Adult, male, domestic longhaired, Felis domesticusThe cat was hospitalized for signs attributable to acute renal failure. The cat had elevated BUN and Creatinine levels. The cat was euthanized for acute renal failure and submitted to the diagnostic laboratory for necropsy.

Gross Description:  

The cat was in good body condition with normal amounts of body fat and only mild postmortem autolysis. The lungs were mildly congested and edematous. The urinary bladder was empty. Cut sections of both kidneys had a diffuse lightly pale appearance in the cortices. No other significant gross changes were observed in the carcass.

Histopathologic Description:

H&E sections of kidney were submitted. There is some mild variability within the slides submitted. Numerous cortical and medullary tubules are moderately dilated(Fig. 2-1). Tubular lining epithelium of these tubules is flattened and attenuated. Occasional tubules contain clusters of necrotic epithelial cells and rare neutrophils. Rare granular casts are present. Variable, but usually low numbers of intratubular irregular shaped greenish brown birefringent crystals (Fig. 2-2) that fluoresce under polarized light are present within cortical and medullary tubules. Occasional crystals also contain variable amounts of basophilic staining material interpreted as partial mineralization (Fig. 2-3). In some sections, the cortical interstitium has multifocal mild infiltrates of lymphocytes and macrophages with mild foci of interstitial fibrosis.

Morphologic Diagnosis:  

Kidney, acute tubular necrosis (nephrosis), multifocal, moderate with nephritis, interstitial, lymphohistocytic, multifocal, mild.


Melamine/cyanuric acid toxicosis

Contributor Comment:  

The findings in the kidney are consistent with acute toxic tubular necrosis (nephrosis). The history of elevated BUN and creatinine also supports the histological findings. The crystals present in scattered tubules have a greenish brown color, fluoresce under polarized light and are generally less in number compared to most cases of ethylene glycol (antifreeze) toxicity. The crystals do not have the typical palisading appearance as usually seen with oxalate crystals associated with ethylene glycol toxicity.

The cat had been eating one of the pet food brands (Menu foods) which had been recently recalled from the market due to suspicion of toxic compounds in the food.6 Current thoughts regarding etiology of this pet food toxicity are focused on melamine. Melamine has been identified as a component in the wheat gluten imported from China which was a component of the recalled pet foods. Current speculation is that the melamine was intentionally added to the wheat gluten to increase the apparent protein concentration of the product. At the time of this report (April, 2007), there is no standardized toxicologic test for melamine in tissue specimens.

Acute toxicity studies of melamine in mice and rats suggest that this compound is of lower toxicity and has been rated as slightly toxic in acute toxicity ratings when administered by oral route.7 The LD50 of this compound in mice and rats are 3.3g/kg and 3.2 g/kg respectively.3,7

The pathogenesis of crystalluria and the formation of bladder stones in rodents in this syndrome is not fully understood.2,3 Melamine is excreted in the dog or rat partly as crystalline dimelaminemonophosphate. This can be isolated from warm urine by precipitation with oxalic acid as crystalline monomelaminemonooxalate.2 In experimental studies, 60-86.5 per cent of the melamine fed to dogs was recovered in the urine in 24 hours.4

JPC Diagnosis:  

Kidney, corticomedullary junction and medulla: Nephritis, tubulointerstitial, acute, multifocal, mild, with tubular necrosis and degeneration, and numerous intratubular crystals, domestic longhair, (Felis domesticus), feline.

