Siobhan O’Sullivan

Animal Health Laboratory, University of Guelph, Guelph, ON.

AHL Newsletter 2024;28(3):14.

Multiple weaner pigs from a small swine operation presented with neurologic clinical signs that included convulsions, hind limb ataxia, and dog sitting. On postmortem examination, the cranioventral lung lobes were dark red and firm. Histopathology confirmed a lymphocytic meningoencephalitis, as well as granulomatous and neutrophilic interstitial pneumonia. In the ileum, submucosal Peyer’s patches were notably depleted of lymphocytes (Fig. 1), and contained rare macrophages with intra-cytoplasmic, botryoid viral inclusion bodies (Fig. 2). Fresh lung from this submission was positive for porcine circovirus-2 (PCV-2) by PCR (cycle threshold [Ct] value 13.38), confirming PCV-2 as the cause of neurologic signs and mortality. 

PCV-2 associated disease (PCVAD) can produce a variety of clinical signs, gross lesions, and histologic lesions, and there can be obfuscating overlap with other viral infections such as porcine arterivirus (PRRSV). Grossly, lymphadenopathy and pneumonia are suggestive of PCV-2 infection, and some of the most characteristic histologic lesions of PCV-2 infection are granulomatous interstitial pneumonia, lymphocytic meningoencephalitis, granulomatous lymphadenitis, and lymphoid depletion, with diagnostic viral inclusion bodies in macrophages (Fig. 1). While PCV-2 is a common diagnosis for cases coming through the AHL, particularly as a contributor to the porcine respiratory disease complex, it is uncommon to see the inclusion bodies on histopathology, presumably as a consequence of the stage of infection. Pigs can be chronic carriers of PCV2, therefore detection of the virus by PCR, especially at a high Ct value, is not sufficient for a diagnosis of PCVAD; compatible lesions, with a low Ct value, or demonstration of the virus in tissue by immunohistochemistry, are the standard for confirming the diagnosis.

PCV-2 is also the causative agent of porcine multisystemic wasting syndrome (PMWS), porcine dermatitis and nephropathy syndrome (PDNS), and abortion. The virus is transmitted between pigs by contact with infectious secretions (respiratory, nasal, ocular etc.) as well as through urine and feces, and possibly through fomites (Fig. 2). Vaccination and biosecurity protocols are advisable for prevention and control.  AHL

Figure 1. Porcine ileal Peyer’s patch. Submucosal Peyer’s patches are indistinct due to

loss of cellularity. H&E stain.

Figure 2. Porcine ileal Peyer’s patch. Macrophages with intracytoplasmic botryoid viral

inclusion bodies (arrows). H&E stain.

References

1. Segales J. Porcine circovirus diseases [Internet]. The Merck Veterinary Manual. 2021.[cited 2024 August 07]. Available from: https://www.merckvetmanual.com/generalized-conditions/porcine-circovirus-diseases/porcine-circovirus-disease [1].

2. Caswell J, Williams K. Respiratory System. In: Jubb, Kennedy, and Palmer's Pathology of Domestic Animals, 6th ed. Maxie MG, ed. Elsevier. 2016;2:527-528.