph node (20×), lymphoid follicle. Focal, marked PCV3-ISH-RNA positive signals.

Conclusions

  • Heart and lungs were the organs with the most histological lesions and in-situ hybridization positive test results.
  • Lymphoid depletion was not as commonly seen as with PCV2.
  • There seems to be a difference in tropism between PCV2 and PCV3.
  • Heart and lungs are great samples to take to investigate a potential PCV3 case but lymphoid organs are needed to differentiate with PCV2.

For a more extensive description of the histological lesions associated with PCV3 infection, read the article on the journal website.

Abstract

Porcine circovirus type 3 (PCV3) has been recently described as a potential cause of abortions and systemic vasculitis in pigs. Although the virus has been detected by real-time PCR in several porcine tissues from countries worldwide, PCV3-associated diseases have not been satisfactorily clarified. The objective of this study was to investigate the association between the presence of PCV3 mRNA detected by in situ hybridization (ISH) within histological lesions and PCV3 DNA detected by real-time PCR in naturally infected pigs. A total of 25 PCV3 PCR-positive cases were analyzed. Formalin-fixed tissues from these cases were evaluated for histologic lesions and for ISH-RNA positive signals for PCV3. The most frequent tissue type with histopathologic lesions was heart, 76.2%, with lymphoplasmacytic myocarditis and epicarditis as the most frequent lesions observed. Lymphoplasmacytic interstitial pneumonia was also a frequent finding, 47.6%. There were also lesions in kidney, liver, spleen and lymph nodes. PCV3-ISH-RNA positive signals were mostly observed in association with lymphoplasmacytic inflammatory infiltrate in various tissues, including arteries. Based on our results, the minimum set of specimens to be submitted for histopathology and mRNA in situ hybridization to confirm or exclude a diagnosis of PCV3 are heart, lung and lymphoid tissues (i.e., spleen and lymph nodes), especially for differential diagnosis related with PCV2-associated diseases.