Journal of Veterinary Diagnostic Investigation Vol. 18 Issue 5, 496-499
Copyright © 2006 by the American Association of Veterinary Laboratory Diagnosticians
Renal tubular necrosis and interstitial hemorrhage ("turkey-egg kidney") in a circovirus-infected yorkshire cross pig
Denise M. Imai1,
Jennifer Cornish,
Robert Nordhausen,
John Ellis and
N. James MacLachlan
Correspondence: 1Corresponding Author: Denise M Imai, Veterinary Medical Teaching Hospital, University of California, One Garrod Drive, Davis, CA 95616
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Abstract
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A juvenile Yorkshire cross pig with rapidly progressive acute renal failure was submitted for necropsy. There was marked edema and disseminated petechiation of both kidneys, producing the "turkey-egg" appearance that is characteristic of exotic diseases such as African and classical swine fever. Microscopic findings included renal tubular epithelial necrosis with extensive interstitial edema and hemorrhage; lymphoplasmacytic, eosinophilic, and histiocytic tubulointerstitial nephritis; and numerous botryoid intracytoplasmic inclusions within the renal tubular epithelium and interstitial macrophages. Porcine circovirus 2 (PCV2) was readily identified within these lesions by both PCV2-specific immunohistochemistical staining and electron microscopy. Tests for African and classical swine fever viruses, as well as bacterial cultures, were negative. The striking renal lesions in this pig were attributed to PCV2 infection and are distinct from those that are typical of other PCV2-associated diseases.
Key Words: Porcine circovirus 2 • renal tubular necrosis • turkey-egg kidney
Porcine circovirus 2 (PCV2), a member of the family Circoviridae and genus Circovirus,11 was initially described in 1996 as the cause of postweaning multi-systemic wasting syndrome (PMWS) in pigs.6,10 Porcine circovirus 2 has since been associated with several other disease syndromes including porcine dermatitis and nephropathy syndrome (PDNS), porcine respiratory disease complex, reproductive failure, granulomatous enteritis, exudative epidermitis, and necrotizing lymphadenitis.2,12 The PCV2 virion is nonenveloped with a genome of circular, negative-sense, single-stranded DNA that includes 6 open reading frames (ORF). Field isolates of PCV2 from disease outbreaks in several countries share a high degree of nucleotide similarity, and genetic determinants of pathogenesis and/or virulence have not yet been identified.5 However, the greatest variation occurs in ORF2, which encodes the major capsid protein of PCV2, and this variation could potentially be responsible for differences in tissue tropism and pathogenesis of individual isolates of PCV2.1,2
The diagnosis of PCV2-associated disease is based on the characteristic clinical signs and lesions, as well as demonstration of the virus within the tissues of affected pigs.1,2,7,12,14 Morbidity in all of the PCV2-associated syndromes is low but mortality is often high amongst affected young pigs (<6 months of age).2 Clinical manifestations of the various PCV2-induced syndromes vary; for example, PMWS is characterized by chronic wasting, whereas PDNS is characterized by acute renal failure with dermatitis. Although renal lesions can accompany both PMWS and PDNS, they are quite different in the 2 syndromes and distinct from the lesions described in this case.1,2,7,12,14
A young (8-month-old) Yorkshire cross pig that was kept as a backyard pet presented to the Veterinary Medical Teaching Hospital with a very brief history of obtundation and lethargy. On physical exam, the pig was thin, obtunded, and ataxic with pale mucous membranes. Over 4 days, results from hematology, serum chemistry, and urinalysis as well as ultrasonographic evaluation of the kidneys confirmed a diagnosis of acute and progressive renal failure. Despite aggressive antibiotic and fluid therapy, the pig died and was submitted for necropsy.
Necropsy examination was limited to the thoracic and abdominal viscera at the owner's request. Tissue samples were collected in 10% buffered formalin, routinely processed, and embedded in paraffin blocks. From these blocks, 4- to 6-µm thick sections were cut and stained with hematoxylin and eosin (HE). Kidney, spleen, and peritoneal and joint fluids were submitted for aerobic and anaerobic microbial culture, and feces were screened for Salmonella sp. by culture. Frozen kidney, spleen, and lymph node were submitted to the National Veterinary Diagnostic Laboratory/Foreign Animal Disease Diagnostic Laboratory (NVDLS/FADDL, Plum Island, NY) for virus-specific polymerase chain reaction (PCR) assays to detect African and classical (hog cholera) swine fever viruses. Additional sections of paraffin-embedded kidney were also evaluated by indirect immunohistochemical staining for PCV2 antigen. Tissue sections were reacted with rabbit anti-PCV2 polyclonal antiseruma followed by incubation with biotinylated goat anti-rabbit antibodyb. Antigen-binding was visualized using a Streptavidin-biotin complexc staining procedure, as previously described.13 Samples of formalin-fixed kidney were also examined by thin-section transmission electron microscopy (EM).
