Most infections of the urinary system are caused by bacteria. The infection usually develops when bacteria enter the body through 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, which 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), and a weakened immune system. Female dogs are more prone to urinary tract infections than male dogs. Older, uncastrated male dogs, however, are prone to bacterial infec-tions of the prostate. Most bacterial infections of dogs cannot be passed to humans.
Treatment of bacterial infections is important for several reasons. The bac-teria 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 or prostate infection can be the cause of an infection in the kidneys, which is a more serious condition. Finally, untreated urinary tract infections in dogs are a common cause of a certain type of stone (struvite) that can form in the urinary tract.
Infection and inflammation of the bladder caused by bacteria is called 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. Occasionally, dogs with a bladder infection may show no signs at all. In these cases, the infection is usually diagnosed during a routine urinalysis. Dogs that are given longterm steroids or that have hyperadrenocorticism (an excess of adrenal gland hormones) are more prone to get urinary tract infections with no signs.
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 given by mouth for 2 weeks (simple infections) or longer, if needed. Your veterinarian may take more urine samples during and after treatment to make sure the medications have cured the infection. In dogs that have repeated infections, your veterinarian may take a urine sample at regular intervals (about every 1 to 3 months) to make sure the infection has not come back.
Longterm or recurring infections may be a sign of an underlying problem that needs to be addressed. Because certain medications can increase the risk of urinary tract infections, be sure your veterinarian knows about all the medications that your dog is being given. Additional tests such as contrast x-rays, ultrasonography, and/or cystoscopy may be needed in order to exclude problems such as cysts, growths, stones, tumors, and birth defects. Blood tests may be needed to diagnose other diseases that may contribute to the risk of infection.
Sometimes, even when nothing else appears to be wrong with your pet, bacterial cystitis simply continues to come back. In these cases, your veterinarian may prescribe low-dose antibiotics for your dog to take on a longterm basis. These medications will help prevent the recurrence of bladder infections, as well as preventing the infection from spreading upwards into the kidneys. If your dog is on longterm, low-dose antibiotics, frequent monitoring (urinalysis and bacterial culture) is usually necessary. Encouraging your dog to urinate frequently during the day may help prevent the infection from coming back.
Kidney Infection (Pyelonephritis)
Pyelonephritis is inflammation of the kidneys. This is usually caused by bacteria in the urinary tract that have climbed upwards into the bladder and then continued into the kidneys. The risk factors for pyelonephritis and those for bacterial cystitis are very similar. Anything that interferes with normal urine flow through the urinary system, such as stones in the kidneys or ureters, can increase the risk for pyelonephritis. In young dogs, birth defects such as ectopic ureters (see Kidney and Urinary Tract Disorders of Dogs: Disorders of the Ureters) can cause pyelonephritis. Dogs at risk for this condition are the very young, the very old, those that have weak immune systems, and 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, a reduced appetite, excessive thirst, or excessive urination. The kidneys might suddenly begin to fail. Dogs with longterm pyelonephritis may have few or no signs (other than excessive thirst and urination), and they are often not diagnosed until their kidneys begin to fail.
Your veterinarian may be able to diagnose pyelonephritis through urine and blood samples. In many cases, ultrasonography or contrast x-rays may be necessary for diagnosis.
Treatment includes longterm antibiotics (4 to 6 weeks), sometimes at high dosages. If your dog is very ill, your veterinarian may give intravenous fluids and injectable antibiotics. In extreme cases, the infected kidney must be removed in order to prevent the infection from spreading to the remaining, healthy kidney. Your veterinarian may take urine samples at regular intervals (about once a month) during and after treatment to make sure the infection does not come back. Dogs with pyelonephritis are at high risk for repeated infections. Because pyelonephritis can be a life-threatening disease, following your veterinarian's recommendations is important.
Animals with short-term 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 dog stable and comfortable.
Interstitial nephritis is another type of inflammation of the kidney. In dogs, sudden onset (acute) interstitial nephritis is often triggered by infectious diseases. The most common cause is leptospirosis (infection by Leptospira interrogans). Leptospirosis is usually spread by wildlife, such as raccoons, opossums, rats, and other small mammals. It can occur in dogs living in rural, suburban, and even in urban areas. Antibiotics are needed to treat the infection. Kidney failure, if it has occurred, may be treated with supportive treatment, including fluids. Humans can also become infected with leptospirosis. Although transmission from dogs to humans is uncommon, if a dog is diagnosed with the infection, the dog owner should consult a physician.
Capillaria plica Infection
Capillaria plica is a small worm that can infect the bladder, and, less often, the ureters and kidneys of dogs. This is an uncommon parasite in pet dogs. The worms are threadlike, yellowish, and 0.5 to 2.5 inches (13 to 60 millimeters) long. This infection is most common in wild animals. Dogs catch the infection by eating earthworms that carry the larvae of the parasite. Most dogs have no signs. Some show signs of excessive urination, inability to control urination, and urinating in abnormal places. The worms' eggs come out in the urine; your veterinarian may be able to identify them when examining a urine sample. The best treatment has not been determined, but several different antiparasitic drugs are available.
Giant Kidney Worm Infection
Giant kidney worms, known as Dioctophyma renale, are a type of parasite that can infect the kidney and the abdomen of dogs. However, they are uncommon parasites in pet dogs. This is one of the largest parasitic worms known and can reach 40 inches (103 centimeters) in length. Female worms are bigger than males. Both sexes are blood red in color. They lay eggs that are barrel-shaped and yellow-brown in color. The urine of infected dogs contains these eggs.
Dogs catch the worm by eating infected raw fish, frogs, or certain common backyard worms (such as earthworms). Once a dog begins digesting the infected fish, frog, or worm, the giant kidney worm makes its way out of the bowels of the dog, into the liver, and finally into the kidneys. Often the worms do not make it all the way to the kidney and end up instead in the abdomen.
Once in the kidneys, the worms cause blockage and destruction of kidney tissues. The right kidney is the one most commonly infected. If both kidneys become infected, kidney failure may result. Other problems that can result from this infection include inflammation of the abdominal cavity, bands of scar tissue in the abdomen or intestines, and liver disease. Signs of the infection include blood in the urine, excessive urination, weight loss, and pain in the abdomen or in the area around the kidneys.
Your veterinarian may be able to diagnose a giant kidney worm infection by identifying eggs in a urine sample. Other tests, such as x-rays, ultrasonography, or abdominal surgery may be needed in order to make the diagnosis. The best treatment is removal of the affected kidney, as long as the other kidney is healthy. To prevent this infection, make sure your dog does not eat raw fish or other animals likely to be infected.
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