Most infections of the urinary system are caused by bacteria. The infection usually develops when bacteria enter the body through of the urethra. The bacteria then travel to the bladder, and in some cases they set up an infection there. Sometimes bacteria continue to move up the urinary tract to the kidneys and can result in kidney infection (pyelonephritis). There are several factors that increase the risk of urinary system infection. These include problems with urine flow (especially not being able to empty the bladder completely during urination), overly dilute urine, sugar in the urine (often a sign of diabetes mellitus), older age, a weakened immune system, and the presence of other diseases. Healthy adult cats are relatively resistant to urinary tract infections.

Treatment of bacterial infections is important for several reasons. The bacteria that cause infections of the urinary tract can become resistant to antibiotics if the infections are not treated properly. Antibiotic resistance can lead to an infection that will not go away. In some cases, an untreated or inadequately treated bladder infection can be the cause of an infection in the kidneys, which is a more serious condition.

Bladder Infection

Infection and inflammation of the bladder caused by bacteria is called bacterial cystitis. Healthy adult cats do not generally get bacterial cystitis. However, cats that are elderly, have compromised immune systems (including those with feline leukemia virus or feline immunodeficiency virus), or those with diseases that affect the whole body (such as diabetes mellitus or thyroid disease) are more prone to bacterial cystitis.

Signs of bladder infection include frequent urination, painful or difficult urination, and urinating in inappropriate places. There may also be blood in the urine. This may be more noticeable at the end of the urine stream. Rarely, cats with a bladder infection may show no signs at all. In these cases, the infection is usually diagnosed during a routine urinalysis.

A urine sample is needed to diagnose bacterial cystitis. The laboratory tests your veterinarian will likely perform on the sample are a urinalysis and a bacterial culture. Treatment consists of antibiotics. Your veterinarian may want to take more urine samples during and after treatment to make sure that the medications have controlled the infection. Longterm or recurring infections may be a sign of an underlying problem that should be addressed.

Kidney Infection (Pyelonephritis)

Pyelonephritis is a bacterial infection of the kidneys. This is usually caused by bacteria in the urinary tract that climbed upwards into the bladder and then into the kidneys. The risk factors for pyelonephritis and those for bacterial cystitis are similar. Stones in the kidney or ureter, which prevent urine from flowing normally, are a common cause. In young cats, birth defects such as ectopic ureters (see Kidney and Urinary Tract Disorders of Cats: Ectopic Ureter) can cause pyelonephritis. Cats at risk for this condition are the very young, the very old, those that have weak immune systems, or those with kidneys that cannot properly balance the amount of water in the urine. In many cases, your veterinarian may not be able to identify what caused the pyelonephritis.

Signs of pyelonephritis include pain in the sides (especially in the area around the kidneys), fever, and a general sense of not feeling well. Other signs include vomiting, decreased appetite, excessive thirst, or excessive urination. Sometimes there are no signs until kidney failure has occurred from the infection. Diagnosis of this condition requires blood and urine tests (urinalysis and bacterial culture). Longterm cases may not show abnormalities on these tests. If this is the case, contrast x-rays or ultrasonography may be needed. Treatment for pyelonephritis includes longterm anti-biotics (4 to 6 weeks), sometimes using high dosages. In some cases, intravenous fluids are given. Rarely, surgery to remove the affected kidney may be required. Because pyelonephritis can be a life-threatening disease, following your veterinarian's recommendations is important.

Cats with pyelonephritis may be able to recover full kidney function, depending on the amount of damage that occurred before treatment. If both of the kidneys have already failed, your veterinarian may be able to do little more than keep your cat stable and comfortable.

Interstitial Nephritis

Interstitial nephritis is a type of inflammation of the kidney. Sudden-onset (acute) interstitial nephritis is uncommon in cats. Chronic (longstanding) progressive interstitial nephritis is common in cats and is more commonly referred to as chronic kidney disease (see Kidney and Urinary Tract Disorders of Cats: Chronic Kidney Disease and Kidney Failure). Infectious diseases that affect the blood vessels (for example, feline infectious peritonitis) or diseases that activate the immune system can cause interstitial nephritis. It is usually not possible to identify the exact cause in an individual cat. Antibiotics or other drugs may be needed to treat an infectious disease, if one is diagnosed. Kidney failure, if it has occurred, may be treated with supportive treatment, including fluids, dietary changes, and other medications.

Capillaria Infection

Capillaria plica is a small worm that can infect the bladder, and, less often, the ureters and kidneys of cats. This is an uncommon condition in pet cats. The worms are threadlike, yellowish, and about 0.5 to 2.5 inches (13 to 60 millimeters) long. This infection is most common in wild animals. House pets catch the infection by eating earthworms that carry the larvae of the parasite. A similar but less common worm called Capillaria felis cati can also infect cats.

Most cats have no signs. Some cats will urinate frequently, become unable to control urination, or urinate in abnormal places. The worms' eggs come out in the urine; your veterinarian may be able to identify them by taking a urine sample. The best treatment has not been determined, but several different antiparasitic drugs are available.

Last full review/revision July 2011 by Scott D. Fitzgerald, DVM, PhD, DACVP, DACPV; Joseph W. Bartges, DVM, PhD, DACVIM, DACVN; Scott A. Brown, VMD, PhD, DACVIM; Sherry Lynn Sanderson, BS, DVM, PhD, DACVIM, DACVN; Melissa S. Wallace, DVM, DACVIM