JPC SYSTEMIC PATHOLOGY
HEMOLYMPHATICS SYSTEMS
April 2021
H-V11

Signalment (JPC #10893-97):  Pig

HISTORY:  None

HISTOPATHOLOGIC DESCRIPTION:  Lymph node: Approximately 80% of the cortex and medulla is replaced by multifocal to coalescing, densely cellular aggregates of epithelioid macrophages and fewer multinucleate giant cells admixed with moderate numbers of eosinophils.  The macrophages and multinucleate giant cells have abundant eosinophilic cytoplasm, and the multinucleate giant cells have up to 20 nuclei that are located either along the cell periphery (Langhans type) or within the center (foreign body type). Occasionally macrophages and multinucleate giant cells contain clusters of 4 - 8 µm diameter, eosinophilic to basophilic, globular, intracytoplasmic viral inclusions (botryoid cytoplasmic inclusion bodies). The remaining lymph node parenchyma is characterized by a paucity of lymphoid follicles and a diffuse decrease in lymphocyte density (lymphoid depletion). 

MORPHOLOGIC DIAGNOSIS:  Lymph node:  Lymphadenitis, granulomatous, multifocal to coalescing, marked, with lymphoid depletion and intrahistiocytic intracytoplasmic eosinophilic botryoid viral inclusion bodies, breed unspecified, porcine.

ETIOLOGIC DIAGNOSIS:  Porcine circoviral lymphadenitis

CAUSE:  Porcine circovirus type 2 (PCV2)

CONDITION:  Postweaning multisystemic wasting syndrome

GENERAL DISCUSSION:

• Porcine circovirus is a small, non-enveloped, single-stranded DNA virus in the Circoviridae family
• PCV2 has been linked to several disorders (Porcine circovirus-associated disease, PCVAD):
  • Postweaning multisystemic wasting syndrome (PMWS)
  • PCV2-associated respiratory disease (aka PCV2 lung disease, proliferative and necrotizing pneumonia)
  • PCV2-associated enteritis (aka PCV2 enteric disease)
  • PCV2-associated reproductive failure (aka PCV2 reproductive disease)
  • Porcine dermatitis and nephropathy syndrome (PDNS)
• Lymphoid depletion is one of the hallmarks of PMWS
  • Affects B-cells, T-cells and NK cells
• Necrotizing lymphadenitis has been found in at least one lymph node in 2-10% of pigs with naturally-occurring PCV2 systemic disease (PMWS)
• In one study, Landrace pigs were more susceptible to PMWS than Durocs or large white pigs
• Severe lesions and death, typical of PMWS, are seen in pigs coinfected with other viruses such as porcine parvovirus (PPV), porcine reproductive and respiratory syndrome virus (PRRSV), pseudorabies virus, or bacteria (Mycoplasma hyopneumoniae) 
  • Potentiation of PPV in PCV2-induced PMWS is associated with excessive production of TNF-alpha
• Mortality may be up to 40% in weanling pigs

PATHOGENESIS:

• Site of replication of virus and exact pathogenesis of lymphoid depletion is still not fully understood
• Activation of the immune system followed by circovirus infection is key in the development of PMWS
• Cell recognition, attachment and entry: PCV2 binds to heparin sulfate and chondroitin sulfate; also enters cells lacking these, so suspect another receptor is also involved
• PCV2 nucleic acids and replication are found in the bone marrow and thymus; but there was is limited evidence supporting that primary lymphoid organ cells are major supporters of PCV2 replication
• Replication also occurs in pulmonary and renal epithelial cells and in endothelial cells
• Immune system dysfunction important in pathogenesis of disease
  • Possible contributing factors: Reduction in MHC-II on antigen-presenting cells, lymphoid depletion
• Proposed mechanisms for lymphoid depletion include: Apoptosis, viral-induced lysis, destruction of lymphocyte precursors in bone marrow, reduced lymphocyte proliferation in secondary lymphoid tissue, and thrombosis (secondary to hypertrophy and hyperplasia of high endothelial venules and overexpression of vWF)
• Acute vascular lesions/endothelial cell damage is caused directly by PCV2; chronic vasculitis is immune mediated (possible type III hypersensitivity reaction)
• Virus is spread via monocyte- macrophage cell lines which contain but do not effectively degrade the PCV2 virus
• Cell mediated proliferative response (granulomatous response) follows lymphoid depletion

