International Harmonization of Nomenclature and Diagnostic Criteria (INHAND): Nonproliferative and Proliferative Lesions of the Rabbit.

Calcium absorption and metabolism in the rabbit is poorly understood. Rabbits normally have a higher blood calcium range than other laboratory animal species and are predisposed to cystic, urethral, ureteral and renal calculi. Rabbits excrete 45–60% excess calcium though the urine as calcium carbonate. Mineralized foci are commonly seen throughout the urinary tract. Urine is normally alkaline, and cloudy to pigmented, caused by the presence of albumin, fine calcium carbonate and ammonium magnesium phosphate crystals. The fact that rabbits are horizontal quadrupeds may also predispose the anterior wall to retention of microcrystals and other particles as compared to humans. Rabbit urine varies in color from creamy yellow to dark red depending on the presence of porphyrin pigments derived from the diet or xenobiotics e.g. antibiotics. Care must be taken to differentiate between red urine caused by porphyrin excretion and hematuria. While modern laboratory animal management practices have limited the incidence of infectious processes in the kidney, inflammatory conditions related to infectious disease may still occur ( Table 71 Table 71. Microscopic Findings of the Kidney: Rabbit Finding Common Uncommon Not Observed but Potentially Relevant Not Applicable Congenital Aplasia X Renal dysplasia X Non-proliferative Abscess X Accumulation, glycogen X Accumulation, hyaline droplets X Accumulation, adipocytes, interstitium X Amyloid, glomerulus/interstitium X Angiectasis X Apoptosis ǂ X Atrophy, glomerulus/tubule X Basophilia, tubule X Basophilic granules X Calculus X Cast (+ modifier) X Chronic progressive nephropathy X Crystals X Cyst * X Degeneration, tubule X Degeneration/regeneration X Dilatation, Bowman’s space/tubule X Edema, interstitium X Erosion X Extramedullary hematopoiesis X Fibrosis, interstitium X Glomerulonephritis X Glomerulopathy, hyaline/mesangioproliferative X Glomerulopathy * X Glomerulosclerosis X Granuloma, Foreign material X Hemorrhage X Hyperplasia/metaplasia, Bowman’s capsule X Hypertrophy, tubule X Immature glomerulus X Inclusion bodies X Infarct X Infiltrate, inflammatory cell [insert appropriate cell type] X Inflammation, interstitium X Interstitial nephritis X Karyomegaly X Mesangiolysis X Metaplasia, osseous/squamous cell X Microabscess X Mineralization * (+ locator) X Multinucleated giant cells X Necrosis, (+ modifier, locator) X Nephropathy, obstructive/retrograde X Nephropathy, alpha2u-globulin X Nephropathy, spontaneous * X Parasite X Pigment X Pyelonephritis X Regeneration, tubule X Single cell necrosis ǂ X Tissue, ectopic, adrenal X Vacuolation *, tubular X Proliferative Non-neoplastic Hyperplasia, oncocyte X Hyperplasia, mesangium X Hyperplasia, juxtaglomerular X Hyperplasia, tubule X Hyperplasia, urothelium X Nephroblastematosis X Proliferative Neoplastic Adenoma X Nephroblastoma X Oncocytoma X Papilloma X Renal mesenchymal tumor X Adenocarcinoma X Carcinoma X Carcinoma, squamous cell/urothelium X Sarcoma, renal X * Terminology with diagnostic criteria or comments described in the text. ǂ Refer to 4 for diagnostic criteria and use of the terms apoptosis and single cell necrosis. ). * Terminology with diagnostic criteria or comments described in the text. ǂ Refer to 4 for diagnostic criteria and use of the terms apoptosis and single cell necrosis. Comments: Renal lesions resembling human polycystic kidney disease were reported in a retrospective evaluation of NZW rabbit kidney tissue 403 . Comments: Rabbits are susceptible to a glomerulopathy induced by corticosteroids 404 , 405 , 406 . The initiating lesion is a glomerulopathy characterized by aneurysmal capillary dilatation with nodular changes of eosinophilia and cellular loss in the glomeruli. Bowman’s space often contains erythrocytes or eosinophilic material. Bowman’s capsule may be necrotic. Ultrastructurally, glomerular capillaries may be occluded with a proteinaceous coagulum, endothelium may be swollen, and podocytes have loss of foot processes. Basement membranes may be thickened or tortuous. Epithelial cells of the glomerular tufts may have hyaline globules, vacuoles or be intensely osmophilic. Renal tubules may have fatty infiltration, hyaline droplets, and cellular necrosis. Tubules contain erythrocytes and protein casts. Clinical pathology parameters may demonstrate increased BUN, glycosuria, albuminuria, and hematuria with clinically red urine. Kidney weights may be increased. It is important to recognize the susceptibility of rabbits to corticosteroid glomerulopathy as it may be confused with a test article-related effect if animals are given corticosteroids for palliative purposes during a toxicology study. ( Figure 55 Figure 55. Kidney, Renal mineralisation, H&E. ) Kidney, Renal mineralisation, H&E. Comments: Mineralized foci are commonly seen in the collecting ducts and medulla of rabbit kidneys. Mineralized foci in the tubules or interstitial areas of the cortex are present in 60% of male and female rabbits evaluated 81 , 407 . Focal mineralization is also occasionally recorded in the urothelium of the urinary bladder. Pathogenesis/cell of origin: Proximal and distal tubules Diagnostic Features: The histological findings are generally recorded separately (basophilia tubules, dilated/cystic tubules, pigmented tubules, interstitial inflammatory cell infiltrate), and