Pyelonephritis is an acute or chronic upper urinary tract disease which involves renal interstitial inflammation, often most severe in the renal pelvis and adjacent medullary tissue, associated with bacterial infection of the kidney. Long term, low-grade pyelonephritis is a common cause of chronic renal disease in cats.
Bacterial urinary tract infection is less common in the cat than in the dog and the importance of ascending infection and obstructive nephropathy in the pathogenesis of pyelonephritis in the cat is not clear.
Pyelonephritis in cats may result in fever, anorexia, lethargy, dehydration, and weight loss. Such clinical signs are not expected in all affected animals and may be present for a very short period, making recognition of the disease difficult. Depending on the extent of renal parenchymal involvement, azotemia, hyperphosphatemia, nonregenerative anemia, and metabolic acidosis also may be found. Unfortunately, urinalysis findings often are nonspecific and urine cultures may be negative in chronic pyelonephritis.
Excretory urography may show dilatation of the renal pelvis and diverticula or decreased renal size.
Cats with ureteral calculi or chronic cystitis can develop pyelonephritis that causes decompensation. It is more likely for both kidneys to be involved, indicating chronic renal disease. One of the major features of pyelonephritis is pyelectasia. The renal pelvis is dilated, but not to the degree as an obstructed kidney. The renal diverticuli tend to look more pointed than rounded, and the papilla is blunted (see image 1). The ureter is usually not traceable past the first few centimeters, though it may be distended all the way to the bladder. You may see some sludge in the renal pelvis or ureter without obstructive calculi (image 2). The hyperechoic halo can also be a sign of pyelonephritis. Kidneys with pyelonephritis also tend to enlarge because of inflammation. This can be hard to tell with kidneys that are already altered in size, but you can compare them to previous measurements if they are available. Hyperechoic or hypoechoic foci in the cortex or medulla can also be a sign of infection. Pyelonephritis can be either unilateral or bilateral.
Treatment consists of bactericidal antibiotics (for four to eight weeks) to eradicate the infection, definitive therapy to remove any predisposing factors (e.g., calculi, obstruction), and fluid therapy to restore and maintain hydration. The prognosis is good if predisposing factors can be eliminated and if treatment is instituted before the development of endstage renal disease.