Both kidneys were uniformly enlarged, pale, and edematous, with prominent petechial hemorrhages disseminated throughout the cortex and medulla resulting in a striking "turkey-egg" appearance (Fig. 1). PCR analyses for African and classical swine fever viruses were both negative. Neither Salmonella sp. nor other pathogenic bacteria were isolated from tissues, peritoneal fluid, or feces. Histologic findings included marked tubular epithelial necrosis with severe interstitial edema; hemorrhage; patchy lymphocytic, plasmacytic, eosinophilic, and histiocytic tubulointerstitial nephritis; and numerous intracytoplasmic inclusions within tubular epithelial cells and interstitial macrophages (Fig. 2). The inclusions were round, homogenous and basophilic (from 2 to 25 µm in diameter) and often formed botryoid clusters (Fig. 2). Scattered small arterioles exhibited a chronic necrotizing vasculitis. Abundant PCV2 antigen was detected within the cytoplasm of tubular epithelial cells and interstitial macrophages by immunohistochemical staining (Fig. 3). Transmission EM confirmed that the inclusions contained paracrystalline arrays of small nonenveloped icosahedral viral particles of approximately 14 to 16 nm in diameter (Fig. 4). Virion arrays were also present within the nuclei of infected cells, although intranuclear inclusions were not identified in HE-stained sections. Additional lesions in the pig included diffuse serofibrinous polyserositis, ulcerative glossitis, bronchopneumonia, necrotizing vasculitis in the spleen, and chronic lymphoplasmacytic enteritis.
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Figure 1 Yorkshire-cross pig, kidney. Marked edema with multifocal cortical petechiation resulting in a "turkey-egg" appearance.
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Figure 2 Yorkshire-cross pig, kidney. Severe tubular epithelial necrosis with lymphoplasmacytic, eosinophilic, and histiocytic tubulointerstitial nephritis, edema, and hemorrhage. HE stain. Bar = 500 um. Inset. Yorkshire cross pig, kidney. Numerous botryoid intracytoplasmic inclusions within tubular epithelial cells. HE stain. Bar = 20 um.
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Figure 3 Yorkshire cross pig, kidney. PCV2-specific immunohistochemical staining shows abundant viral antigen within interstitial macrophages, tubular epithelial cells, and sloughed intraluminal cellular debris. Bar = 100 um.
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Figure 4 Thin-section transmission electron microscopy demonstrates a paracrystalline array of icosahedral viral particles within an intracytoplasmic inclusion. Bar = 100 nm.
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PCV2 infection was confirmed in this pig on the basis of the unique histologic appearance of the viral inclusions,2,7,8,12 the EM appearance of the viral particles,3,8 and the abundant viral antigen that was identified by immunohistochemical staining of sections of kidney. Furthermore, PCV2 infection was likely responsible for acute renal failure in the pig because of the coincident distribution of abundant PCV2 antigen within the lesions. African swine fever, classical swine fever, and salmonellosis were all excluded as potential causes of the remarkable "turkey-egg" appearance of the kidneys in this pig by virus-specific PCR assays and bacterial culture. Although cortical microhemorrhage occurs in pigs with PDNS, this case lacks the characteristic exudative glomerulonephritis that defines PDNS.2,12,14 Although this pig had gross lesions consistent with dermal infarction on the pinnae and the plantar aspects of both hocks, these are 0not the characteristic sites of dermal infarction (hind limbs, flanks, and perineum) that typically occur in PDNS.2,12,14 Furthermore, although PCV2 antigen and immune complexes are abundant in PDNS, intracytoplasmic inclusions are usually absent, whereas they were very prominent in this case. Similarly, the multinucleated giant cells that are characteristic of other PCV2-associated syndromes were not present in the tissues examined from this pig.2,7,12 Specifically, a granulomatous interstitial nephritis that results in a "white-spotted kidney," and the presence of multinucleated cells within lymphoid tissue, are features of some cases of PMWS.1,4,9,12
In summary, the renal lesions in this pig were not typical of either "classical" PWMS or PDNS and were characterized by widespread necrosis and PCV2 infection of renal tubular epithelium with accompanying interstitial edema and hemorrhage. Of importance, the striking "turkey-egg" appearance of the kidneys in this pig raised immediate concerns at necropsy that this was a case of either African or classical swine fever. These findings emphasize that PCV2 infection must be included in the differential diagnosis of these important exotic diseases.
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Acknowledgments
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The authors would like to acknowledge the excellent assistance provided by the staff of the Anatomic Pathology service and Histopathology Laboratory at the Veterinary Medical Teaching Hospital and the California Animal Health and Food Safety Diagnostic Laboratory, University of California, and Dr Pamela Hullinger for assistance in coordination of sample submission to the Foreign Animal Disease Diagnostic Laboratory, Plum Island.
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Sources and manufacturers
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From the Veterinary Medical Teaching Hospital (Imai, Cornish, MacLachlan), and California Animal Health and Food Safety Laboratory (Nordhausen),Schoolof Veterinary Medicine,University of California, CA95616, Department of Microbiology(Ellis),Western College of Veterinary Medicine, Saskatoon, Saskatchewan, Canada. 
a. Rabbit anti-PCV2 polyclonal antiserum, obtained from Iowa State University, Ames, IA.
b. Biotinylated goat anti-rabbit antibody, obtained from DakoCytomation, Carpinteria, CA.
c. Streptavidin-biotin complex staining procedure, obtained from DakoCytomation, Carpinteria, CA.
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