Renal disease in avian species is a relatively common occurrence in clinical practice and can be caused by a number of disease processes. Just as in other metabolic diseases (i.e. liver disease) determining a definitive diagnosis in a timely manner and administering appropriate therapy is crucial to the patient's survival. Unfortunately, kidney diseases in birds often carry a poor prognosis as many cases are diagnosed after they become chronic disease. This article will discuss methods of recognizing and diagnosing clinical renal disease in avian species including clinical signs, laboratory testing, diagnostic imaging, biopsy, histopathology and culture.

Clinical Signs

Clinical signs associated with kidney disease are highly variable and may be attributable to any number of disease processes. Clinical signs may include lethargy, depression, polyuria, polydipsia, dehydration, weakness, ataxia, lameness, weight loss, diarrhea and neurologic signs.

Hematological data (CBC) allows the clinician to screen the hematopoietic system for abnormalities indicative of disease. However, biochemical assays are often more valuable when attempting to diagnose renal disease/failure in birds. The clinician should remember that consistently repeatable abnormalities in the biochemical panel are more diagnostic than a single value outside of reference ranges.

Serum and Plasma proteins and dysproteinemias

Changes in plasma protein levels, in particular hypoproteinemias, have been associated with renal disease; however, few studies have evaluated serum protein levels in avian species with confirmed renal disease.1

Phosphorous

Elevations in phosphate levels are commonly seen in mammalian species with renal disease. In avian species alterations (increase) in phosphorous levels do not occur consistently with renal disease and are of little diagnostic value.2

Urea Nitrogen

Unlike mammals, urea nitrogen (UN) occurs in only small amounts in avian plasma, is excreted by glomerular filtration (100%), and is not a particularly useful indicator of renal function in birds.2 However, there is some indication that urea is significantly affected by the patient's hydration status as it is 99% reabsorbed in the tubules during periods of dehydration and may be the single most useful indicator of prerenal (dehydration) causes of kidney disease in birds.1,3

Uric Acid

Uric acid (UA) is the major end product of protein breakdown in birds. It is produced and secreted in the liver, kidney and pancreas and eliminated by tubular secretion independent of glomerular filtration, water resorption and urine flow rate.1,3 Plasma uric acid levels are not highly dependent upon the patient's hydration status and reflect the functional capacity of the renal proximal tubules.4 Therefore, hyperuricemia may be an indicator of renal disease in avian species. Unfortunately, consistent hyperuricemia is most often seen late in renal disease thus somewhat limiting the diagnostic value of this test.2 Additionally, normal uric acid levels do not necessarily guarantee that the kidneys are healthy.2,5 Other factors may also affect uric acid levels such as age (juvenile birds may have lower UA levels than adults), diet (granivorous birds have 50% lower UA concentrations than carnivorous species), post-prandial increases in uric acid (carnivorous species), hyperuricemia associated with ovulation, and the presence of purine precursors from degraded body proteins.2.6-8 Hyperuricemia may also be associated with disease conditions such as articular gout (UA plasma levels may not always rise in association with visceral gout), severe tissue damage, starvation, hypovitaminosis-A induced damage to renal epithelium, nephrotoxic drugs and hypervitaminosis D3 induced renal damage may result in elevated plasma uric acid levels.2,9-12 Plasma uric acid levels within the range of 10-20 mg/dl should always be reevaluated.5

Plasma Urea:Uric acid ratios may be used to define pre-and post-renal azotemia because reabsorption of urea is disproportionally higher than uric acid, and therefore, this ratio should be high during dehydration and ureteral obstruction.¹³ Urea:uric acid = Urea [mmol/L] ×1000/Uric acid [μmol/L]

Creatinine

Creatinine is of questionable value in evaluating renal function in avian species because birds reportedly excrete creatine before its conversion to creatinine.^13,14 Elevations in creatinine have been associated dehydration (pigeons) renal trauma or nephrotoxic drugs.¹³