the term nephropathy is only used when at least three of the aforementioned components are present. A spontaneous nephropathy syndrome is commonly seen in clinically normal apparently healthy rabbits from colonies free from Encephalitozoon cuniculi . There are no clinical signs accompanying these lesions and no evidence of progression/greater severity of the findings on longer term studies i.e. the lesion is not thought to be progressive unlike the nephropathy in rats. It is considered to be a syndrome particular to the NZW rabbit as similar findings have not been recorded in mixed breed pet rabbits. The renal findings are observed in 80% of apparently healthy young rabbits, more frequently in females than males 81 . Spontaneous findings of mineralization, tubular basophilia and dilatation have been reported previously in young laboratory rabbits less than 1 year of age 408 , 409 , but the incidence of this lesion seems to be increasing in the NZW population. Basophilic tubules, dilated/cystic tubules, pigmented tubules, interstitial inflammatory cell infiltration, and mineralization have been reported in juvenile NZW rabbits as little as 8 weeks old (Bradley, unpublished data). Study pathologists should exercise caution when interpreting kidney findings in apparently healthy rabbits as these renal nephropathy findings may mask nephrotoxic effects. ( Figure 56 Figure 56. Kidney, Nephroblastoma, H&E. ) Kidney, Nephroblastoma, H&E. Comments: Vacuolation of the proximal convoluted tubule epithelium is a regular finding in young non-pregnant females (35%). These vacuoles stain positively with Oil red O stain for neutral lipids. Vacuolation of the proximal convoluted tubule epithelium has been recorded in juvenile NZW rabbits as little as 8 weeks old (Bradley, unpublished data) ( Table 72, 73, 74 Table 72. Microscopic Findings of Ureter: Rabbit Finding Common Uncommon Not Observed but Potentially Relevant Not Applicable Congenital Aplasia, ureter X Non-proliferative Abscess X Apoptosis ǂ X Calculus X Crystals X Dilatation X Edema X Erosion/ulcer X Fibrosis X Hemorrhage X Infiltrate, inflammatory cell [insert appropriate cell type] X Inflammation X Single cell necrosis ǂ X Proliferative Non-neoplastic Hyperplasia, urothelium X Metaplasia, glandular/squamous cell X Proliferative Neoplastic Papilloma, squamous cell/urothelial X Adenocarcinoma X Carcinoma, squamous cell/urothelial X ǂ Refer to 4 for diagnostic criteria and use of the terms apoptosis and single cell necrosis. Table 73. Microscopic Findings of the Urinary Bladder: Rabbit Finding Common Uncommon Not Observed but Potentially Relevant Not Applicable Congenital Diverticulum X Non-proliferative Abscess X Angiectasis X Apoptosis ǂ X Calculus X Crystals X Dilatation X Edema X Erosion X Fibrosis X Hemorrhage X Hypertrophy, urothelium X Inclusions, urothelium X Infiltrate, inflammatory cell [insert appropriate cell type] X Inflammation X Metaplasia X Mineralization, urothelium X Necrosis X Parasite X Proteinaceous plug X Single cell necrosis ǂ X Ulcer X Uropathy, obstructive X Vacuolation, urothelium X Proliferative Non-neoplastic Hyperplasia, urothelium X Metaplasia, glandular/squamous cell X Proliferative Neoplastic Proliferative lesion, mesenchymal X Papilloma, squamous cell/urothelial X Adenocarcinoma X Carcinoma, squamous cell/urothelial X ǂ Refer to 4 for diagnostic criteria and use of the terms apoptosis and single cell necrosis. Table 74. Microscopic Findings of the Urethra: Rabbit Finding Common Uncommon Not Observed but Potentially Relevant Not Applicable Non-proliferative Abscess X Apoptosis ǂ X Congestion X Edema X Erosion X Fibrosis X Hemorrhage X Hypertrophy, urothelium X Infiltrate, inflammatory cell [insert appropriate cell type] X Inflammation X Metaplasia X Obstruction X Plug, proteinaceous X Single cell necrosis ǂ X Proliferative Non-neoplastic Hyperplasia, urothelium X Metaplasia, glandular/squamous cell X Proliferative Neoplastic Proliferative lesion, mesenchymal X Papilloma, squamous cell/urothelial X Adenocarcinoma X Carcinoma, squamous cell/urothelial X ǂ Refer to 4 for diagnostic criteria and use of the terms apoptosis and single cell necrosis. ). ǂ Refer to 4 for diagnostic criteria and use of the terms apoptosis and single cell necrosis. ǂ Refer to 4 for diagnostic criteria and use of the terms apoptosis and single cell necrosis. ǂ Refer to 4 for diagnostic criteria and use of the terms apoptosis and single cell necrosis. Ethical Practices: All procedures used to prepare macroscopic and microscopic images of animal specimens for this article were performed in accordance with regulations and established guidelines for humane treatment of research animals and were reviewed and approved in advance by the relevant Institutional Animal Care and Use/Ethics Committee. Declaration of Conflicting Interests: The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article. Funding: The author(s) received no financial support for the research, authorship and/or publication of this article. The authors wish to express their thanks to the INHAND GESC, and the BSTP, ESTP, JSTP and STP membership for comprehensive reviews, excellent comments and helpful edits. We also thank Dr Rupert Kellner for manuscript review and Ms Beth Mahler, Ms. Emily Singletary, and Ms. Maureen Puccini from EPL Inc., for image editing. Photographs used in this document were either provided from the coauthors or are as acknowledged in the figure legends.