Conference Comment:  

On March 16, 2007, Menu Foods Inc. issued a recall on more than 60 million containers of pet food that was manufactured between December 3, 2006 and March 6, 2007.6 This recall occurred due to numerous instances of animal deaths attributed to food related nephrotoxicosis.5 Over the course of several months, this recall expanded to include several major commercial pet food companies and affected large numbers of dogs and cats in the United States.1 The toxic compounds contaminating wheat flour were isolated as melamine and cyanuric acid.1,4,5 Both of these compounds are considered relatively nontoxic when administered separately, but when combined, they form insoluable crystals nearly identical to the ones found in the cases of melamine associated renal failure (MARF).4,5

Up to three different crystals have been identified in the kidneys of animals affected by MARF: calcium oxalate monohydrate, calcium phosphate, and melamine-containing. On H&E, melamine-containing crystals within the lumen of renal tubules are up to 80_m in diameter, birefringent, pale yellow to brown, and vary from fan-shaped to starburst radial spokes arranged in concentric circles.4,5 Calcium oxalate crystals are also birefringent on H&E, but have a smoother surface and a slight blue tinge due to a prismatic effect.5 Calcium phosphate crystals on H&E appear as non-birefringent, basophilic particles within the lumen of renal tubules as well as within the walls of blood vessels (not apparent in the present WSC case).5 Staining characteristics of the crystals are listed below.

Staining characteristics of melamine-containing, calcium oxalate, and calcium phosphate crystals.5
StainMelamine-ContainingCalcium OxalateCalcium Phosphate
Oil Red O (72 hour)PositiveNegativeNegative
Von KossaNegativePositivePositive
Alizarin Red S (pH 4.1-4.3)NegativeNegativePositive
Hematoxylin and EosinPale yellow-brown, radiating spokes, birefringentColorless, prismatic effectBasophilic

Prolonged formalin fixation results in dissolution of the melamine-containing crystals within 6 weeks.1 Therefore, it is recommend that fixation in formalin be kept to a minimum or preserved in 100% (absolute) ethanol. Although more commonly associated with cases of ethylene glycol toxicity, the calcium oxalate crystals in cases of MARF are likely the result of a secondary oxalosis.1,5

It was brought to our attention by Dr. Wayne Corapi at Texas A&M University College of Veterinary Medicine that the intratubular crystals in WSC 2004-2005, Conference 12, Case 3, are histomorphologically similar to the melamine-containing crystals recently identified in the kidneys of cats and dogs that were fed pet food on the Menu Foods recall list manufactured between December 3, 2006 and March 6, 2007. Upon reviewing the case and performing special stains, we concur with Dr. Corapi and believe it is a case of pet food-associated nephrotoxicosis with melamine-containing crystals. This association with the outbreak of renal toxicity in Asia was also reported by Puschner et al.4 and Brown et al.1 Conference participants are encouraged to review the WSC 2004-2005 case and compare it with the crystals presented in the current case. 


1. Brown CA, Jeong KS, Poppenga RH, Puschner B, Miller DM, Ellis AE, Kang KI, Sum S, Cistola AM, Brown SA: Outbreaks of renal failure associated with melamine and cyanuric acid in dogs and cats in 2004 and 2007. J Vet Diagn Invest 19:525-531, 2007
2. Lipschitz W, Stokey E: The mode of action of three new diuretics: Melamine, adenine and formoguanine. J Pharmacol Exp Ther 83:235-249, 1945
3. Melnick RL, Boorman GA, Haseman JK, Montali RJ, Huff J: Urolithiasis and bladder carcinogenicity of melamine in rodents. Toxicol Appl Pharmacol 72:292-303, 1984
4. Puschner B, Poppenga RH, Lowenstine LJ, Filigenzi MS, Pesavento PA: Assessment of melamine and cyanuric acid toxicity in cats. J Vet Diagn Invest 19:616-624, 2007
5. Thompson ME, Lewin-Smith MR, Kalasinsky VF, Pizzolato KM, Fleetwood ML, McElhaney MR, Johnson TO: Characterization of melamine-containing and calcium oxalate crystals in three dogs with suspected pet food induced nephrotoxicosis. Vet Path 45, 2008 (in press)
6. Pet Food Recall (Melamine)/Tainted Animal Feed. originally accessed on April 23, 2007
7. NTP Acute Toxicity Studies for Melamine originally accessed on April 15, 2007

A virtual slide is not available for this case.

Fig. 2-1 Kidney

Fig. 2-2 Kidney

Fig. 2-3 Kidney

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