TYPICAL CLINICAL FINDINGS:

• Most infections with PCV2 are subclinical
• Wasting or unthriftiness, dyspnea, visibly enlarged lymph nodes, and, less frequently, pallor, diarrhea, and jaundice
• Pigs 5 to 18 weeks old are most commonly affected

TYPICAL GROSS FINDINGS:

• Lymph nodes: Marked lymphadenopathy, especially involving inguinal, mesenteric, bronchial, and mediastinal lymph nodes
• Lung: Failure to collapse and increased firmness to diffuse mottling with areas of consolidation in cranioventral areas
• Liver: Mild icterus to marked atrophy
• Kidney: Enlarged, tan, waxy kidneys with cortical petechiae or white spots
• Gastrointestinal tract: Catarrhal enteritis; inconsistent findings of pallor, edema, and nonhemorrhagic ulceration of the pars esophagea of the stomach and fluid-filled, thin-walled intestines, especially the ileum and spiral colon
• Reproductive: Sows infected late in gestation produce mummified, stillborn, or weak piglets
• Skin: Multifocal to coalescing dermal and epidermal ulcers with black centers
• Spleen: Occasional splenic infarcts
• Heart: Myocarditis in newborn piglets and fetuses
• Brain: Cerebellar hemorrhage
• Muscle: Granulomatous necrotizing myositis

TYPICAL LIGHT MICROSCOPIC FINDINGS:

• Lymph nodes: Lymphoid depletion with expansion of paracortical zones by infiltration of large histiocytic cells and multinucleated giant cells; clusters of variably sized, intensely basophilic OR eosinophilic, intracytoplasmic botryoid viral inclusion bodies within histiocytes
• Lung: Multifocal to diffuse granulomatous bronchointerstitial pneumonia, with flooding of alveoli with macrophages and occasionally peribronchiolar fibrosis, occasionally proliferative and necrotizing pneumonia or necrotizing bronchiolitis
• Liver: Lymphohistiocytic hepatitis, apoptosis of hepatocytes, periacinar necrosis, progressing to widespread hepatocellular loss
• Kidney: Exudative glomerulonephritis and lymphohistiocytic interstitial nephritis and vasculitis with hyaline droplet formation in the glomerular mesangium; often with profound crescent formation (Cianciolo, Wednesday Slide Conference 2020-2021 Conference15, personal communication)
• Gastrointestinal tract: Granulomatous enteritis, lymphocytolysis, follicular atrophy in gut-associated lymphoid tissue, botryoid viral inclusions in macrophages
• Muscle: Granulomatous necrotizing myositis
• Central nervous system: Cerebellar lymphohistiocytic vasculitis with neuronal degeneration and necrosis and laminar spongiosis
• Segmental or circumferential fibrinoid vasculitis in various organs (lymph node, spleen, and choroid plexus

ULTRASTRUCTURAL FINDINGS:

• Cytoplasmic inclusions are dense, non-membrane-bound paracrystalline arrays of tightly packed icosahedral, 14-17 nm diameter, nonenveloped virus particles; they are most numerous in the perinuclear cytoplasm
• Intranuclear inclusions (less commonly visible) are non-membrane-bound and are often associated with reticulated nucleoli or aggregates of heterochromatin; some are irregularly shaped aggregates of indistinct, circular 10-12 nanometer diameter virus-like particles; others are 0.1-1.0 micron diameter, round to ring shaped, dense, and finely granular with sharply demarcated margins
• PCV2 is a DNA virus and thus replicates within the nucleus; the presence of intracytoplasmic viral inclusions is thought to be the result of phagocytosis of PCV2-infected cells

ADDITIONAL DIAGNOSTIC TESTS:

• Immunocytochemistry and in situ hybridization assays: Used commonly in diagnosis of PCVAD, but must have concurrent clinical and histological evidence
• PCR and rtPCR tests are available for PCV-2 and recently reported PCV-3, but correlation with lesions is critical since subclinical infection is common
• Definitive diagnosis is made by detection of viral antigen and/or nucleic acid associated with lesions in diseased pigs
• Recombinase polymerase amplification (RPA) is a recently reported methodology with a high rate of agreement with conventional PCR and rtPCR (Wang, J Vet Diagn Invest. 2016)

