Abstract
Renal failure is a major cause of morbidity and mortality in iguanas.2 Conventional mammalian diagnostics are not applicable for the diagnosis of renal failure in lizards. Urinalysis is unrewarding because of the inability of the reptilian kidney to concentrate urine (metanephric kidneys) and contamination of urine passing through the cloaca before storage in the bladder. Blood urea nitrogen and creatinine concentrations do not reflect renal failure in reptiles since they are uricotelic.3 Radiographic or ultrasound imaging of kidneys is unrewarding in iguanid lizards due to the kidneys’ location deep within the pelvic girdle, and the lack of functional information. Excretory urograms are of limited value for patients with poor renal function because of the poor contrast localization within a diseased kidney. Additionally, preliminary excretory urogram studies in clinically healthy Hermann’s tortoises (Testudo hermanni boettgeri) had only a 50% success rate of kidney visualization.6
Renal failure is currently diagnosed in reptiles by increased plasma uric acid concentrations and plasma calcium:phosphorus ratios <1. These are not specific or reliable indicators, and occur late in disease.1 Renal biopsy is the definitive test for diagnosing renal failure; however, indications to perform a biopsy, as previously stated, are not usually apparent until late in disease, if at all.
The goal of this project was to evaluate potential early diagnostic indicators of renal failure in iguanas. We have two hypotheses for early diagnostic indicators of renal failure. Hypothesis one assumes nuclear scintigraphy can be used to evaluate functional renal mass in iguanas as it can for mammals. Hypothesis two evaluates the theory that secondary renal hyperparathyroidism occurs in reptiles with renal failure. We established normal values for these two hypotheses using 10 healthy, adult, non-folliculogenic iguanas (0.91 to 2.77 kg; seven male, three female) fed a standardized diet and maintained with full-spectrum lighting. Renal biopsies confirmed that this population did not have renal failure.
Hypothesis one was evaluated using 99mTc-dimercaptosuccinic acid (99mTc-DMSA) as a renal imaging agent. Dynamic images were acquired for 6 min with static imaging out to 20 h following an injection of 1.83±0.27 mCi in the ventral tail vein. 99mTc-DMSA is a cortical tubular agent and can be used as an indicator of functional renal mass. Animals in renal failure will have diminished 99mTc-DMSA uptake.4 99mTc-DMSA has also been used to evaluate renal morphology in both man and dogs. The uptake of 99mTc-DMSA at 20 h was 14.3±2.9% as measured on the ventral view and 12.7±2.8% on the lateral view.
There are no commercial assays available that can detect changes in iguana parathyroid hormone directly. Therefore, hypothesis two was evaluated by analyzing total and ionized calcium, phosphorus, total and ionized magnesium, vitamin D3, uric acid, and concomitant evaluations of standard radiographs and dual energy X-ray absorptiometry (DEXA) for bone density assessment. The use of DEXA technology in human medicine is most noted in osteoporosis research but has expanded to include research in renal disease.7
The complete blood count and serum chemistry values were similar to previously published reports.5 The other mean plasma values of interest were ionized magnesium (0.82±0.12 mmol/L); total magnesium (2.16±0.35 mEq/L); ionized calcium (1.68±0.11 mmol/L); total calcium 13.6±2.59 mg/dl); vitamin D3 (154.9±80 nmol/L). DEXA scans determined the mean total bone mineral density of healthy iguanas to be 0.56±0.06 g/cm2; and the total bone calcium to be 1.34±0.04 g calcium expressed as a percentage of body weight.
This study standardized values of DMSA uptake, bone quality (DEXA), and plasma values relevant to renal secondary hyperparathyroidism, in a population of iguanas with healthy kidneys. DMSA appears to be a promising indicator for kidney health in iguanas. Further studies need to be performed to evaluate all of these parameters in a population of renal failure iguanas.
Literature Cited
1. Barten, S.L. 1996. Lizards. In: D.R. Mader (ed.). Reptile Medicine and Surgery. W.B. Saunders Co., Philadelphia, Pennsylvania. Pp 324–332.
2. Boyer, T.H. 1998. Emergency care of reptiles. In: A.E. Rupley (ed.). The Veterinary Clinics of North America. Exotic animal practice. W.B. Saunders Co., Philadelphia, Pennsylvania. Pp 191–206.
3. Campbell, T.W. 1996. Clinical pathology. In: D.R. Mader (ed.). Reptile Medicine and Surgery. W.B. Saunders Co., Philadelphia, Pennsylvania. Pp 248–257.
4. Daniels, G.B., S.K. Mitchell, D. Mawby, J.E. Sackman, and D. Schmidt. 1999. Renal nuclear medicine: a review. Veterinary Radiology & Ultrasound. 40:572–587.
5. Divers, S.J., G. Redmayne, and E.K. Aves. 1996. Hematologic and biochemical values of 10 green iguana (Iguana iguana). Vet. Rec.138:203–205.
6. Kolle, P., and S. Reese. 1999. Intravenous Urography and cystography in tortoises. Proceedings, Association of Reptilian and Amphibian Veterinarians. 1999:141–142.
7. Parfitt, A.M. 1998. A structural approach to renal bone disease. J. Bone Mineral Res. 13:257–266.