Pyelonephritis is an inflammation of the kidney, and is most often due to a bacterial infection that has made it's way from the lower urinary tract (urinary bladder) to the kidney. There may be factors that enhance the susceptibility to infection such as congenital abnormalities, metabolic disorders or systemic immunosuppression; however, no underlying disorders need exist.

The clinical signs associated with pyelonephritis may be mild, or may even go unnoticed, although pyelonephritis can lead to kidney failure, sepsis (infection throughout the bloodstream) and even death, if not addressed. Depending on the specific case, certain diagnostics and therapeutics are usually recommended and tailored to each individual.

Several diseases or disorders can present similarly and need to be differentiated from pyelonephritis. These include:

Urolithiasis (stones) anywhere throughout the urinary tract

Lower urinary tract infection

Chronic kidney failure

Bacterial prostatitis (inflammation of the prostate)

Metritis (inflammation of the uterus)

Other causes of fever and painful abdomen such as pancreatitis (inflammation of the pancreas) or peritonitis (inflammation of the abdominal cavity), as a percentage of animals with pyelonephritis present for abdominal pain

Other causes of increased thirst and increased urination include hyperadrenocorticism (Cushing's disease), diabetes mellitus, kidney disease and liver disease