DIFFERENTIAL DIAGNOSIS:

Histologic lesion of lymphoid depletion in lymph nodes:

• Porcine reproductive and respiratory syndrome virus (porcine arterivirus)
• African swine fever virus (asfarvirus) results in severe lymphoid necrosis
• Classical swine fever (porcine pestivirus) – lack of mature lymphoid tissue; necrosis is less common

COMPARATIVE PATHOLOGY:

Other important circoviruses:

• Chick Anemia Virus Disease: Acute, immunosuppressive disease of young chickens characterized by anorexia, depression, anemia, and atrophy of lymphoid organs (Note: no ICIBs)
• Psittacine beak and feather disease (I-V04): Feather loss, abnormal feathers, and beak abnormalities in psittacines; basophilic intracytoplasmic and rare intranuclear inclusions in follicular epithelium, feather pulpitis, follicular epidermal basal and intermediate layer necrosis
• Pigeon circovirus, canary circovirus, gull circovirus, goose circovirus: Feather dystrophy, lymphoid depletion in bursa of Fabricius and clusters of basophilic circoviral cytoplasmic inclusions present in macrophages of lymphoid organs
• Canine circovirus has been reported in association with enteric disease in dogs (Van Kruiningen, J Vet Diagn Invest. 2019; Thaiwong, Vet Pathol. 2016)
• Porcine circovirus 3 has been identified and reported in association with lesions in some pigs, but the role of this virus in disease is still under investigation

REFERENCES:

1. Brown DL, Van Wettere AJ, Cullen JM. Hepatobiliary system and exocrine pancreas. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 6^th St. Louis, MO: Elsevier; 2017: 441.
2. Cantile C, Youssef S. Nervous system. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 1. 6^th St. Louis, MO: Elsevier; 2016:382-383.
3. Caswell JL, Williams KJ. Respiratory system. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 2. 6^th St. Louis, MO: Elsevier; 2016:527-529.
4. Cianciolo RE, Mohr FC. Urinary system. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 2. 6^th St. Louis, MO: Elsevier; 2016:412-413.
5. Foster RA. Female Reproductive System and Mamma. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 6^th St. Louis, MO: Elsevier; 2017: 1188.
6. Franzo G, et. al. Development and validation of direct PCR and quantitative PCR assays for the rapid, sensitive, and economical detection of porcine circovirus 3. J Vet Diagn Invest. 2018; 30(4):538-544.
7. Konradt G, Cruz RA, Bassuino DM, et al. Granulomatous necrotizing myositis in swine affected by porcine circovirus disease. Vet Pathol. 2018; 55(2):268-272.
8. López A, Shannon A. Martinson SA. Respiratory system, mediastinum, and pleurae. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 6^th St. Louis, MO: Elsevier; 2017: 541.
9. Schlafer DH, Foster RA. Female genital system. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 3. 6^th St. Louis, MO: Elsevier; 2016:440.
10. Thaiwong T, Wise AG, Maes RK, Mullaney T, Kiupel M. Canine circovirus 1 (CaCV-1) and canine parvovirus 2 (CPV-2): Recurrent dual infections in a Papillon breeding colony. Vet Pathol. 2016; 53(6):1204-1209.
11. Uzal FA, Plattner BL, Hostetter JM. Alimentary system In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 2. 6^th St. Louis, MO: Elsevier; 2016:116.
12. Valli VEO, Kiupel M, Bienzle D. Hematopoietic System In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 3. 6^th St. Louis, MO: Elsevier; 2016:210-212.
13. Van Kruiningen HJ, et. al. Canine circoviral hemorrhagic enteritis in a dog in Connecticut. J Vet Diagn Invest. 2019; 31(5):732-736.
14. Wang J, Wang J, Liu L, Li R, Yuan W. Rapid detection of porcine circovirus 2 by recombinase polymerase amplification. J Vet Diagn Invest 2016; 28(5):574-578.
15. Zachary JF. Mechanisms of microbial infections. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 6^th St. Louis, MO: Elsevier; 2017: 219-220.

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